Tecumseh Engine Rebuild for Mini Bikes
and Conversion from Snow Blower Engine to Mini Bike Engine format.

I buy and collect minibikes. 11/01/19. Email: cfh@provide.net

Table of Contents Chapters.



1. A Tecumseh Flat Head for Your 1970s Minibike - Introduction.
    Say you have a vintage 1970s mini bike and you want a proper flat head motor for it. How would someone go about doing that? First, let's talk a bit about minibike engine history...

    In late 1969, Briggs and Stratton decided they would no longer sell their flat head motors to minibike makers. They just didn't want to be associcated with minibikes (probably more legal reasons than anything.) But regardless, this really opened the door for Tecumseh to supply motors. And just about all the mini bike makers took Tecumseh up on this, and utilized their engines for minibikes during the 1970s.

    But the problem with 1970s motors is time, and kids, and mini bikes. After all, who rode mini bikes during the 1970s... crazy kids! And they probably didn't treat these motors very well. That's why when you see a 1970s vintage minibike for sale today, often it is missing the motor.

The Tecumseh minibike motors of 1968-1970: H25, H30, H35, HS40, H50. The best minibike motor, the HS50, wasn't introduced until 1972. The HS40, introduced in 1968, was also a very popular minibike engine. Noticed the "balloon" blower housing Tecumseh logo, which was replaced about 1971 with the "dual flags" Tecumseh logo.

    Yes there are modern motor replacements. The Harbor Freight $100 "Predator 212" motor has put a good number of old minibikes back on the road. But the "look" of the 212 motor is completely different than the old school Tecumseh flathead. For example the cylinder isn't vertical, it's more horizontal. In fact, there's really nothing that looks even similar on a Predator 212 to a classic Tecumseh motor. And if you're going for that vintage look, the modern Predator 212 style overhead value motor just isn't going to cut it.

This is what we are aiming to get to... a minibike ready Tecumseh motor in white (or black.) This particular Tecumseh HS50 motor came off a 1970s Ford snowblower.

Or if you prefer in black... a minibike ready Tecumseh HS50. This particular motor came off a 1970s Arien snowblower.

    If your 1970s Tecumseh minibike motor, which is now 40+ years old and abused by every 10 year kid in the neighborhood, is dead... How do you get a decent motor to restore your vintage minibike? Well the short answer is, Snowblowers!

    I guess it depends where you live, but here in the midwest, old snowblowers are pretty common (and cheap.) And generally speaking, at least here in the midwest, a 20 or 30 or 40 year old snowblower motor tends to be pretty low usage. Why is that? Well they were only used during snow times (no dirt or high temperatures). And in the midwest (I'm in Michigan), we don't tend to get a lot of snow (maybe 30 inches a year.) So these old snowblower flathead Tecumseh HS50 engines are great donors for a vintage minibike.

    That said though, snowblower flathead Tecumseh engines need some work to make them minibike-ready. And that's what we are going to discuss in this document. Obviously you need some tools to do this, and some general mechanical knowledge. I will assume you have these things! Also you'll need some parts. Most is available from Ebay or some of the online small motor warehouses. Also note some abbreviations like SBH (short block horizontal), BB (ball bearing), HS (horizontal small frame), BCR (bump compression release), MCR (mechanical compression release).

    The Famous Tecumseh HS40 Engine.
    We should talk about the Tecumseh HS40 motor. It was the mainstay of minibike motors during the 1970s. The Tecumseh 4hp HS40 was introduced in 1968, but really did not get traction with minibike manufacturers until about 1970. At that point, the large frame H50 motor was out of favor (due to it's extremely wide body format), and the much narrower HS40 just fit better in the wave of mini-cycle style bikes made in the 1970s (like say the Rupp Roadster, etc.) Since there was no small format 5hp motor (until 1972 when the HS50 was introduced), the HS40 really was the go-to motor for a great number of minibike/minicycle makers.

A 1970 Tecumseh HS40 restored and ready for use on a 1970 Rupp Roadster. Notice the motor color (silver), which is what Rupp specified for their 1970 HS40 engines. Also notice the "balloon" logo, as used before 1971 on most Tecumseh engines. The black HS40 engine in the background is a restored HS40 motor for a 1971 or 1972 style Rupp minibike.

    The early 1968-1970 Tecumseh HS40 motors had some small differences like a mechanical compression release (MCR) camshaft (as the motor starts, the compression release swings out of the way). The cam had a huge intake lobe on it and they compensated with a shorter lifter so they had to clearance the crankshaft for it to fit. After this they changed to a bump compression release (BCR). Frankly the BCR style can suck power out of an engine. Sure it helps with starting, but it doesn't go away like MCR after the engine starts. The bump can easily be ground off the cam though.

    With that all said, you will see a lot of references to the HS40 motor in this document. Not because I recommend getting a HS40 motor from a snowblower... if you're doing that you're better off with an HS50 snowblower motor. But a lot of people like to rebuild original HS40 minibike motors from 1969-1975 for their originality. Also if you have a minibike that has lights (like say a Rupp Roadster), an original HS40 motor with an alternator is really the way to use lights (I have yet to see a snowblower with an HS50 motor and an alternator!) Because of this, you will see a lot of HS40 references in this document.

    But again, if you need a motor (you have nothing), and you want a Tecumseh flathead for your vintage 1970s minibike, converting a snowblower HS50 motor to minibike format is really the way to go!


2. Finding a Good Snowblower Donor.

    Finding a good used (old) snowblower (if you're in a snow area), should be pretty easy. Craigslist (especially during the summer) should net a good donor snowblower. Ideally you want a Tecumseh HS50 (5hp) motor (introduced in 1972) on a nasty looking snowblower, but a Tecumseh HS40 (4hp) motors work great too (they were introduced in 1968.) This is usually pretty easy to identify from the snowblower model number. For example, a Toro 521 is a 21" wide snowblower with a 5hp motor. Just make sure it's a Tecumseh flat head (HS50 or HSSK50 or LH195), and you're probably golden. Likewise if you find a 4hp Tecumseh, the same model designations apply. That is, an Arien 420 will be a 4hp motor on a 20" wide snowblower.

The Tecumseh H50 (left) and HS50 (right). There's a pretty dramatic size difference in the two! The H50 is very wide, too wide for most minibike frames that weren't specifically designed for an H50.

    Note because I'm no longer 10 years old and 100 pounds, I try to get 5hp flathead motors. But frankly the Tecumseh HS40 motor (4hp) works pretty darn well too. One thing I would say though, don't get a Tecumseh H50 engine. The H50, used before Tecumseh introduced the HS50 in 1972, is too wide of a format for many minibike applications (though some minibikes did originally use the H50 in teh 1968/1969 time frame.) The HS frame size is just so much nicer of a motor to use in nearly every minibike wanting a flathead.

The Tecumseh HSSK50 motor specs for snowblowers. Note the cast iron cylinder sleeve.

    Also do not discount the newer HSSK50 or LH195 Tecumseh snow blower motors. Though the pull start looks slightly different, theses winter application motors are very good. So good infact, some have a cast iron cylinder (opposed to aluminum.) These are darn good motors.

    In the 2000s Tecumseh came out with the LH195 motor. Basically it's a HSSK50 with a different cam and lifters. They advertise this motor as 5.5hp (instead of the HSSK50's 5hp rating.) This may have happened because the cam is cut differently. The cam used on these is part #37040. Late production models of the LH195 will have a *plastic* cam. Obviously you don't want that! The plastic cam needs lighter duty valve springs too. So if you're good at looking at valve springs through a removed breather assembly, you can tell if it's a metal or plastic cam. If you get an LH195 with a metal cam, that's a great motor. It uses all the same parts as an HSSK50 (or late model HS50) engine (aside from the lifters and cam, be it metal or plastic.)

    Generally I avoid 3hp and 3.5hp (Tecumseh H30 and H35) engines. Often they have a 5/8" PTO shaft. And they are usually "side poppers" (exhaust on the PTO side of the engine). Side exhaust will really limit your exhaust options, as the common "Taylor" minibike exhausts don't exist for side poppers. Also personally I don't like exhaust blowing on my left leg! And 3hp and 3.5hp are just too whimpy. If you're going through all this trouble to re-do a motor, it's better to have a 4hp or (better yet) 5hp motor as your starting point. Note they did use side popper engines on a few minibikes in the 1970s, but for 98% of all mini bikes, rear exhaust engines were the norm. It's the same amount of work to restore a H30 motor as it is an HS50 engine. So why not get the best starting point with the more power.

A 1970s Yardman 521 snowblower with a nice Tecumseh HS50 motor.

    Note you do not want the "combo" snowblower plastic models. Generally these will have a tapered PTO output shaft, which won't work on a minibike. Avoid these models unless they are dirt cheap (or you can see the PTO shaft, and make sure it's not tapered.) You can convert a tapered shaft motor to a fixed width, but it's more work and expense. Hence you're better off not getting a "plastic" snowblower (unless it's dirt cheap or free!)

A 1980s Craftsman "plastic" snowblower with a Tecumseh HS50 motor. Generally avoid this type of snowblower, as the PTO shaft will almost certainly be tapered.

    Do we care if the motor runs? Heck no! In fact, I would say a non-running snowblower is probably an advantage, as the snowblower will be cheaper. We are going to rebuild the motor anyway. I mean if it runs that's great. But it's not necessary.

A 1970s snowblower with a Tecumseh HS50 motor. Looks like junk, but this is the perfect motor donor candidate for a vintage 1970s minibike.

    Tecumseh made flathead motors until 2006 (Tecumseh went out of business in late 2008.) The newer 1990s style flathead motors are not labeled as HS50 but are HSSK50 and LH195 models. They have a newer style "cyclone" or "pull lite" pull starter (introduced around 1987). They are great flathead motors too, though they don't have quite the same look as the old style "four leg" pull start motors. If you get a snowblower that is an HSSK50 motor (or LH195 which is 5.5hp), don't turn it down. They are great flathead motors, and all the info here applies to them too. Also some HSSK50 motors have a cast iron cylinder sleeve (instead of aluminum) for increased life.

    The newer flathead cyclone motors do have some advantages. For the most part, they are really an older HS50, but with some modifications to make them emit less hydrocarbons. That doesn't effect their power. In fact, because they are newer, chances are pretty good that they will have lower time (usage). Also they won't be a "points and condenser" motor, but will have electronic (solid state CDI) ignition... which frankly is a really nice feature (one less thing you'll have to worry about, and less parts to buy/adjust/maintain.)

Here's a "cyclone" pull start HSSK50 snowblower flathead motor. It does have a slightly different look than the old school 4-leg pull start HS50 motors. But it's still a great motor and works wonderfully on a minibike. Note the pull handle will get replaced with the proper style.

Here's two converted snowblower Tecumseh 5hp engines. The one on the left is an "old school" 4-leg pull start 1982 model HS50. The one on the right is a 2001 Tecumseh HSSK50 motor with the "cyclone" pull start. Note after the conversion to minibike format they don't tend to look that much different. And both work great on a mini bike. And if you really want, the old style 4-leg pull start blower housing/starter for the HS50 will fit right on the newer HSSK50 engines. This gives the 'old look', but on a newer motor.

    If you can't find a number identifying which model a motor may be, you can always remove the head and measure the bore.
    • HS50 = 2.812"
    • HS50 (newer HS50/HSSK50/LH195 motors starting mid-1990s)* = 2.795"
    • HS40 = 2.625" (2 5/8", note the fatter H50 also used this size, but had a longer bore.)
    • H35 = 2.500" (pre-1990s)
    • H25/H30 = 2.3125" (pre-1990s)
    * The smaller HS50 bore also came about with a smaller exhaust valve. The smaller exhaust valve reduces knocking (pre-detonation) by reducing heat in the combustion chamber. Since the exhaust valve glows cherry red, it can ignite the fuel in the combustion chamber before the spark plug does. In normal (cam and carburetor) configuration, a smaller exhaust valve is a good thing, hence Tecumseh made that change in the mid-1990s.

    Don't forget Sears (Craftsmen) snowblowers either! Though Sears uses their own "143" number (instead of a Tecumseh number), most Sears/Craftsmen snowblower engines were made by Tecumseh. You can cross reference the 143 craftsmen number to figure out what the Tecumseh number may be (and the size/hp of the motor.) A link to that cross reference is at the top of this document.

    In the 2000s Tecumseh came out with the LH195 motor. Basically it's a HSSK50 with a different cam and lifters. They advertise this motor as 5.5hp (instead of the HSSK50's 5hp rating.) This may have happened because the cam is cut differently. The cam used on these is part #37040. Late production models of the LH195 will have a *plastic* cam. Obviously you don't want that! The plastic cam needs lighter duty valve springs too. So if you're good at looking at valve springs through a removed breather assembly, you can tell if it's a metal or plastic cam. If you get an LH195 with a metal cam, that's a great motor. It uses all the same parts as an HSSK50 (or late model HS50) engine (aside from the lifters and cam, be it metal or plastic.)

    After you have found your donor snowblower, remove the motor. It should be pretty obvious what is needed to be done for this task. The most difficult task will be getting the pully off the PTO output shaft. I personally often use a 3-leg flywheel puller for this task. Not necessary but it helps a lot.


3. Dating a Tecumseh Engine.

    Tecumseh used a pretty consistent method to dating their motors. The only problem with their method is the lack of a decade in the serial number/year number. But based on other information, you can generally tell what decade the motor was made. Some things to think about include:
    • Blower Housing numbers on the side of housing, on a two rivet tag: pre-1973.
    • Blower Housing numbers stamped into metal on top near spark plug, 1973 and later.
    • Sticker with model/serial number on side of blower housing: 1995 and later.
    • Small metal tag with numbers, bolted to fins on side of the cylinder: pre-1975. But sometimes this is seen with short/long block numbers into the 1990s.
    • Blower Housing Shape: a rounded blower housing (rounded as the metal bends towards the spark plug): pre-1973.
    • Blower Housing Shape: starting about 1973 most horizontal shaft motors had a more squared blower housing.
    • Four leg pull start: pre-1987.
    • Cyclone "pull lite" style pull start: 1987 and later.
    • Points/Condenser ignition: pre-1984.
    • CDI solidstate ignition: 1984 and newer.
    • Motor block painted: pre-1995.
    • Naked motor block: 1995 and newer.
    • Alloy flywheel: pre-1975.
    • Steel flywheel: 1975 and later.

    The numbers on the blower housing is what identifies the motor type and it's date. The first set of numbers are the motor type like "HS50", for example. The second set of numbers are the actually motor spec number. This is important information when ordering parts. The third set of numbers is the serial number. But really it's the manufacture date. The first number is the last digit of year. The next three numbers are the day (from 001 to 365). Sometimes there's a letter after this number, which gives further info like shift/plant, etc.

    The problem with the Tecumseh date scheme is that no decade is provide in the date code. Hence the above bullet points help figure out exactly when a particular motor was made.

Dating a Tecumseh motor: On the side of the blower housing is a two rivet tag with the model/spec/serial number. This style and location of numbers was done on pre-1973 motors. This motor is an H35 spec# 452716, made on the 118th day of 1970.

Dating a Tecumseh motor: On the top of mid-1970s Tecumseh motors should be impressed numbers, as shown here. This is a HS50 spec# 67175d. Newer motors (like 1990s till 2008) used a sticker with a bar code identifying the motor number. In the picture below you can see on the left "HS50", which is the model/size of the motor. If you want to know the year, look at the number after "SER" (the third set of numbers). In the example above the numbers are "0183b". The "0" is the last digit of the year, and the "183" is the day of the year (from 001 to 365) that the motor was made, and the "B" is the factory the motor was made. So the above motor was made on July 2, 1980. So how did we know it was 1980 and not 1990? Well the decade can be a bit tricky, but for the most part you can identify the decade from the snowblower it was removed, or the style of the engine (this engine was points/condenser.) So this motor could not be 1990, and it couldn't be 1970 (the HS50 model didn't exist in 1970), so it had to be 1980.

Serial number mounted on a tag. This happened on some motor pre-1975. First number is the basic model (HS50). The '67023' number tells us the specifications of the motor. And the number '2270b' tells us the motor was made on the 270th day of 1972.

Serial number mounted on a tag. This happened on some motor pre-1975. First number is the basic model (HS50). The '67011' number tells us the specifications of the motor. The number '2175b' tells us the motor was made on the 175th day of 1972. The second tag shows that this motor was sold as a short block. The motor may have been damaged, and a new short block was used to replace the original block.

    Craftsmen Tecumseh motors used their own system of numbers. You can cross reference the Craftsmen 143 number to a standard Tecumseh model/spec number using this cross reference chart to figure out which exact Tecumseh motor you have.

Craftsmen serial number on top of motor. Using the Tecumseh/Craftsmen cross reference document, the "143" number translates to a Tecumseh H35 spec# 45592R, made on the 133rd day of 1986.

The new style Tecumseh date tag. This HSSK50 motor was made on the 162nd day of 2001.


4. Is Your Snowblower Engine a Good Minibike Engine?

    With the flathead Tecumseh motor out of the snowblower and on a table, you can evaluate what is needed to be done to make it "minibike ready."

A 1980 Tecumseh HS50 snowblower motor.

    One thing you'll see on a lot of snowblowers is weird colors. Minibike motor colors are generally white or black (or possibly silver, like on MTD or 1970 model Rupps.) Orange (a common snowblower motor color) was never used on a minibike. Not that this is a big deal, but it's something to think about. If that bothers you, get a different snowblower! Generally Toro and Ariens will have orange motors.

Note you can see the serial number of the motor in this picture. Also note the snow guards on the gas tank and the pull start. These will be removed for our minibike usage.

Here's a picture of the throttle assembly and the snowblower carburetor. These both will be changed to minibike style, and an air cleaner assembly added. This style muffler can be used on a minibike, though traditionally, it's not really a minibike muffler.

Here's a Tecumseh HS40 of a Toro snowblower. The orange color is not a deal breaker. We can fix that color...


5. The PTO Output Shaft and Extended Cam Shaft.
    One of the most important features about a donor motor is the PTO (power take off) output shaft. That's the shaft which you ulitimately mount the clutch upon, and powers the minibike. The PTO needs to be the correct size. The most common PTO size is 2.25" long and 3/4" diameter. This is the "standard" mini bike shaft format. Longer is OK (heck you can always cut the shaft, and if using a Torque Converter, longer is an advantage.) But you never want shorter than 2.25" in length. Note some Rupp HS40 motors have 2.75" in length. And some snowblowers also use that length too. If you are running a torque converter, the longer length PTO is a plus (though no required.)

A 1978 Tecumseh HS40 snowblower motor with 1" wide and 2.75" long PTO shaft.

    Tecumseh 4 and 5hp motors generally will have either a 3/4" or 1" PTO shaft. Some Tecumseh 3 and 3.5hp motors can have a 5/8" PTO. I would say to avoid motors with a 5/8" PTO. Not because you can't use them, but it just limited the number of clutch/torque converter options you have. The 3/4" size is most desirable, but don't discount a 1" PTO shaft. It will limit your clutch options a bit too, but it's still workable (and if you have a metal lathe, you can turn a 1" shaft down to 3/4".) But for the most part, having a 3/4" PTO with no taper is the ideal format for a minibike.

A 1970s Tecumseh H35 snowblower motor with 3/4" PTO and a secondary cam shaft PTO.

    Note some Tecumseh snowblower motors will have a secondary camshaft extended PTO (used on snowblowers for reverse.) What Tecumseh did is lengthen the cam shaft so it exits out the side case, and put another gear/pully upon this smaller shaft. Though not ideal, this is not a show stopper. The simple reason is that the extended cam shaft can be cut (externally), and the motor used as a single PTO shaft engine. The downside is this is another seal and another place for the engine to potentially leak oil. But for the most part, it's fine to just cut it clean externally.

Cutting the secondary extended cam shaft PTO using a hacksaw blade.

    The easiest way to get rid of the secondary small cam shaft PTO is to start the motor, and use a hacksaw blade to cut the shaft! This works amazingly well and is quick and easy. A more time consuming way to remove the secondary PTO shaft is to "split the case." That is, remove the side case, and remove the cam (easier than it sounds.) Then the cam shaft can be cut to .6" from the main gear to the end of the cam (to get rid of the extended secondary PTO.) Unfortunately you will have to get a different side cover, one with only the one main PTO hole. Or you can use a 7/8" freeze plug to fill the hole. A different side sump cover is easy to get as most Tecumseh HS40 and HS50 motor from pretty much any era will work (they are generally interchangeable.) Note that H30 and H35 side covers are generally different - the PTO bushing is only 7/8" diameter, where on HS40/HS50 Tecumseh engines have a PTO bushing that is 1" diameter. Another variable is the location of the oil fill (single or double low, or high). Generically the best minibike single shaft side cover is Tecumseh #32700, but like I said, others will work too (with the oil fill position being the main variable.) Don't discount high oil fill PTO covers - though not a minibike thing, they sure are convenient to use!

Here's a Tecumseh H35 motor that had a second cam shaft PTO. I cut it off externally (using the running motor and hacksaw blade to do so), and restored the motor as usual. You can't even tell the second small PTO was even there! Note this motor has a bushing style side case (as most motors, but not all, will have.)

    Note if you remove the side case and keep the small PTO (say when you're removing the governor), you may have problem with the seal around the small cam shaft PTO. That seal does not like to have the cam removed, meaning you can end up with a leak. I mention this because if you remove the internal governor parts (which requires spliting the case), you may have problems with this seal.

Here's a cam that is being measured. It shows .580" to .600" length on the end going into the side cover.

    If you remove the side case (for any reason), my suggestion is to cut the end of the cam shaft length to .580" to .600", and get another side case without the small PTO cam shaft hole. Alternatively, you can use a 7/8" .885 "freeze plug" (aka steel cup expansion plug) and some sealer, and seal the side case cam shaft hole (after you cut the cam end to .580" in length.) These are available from Dorman Products or Advance Auto Parts. Note the exact length of the end of the cam is approximate... this was originally cut on a bandsaw at the factory, and the measurement varies. So keep the measurement at .580" to .600" and it should work out fine.

A freeze plug installed on an H35 side sump cover, where an extended cam shaft once came through. I got a little over-zealous with the hammer when installing it, and slightly dented the freeze plug. That's because I didn't compare it's size to the side cover hole size... there's a .005" difference. Use a bench grinder and lightly take some meat off the freeze plug sides so it's installs easier. Note I installed this freeze plug convex... probably should have done it concave, but either way will work.

    To install the freeze plug, you can do it with the engine assembled, or with the side cover off (your choice.) Personally I find it's easy with the engine assembled. Use a dial caliper on the freeze plug and compare it to the hole in the side case. I have found the freeze plug is usually about .005" of an inch bigger. That's actually quite a bit bigger... for this reason I take the freeze plug to a fine stone bench grinder, and take a little meat off the side of the plug. Then I can nylon hammer the freeze plug easily into the side case cam hole. Afterwards I used a bit of Permatec Ultra Gray gasket compound around the freeze plug, just to make sure it does not leak. Some people use JB Weld for this... but either will work. Also not you can mount the freeze plug either way (concave or convex.) I guess if you really want to be accurate, concave is the proper mount.

This 1974 Tecumseh HS50 engine has a metal tag with the identification number and serial number. This motor's number of "4 079b" indicates it was made on the 79th day of 1974. Also note this HS50 motor has a ball bearing side case (opposed to an aluminum bushing.) You can tell this because the seal is indented into the side case about a 1/8 of an inch. On bushing style side case, the seal is flush with the outside of the side case.

    Note some HS40 and HS50 motors can have a ball bearing side case (opposed to an aluminum bushing.) These are easy to identify because the side case seal is indented about a 1/4 of an inch, opposed to a bushing side case seal which is flush with the side case.. The ball bearing side case is less prone to wear and is generally considered to be a good thing for a mini bike motor. Some makers, like Rupp, used ball bearing side cases on their HS40 motors too, for better wear properties. All of the early (pre-1975) HS50 motors seem to used a ball bearing side case. The only downside to a ball bearing side case is if you need to remove the side case. Removing a ball bearing side case is more eloborate - the seal has to be pried out, and a "C" clip exposed and removed to get the side case off. Hence you best have a spare side case seal #28540, if you are going to remove a ball bearing side case.

On ball bearing side covers, the external seal must be pried out before removing the side case. I use a flat head screwdriver to get the seal out. This will almost certainly ruin the seal (the lower seal was the one pried out, the upper seal is new.) Because of this, have a new seal #28540 handy to replace it when re-assemblying the case. Note in the picture below the blue arrow shows the "C" clip. This *must* be removed to get a ball bearing side cover off!

    After the "C" clip is removed, the side cover will come right off. If you look at the inside of the side cover, you can see why the "C" clip had to be removed - the ball bearing is held in place with two screws/washers. If you try and pry the side cover off without removing the exterior "C" clip, you will destroy the ball bearing mount screws/washers.

Here's the ball bearing side case removed. Note the ball bearing is held in place with two screws and oversized washers. This is why you can't remove the side cover without taking out the "C" clip (exposed from removing the seal, as show in the picture above.)

    Note that the crankshaft used in motors with side case ball bearings is different. The area that contacts the ball bearing is smaller than a crankshaft used in a bushing side case motor. Bushing motors had a .9985" surface contacting the bushing. On ball bearing side case motors the crankshaft had .785" surface contacting the ball bearing. I guess in theory you could turn a bushing crankshaft down and a C clip groove width of .0745", making it a ball bearing crank.

A ball bearing crankshaft (installed) versus a bushing side case crankshaft. Notice the difference is diameter in the portion of the crankshaft that goes through the side case. Diameter is .785" on the ball bearing side case, version .998" on the bushing side case crankshaft.

Specs from Tecumseh on the Ball Bearing HS50 crankshaft.


6. Ignition - Points/Condenser or Electronic, and Flywheel material.
    Prior to about 1984, all Tecumseh motors used points and condenser for the ignition. Tecumseh officially switched to electronic (solid state CDI) ignition on August 1st 1984 (though on some models it took to 1985 to fully implement the change.) This new system does not need points/condenser. This change is a good thing for the most part... Less parts to wear and maintain. But the classic minibike motor used points/condenser.

A 1974 Tecumseh HS50 engine with a stock aluminum flywheel. Prior to 1975, Tecumseh used aluminum for their flywheel material (opposed to steel.)

    Also flywheel options are different between the two systems. With the electronic ignition, all the flywheels will be steel. With points and condenser, most flywheels are also steel, but original minibike motors uses aluminum flywheels. The advantage to aluminum is faster RPM gains when you hit the gas. The advantage to steel is once you are to the desired RPM the momentum keeps you there better. The year Tecumseh went to steel for all flywheels seems to be 1975. Prior to 1975 and you'll find their engines (be it HS40 or HS50) will be aluminum.

    In the end, I'm not sure the flywheel material really matters all that much. All snowblowers will have a steel flywheel (often with "teeth" to accomodate electric start.) So frankly that choice is made for you. And you will, "get what you get", when it comes to electronic versus points/condenser. You can't change the system, so you'll have to deal with what you have. Also changing a steel flywheel to an alloy flywheel is harder than you may think. I mean you can do it, but it's tricky. So I don't sugguest that avenue, if you were thinking about that...

electronic ignition on the left, points/condenser on the right. Blue arrow shows external ignition coil on electronic ignition engines.

    It's easy to tell the two systems apart. The coil (blue arrow in the above picture) is external on electronic ignitions. On points/condeser motors, the coil is internal (behind the flywheel). That means maintaining the points/condenser requires removing the flywheel. We'll talk about that later, but if you have a points/condenser motor, you will have to remove that flywheel to maintain those parts.

    Note flywheels are not necessarily interchangeable. The flywheel side of the crankshaft changed about 1981. The 'points condenser' crankshafts before 1981 had a .541" diameter end for the flywheel. The 1981 and later crankshafts had a .644" diamenter end for the flywheel. This change happened almost the same time as CDI (electronic) ignition came about (in 1985). But there were some points/condenser motros made with the larger crankshaft ends in the 1981-1984 era. These flywheels are not interchangeable between the two sizes. And they certainly are not interchangeable between points/condenser and CDI model engines.


7. Removing the Snow Parts.
    At this point it's a good idea to remove unnecessary parts from the motor. Here's a list of what to remove:

    • Remove the snow shields from around the carburetor, discard (keep screws).
    • Remove the carburetor, discard (keep screws)
    • Remove the blower housing (with pull start attached), keep.
    • Remove the pull starter, keep. If it has a snow sheild around the top, discard.
    • Remove PTO shaft pully, discard. Usually need a pully remover to do this.
    • Remove the gas tank. If there is a snow shield, remove, discard.
    • Remove the fuel line, discard.
    • Remove the mufflier, keep.

The Tecumseh 'snow box' seen on snowblowers, and keeps snow off the carb.

With these parts removed, you basically have a "long block" motor. Now we can deal with building the motor back up with minibike usable parts.

Parts are removed from the motor, making it a long block. Ready for some serious work!


8. The Governor - Remove or Keep?
    The governor's job is to keep the motor from sustained high RPMs. These Tecumseh motors are designed for a maximum RPM of 3600. And frankly, they don't like to stay at that RPM level for any length of time (more than say 5 seconds.) As a kid I was always asked on my minibike, "did you remove the governor so it would go faster??" I was never sure what the governor was, much less how to remove it! Good thing too. Because a de-governor motor and a 10 year old are almost for sure going to throw the connecting rod, pretty much ruining the motor.

The Tecumseh side case removed. Shown is the plastic gear and "wings" which push on the throttle arm and lower the RPM if the motor revs too long.

    The way the governor works is as the user hits maximum RPM (3600), internally a small plastic wheel pushes out "wings", which push on a rod connected to the throttle arm. This backs the RPMs down. This happens automatically, without any user control. So you'll get the full 3600 RPMs at inital full throttle, but only for a second or two. Then the motor backs it down to about 3000 RPM (or so.) This saves the motors from throwing the connecting rod.

The parts that are removed from the side case to remove the governor. The "wings" are clearly shown in this picture.

    So how does the motor "throw the rod". Well the stock connecting rod is cast aluminum, and frankly not that robust. What probably happens is the rod's journals get starved for oil at high RPM and heat up. This compromises the rod and it breaks - often throwing the rod pieces through the side of the motor's case!

Here is the internal lever that rides on the plastic governor gear and which is influenced by the governor's "wings", to lower the RPM. This internal lever is connected to the external throttle linkage, and ultimately the carb's throttle. Sometimes I remove these parts too (blue arrows), and tap the case hole 10-32 and put a bolt in the hole. When using a slide carb these parts just get in the way.

    With this in mind, do you want to remove the governor? Frankly on my motors I always remove the governor. The internal plastic cam and it's pieces can also break, ruining a motor. And frankly I'm smart enough to know not to leave a motor at full RPM for extended periods. Basically me, as the rider, is the governor! Also if you're using a torque converter on your mindbike, it provides some level of protection too (as the torque converter automatically changes gears, this changes the motor load and RPM.)

Want to really 'bulletproof' the Tecuseh motor with no governor? Get an aluminum billet connecting rod from Arc #6282. Advertised for the Tecumseh OHV 5.5hp motor, this rod works great in both vintage Tecumseh HS40 and HS50 engines. At $65 it's good assurance you won't throw a rod in a non-governor HS40 or HS50 Tecumseh motor. In the picture below you can see the Arc 6282 rod (lower) is a lot more robust than the (upper) stock Tecumseh rod.

Here's an HS50 engine with an Arc 6282 rod installed.

    If you are going to remove the governor, you will have to "split the case." Frankly I don't even remove the oil to do this. Just put the motor on its side (flywheel down), and remove the side case screws. Then gently tap the side case with a rubber mallet, and the case should slide off (assuming the the motor does not have a crank ball bearing on the side case, which snowblowers do not use.) Then the parts can be removed from the side case (as shown in the above picture.) It's just two E clips to remove the parts. Also don't forget to remove the thin washer after the gears are removed! Note on newer Tecumseh motors, the governor parts have to be cut off (they pressed on the parts and don't use E clips.)

Engine on its side, side cover removed. Now you can remove the governor arm.

    When replacing the side cover, always use a new gasket #27677. Scrap the old one off with a razor blade. One thing I really hate is leaking engines. So I clean both the engine and side cover gasket area with Naptha, and apply a very thin coat of Permatex Ultra Gray gasket maker. I use a thin layer of the Permatex Ultra Gray on both the engine and side cover faces. Then put the gasket on the engine, and replace the side cover. But don't screw it down just yet. Just push the cover on, and let it dry for an hour. Then screw the side sump cover down to 115 inch/pounds or 9.5 foot/pounds.

Permatex gasket maker. I use it with a new gasket #27677.

    I have seen some people use the Permatex gasket maker with no gasket. Personally I'm not comfortable doing that, but I have seen it. I perfer to use a new gasket #27677 and the Permatec. I've found that to work best.

Here's a motor where I removed the governor internally, but also removed the external throttle control arm. The hole was tapped with a 10-32 tap, and a 3/4" 10-32 screw/washer installed to plug the hole.

    Additionally sometimes I remove the internal metal arm and external throttle arm. If I'm using a non-stock slide carb, I don't need the throttle arm. Hence removing the internal governor arm and external throttle arm is ideal. In this case I tap the hole where the governor arm was located with a 10-32 tap, and put a 3/4" long 10-32 bolt and washer into the hole to plug it. One less spot for oil to leak! But don't do this if you are using the stock carburetor set up - you will need that throttle arm.

Here's a snowblower motor that "threw a rod." Nice hole in the side case! I'm not entirely sure why this happened, but it was probably because the snow blower did not have enough oil.

    If you are really worried about "throwing a rod" on your non-governed Tecumseh, consider using a billet aluminum connecting rod. At $65 for the Arc #6282 rod, it's good piece of mind. The rod is 3.484" Tecumseh 5.5 OHV Stock Length with a solid dipper. Designed for the Tecumseh overhead value 5.5hp motor, it happens to work just fine in vintage Tecumseh HS40 and HS50 motors. Yes you will have to remove the piston from the motor and do some work to install the rod. But if you already have the case split, it's not a huge job.

Here's the same motor as above's piston and rod... Note where the connecting rod attached to the crankshaft melted! You can see where the aluminum rod attached to the steel crankshaft that the rod's aluminum melted and broke from lack of oil. This could have happened due to high RPM... but the more likely culprit was the oil level was too low.

    Now and then you many have to replace the seals that go on the side case (or behind the magneto on the other side of the motor.) Also you will need a new side cover gasket #27677. The side cover should use a torque wrench and the bolts tightened to 115 inch/pounds or 9.5 foot/pounds. Here's some other part numbers that may be useful:
    • Bushing side case seal: #27897 (typical of a snowblower motor)
    • Ball bearing side case seal: #28540 (as used on Rupp Tecumseh motors)
    • Side case ball bearing (as used on Rupp Tecumseh motors): #28458
    • Magneto side case seal: #32600 (or #27876 ?)
    • Side case (sump) gasket: #27677

    Also don't forget if you have a ball bearing side case motor (like say a Rupp HS40 or a pre-1975 HS50 motor), you will definitely need a new side case seal #28540. That's because you have to pry out the original seal to expose a "C" clip, and remove the "C" clip, before the side case will come off the motor.


9. Tecumseh Part Numbers.
    Depending on your skill set and what your motor have or need, there are some other parts that may be helpful to have. Note when I say "points" models I generally mean 1984 and before. And likewise "no points" is 1985 and later. On crankshafts the change in 1982 brought about a larger flywheel nut and less taper on the flywheel side of the crank. This kind of corresponded with the change from points/condenser to electronic ignition, not perfectly, but roughly (technically though the change from points to electronic ignition was August 1st 1984.) In 1983/1983 there were some motor with both the less taper crankshaft and points, but this is happened only for a short time. Note the early pre-1982 crankshafts with more taper used a 7/16-20 nut, and the later crankshafts with less taper used a larger 1/2-20 nut. Also on HS50 motors they changed the cylinder bore at some point (2.795" newer HS50, 2.812" older bore), I believe in the 1990s. This change happened when they decreased the exhaust value size slightly to accomodate EPA emission regulations.

    Another note... HS40 and HS50 do *not* use the same crankshaft. The counterweights are different (though pretty much every other dimension is the same.)

    Note the change during the 1990 from the old style thick rings to thinner rings. The thinner rings used less oil and hence less emmisions. Sometimes these are known as the "UK version", I guess because the change was implemented overseas first. The thin ring piston is different too, with less side shirt and obviously thinner gaps for the rings. But the newer thin ring style piston/rings for HS40 works fine in the old vintage 1970s and 1980s HS40 motors. And in some ways is more desirable because there is less cylinder wall contact with the piston and rings.

    On the HS50 motors, the newer thin ring piston are slightly different in size (as they are really HSSK50/HSSK55/LH195 piston/rings), and will *not* work in 1972-1990 HS50 motors. Why? Because the piston size is different. With the newer thin ring piston, they changed the piston size from 2.812" (old thick ring piston) to 2.795" (thin ring piston.) So these, unlike the HS40, are not interchangable.

    Tecumseh motor bores:

    • HS50 = 2.812" thick ring piston (1972 to 1980s)
    • HS50/HSSK50 = 2.795" thin ring piston (1990s and later)
    • HS40 = 2.625" (note the fat body H50 also used this size, but had a longer bore.)
    • H35 = 2.500" (pre-1990s)
    • H25/H30 = 2.3125" (pre-1990s)

On the left is the old style thick ring piston. At the right is the newer UK style thin ring piston. Notice the wrist pin is set in, for decreased piston side wall area. This and the thinner rings gives less wear properties.

    It's important to note the difference in the thick and thin rings. Because on the internet, these part numbers get cross referenced. You need to know what is what, so you get the correct rings/piston. Especially if you buy pistons or rings on ebay. The part numbers just get crossed, and you may not order what you really want. I highly suggest you look closely at the pictures, as it's easy to identify a thin or thick ring piston and thin or thick rings.

    • Thick Rings: two top compression rings are .092" thick. Bottom oil ring is .153" thick.
    • Thin Rings: two top compression rings are .060" thick. Bottom oil ring is .121" thick.

    • HS50/HSSK50 crankshaft 3/4" PTO new style (no points): #34740
    • HS50 crankshaft 3/4" PTO old style (points): #33677 or #34728
    • HS50 crankshaft 3/4" PTO old style (points) ball bearing case: #33676 (1972 to 1980)

    • HS40 crankshaft 3/4" PTO new style CDI (no points): #34734
    • HS40 crankshaft 3/4" PTO old style (points): #32877
    • HS40 crankshaft 3/4" PTO, long, old style (points), ball bearing: #33080 (Rupp style)

    • HS40/HS50 stock connecting rod: #32875
    • HS40/HS50/HSSK50 valve lifter (intake/exhaust the same): #27241
    • LH195sp valve lifter (intake/exhaust same, longer than hs50 lifter): #37670
    • HS40/HS50 valve spring (intake/exhaust the same): #31672
    • HS40/HS50 valve spring cap retainer lower (intake/exhaust the same): #31673
    • HS40 valve spring cap upper (intake/exhaust the same): #31671, 27883
    • HS50 valve spring cap upper (exhaust is different, bigger hole): #27882
    • HS40/HS50/HSSK50/LH195sp intake valve: #32644
    • HS40/HS50 exhaust valve (large): #29313
    • HSSK50/LH195sp exhaust valve (smaller): #36472

    • HS40/HS50/HSSK50 cam bump compression release (BCR): #33158
    • HS40 cam mechanical compression release (MCR): #32701
    • H30/H35 cam (no compression release): #31317
    • H30/H35 cam (bump compression release): #33149
    • H30/H35 cam (mechanical compression release): #32197
    • LH195sp cam mechanical compression release (MCR): #37040 (can not be used on HS40/HS50). Note this cam is available in metal or plastic! If you're looking on ebay and it comes with valve springs, don't buy it! The lighter duty valve springs are for the plastic cam (also there are "splines" behind the exhaust lobe of the plastic cam.) The metal version of 37040 uses standard HS50 valve springs.

    • HS40 head gasket: #33015
    • HS50 head gasket: #33554 or #36443
    • HS40/HS50 head: #33016
    • HS40 gasket set: #33240
    • HS50 gasket set: #33683 (or #36444 with two less gaskets)
    • HS40/HS50 side cover gasket: #27677
    • Note that a late 1969 and later HS40 and all HS50 heads are identical. But the head gaskets are not the same (due to difference sizes in combustion chambers).
      ---
    • HS50 piston/pin/rings assembly (thick rings): #34535
    • HS50 piston/pin/rings assembly (thick rings): #34536 (.010 over)
    • HS50 piston/pin/rings assembly (thick rings): #34537 (.020 over)

    • HS50 piston/pin (thick rings): #33562
    • HS50 piston/pin (thick rings): #33563 (.010 over)
    • HS50 piston/pin (thick rings): #33564 (.020 over)

    • HS50 piston rings (thick): #33567
    • HS50 piston rings (thick): #33568 (.010 over)
    • HS50 piston rings (thick): #33569 (.020 over)
      ---
    • HS50 piston/pin/rings assembly (thin rings): #40004 (replaces #36073)
    • HS50 piston/pin/rings assembly (thin rings): #40005 (.010 over, replaces #36074)
    • HS50 piston/pin/rings assembly (thin rings): #36075 (.020 over)

    • HS50 piston/pin (thin rings): #36070
    • HS50 piston/pin (thin rings): #36071 (.010 over)
    • HS50 piston/pin (thin rings): #36072 (.020 over)

    • HS50 piston rings (thin): #40006 (replaces #36076)
    • HS50 piston rings (thin): #40007 (.010 over, replaces #36077)
    • HS50 piston rings (thin): #36078 (.020 over)
      ---
    • HS40 piston/pin/rings assembly (thick rings): #34520
    • HS40 piston/pin/rings assembly (thick rings): #34521 (.010 over)
    • HS40 piston/pin/rings assembly (thick rings): #34522 (.020 over)

    • HS40 piston/pin (thick rings): #32603
    • HS40 piston/pin (thick rings): #32604 (.010 over)
    • HS40 piston/pin (thick rings): #32605 (.020 over)

    • HS40 piston rings (thick): #34854 (or #33315)
    • HS40 piston rings (thick): #34855 (.010 over)
    • HS40 piston rings (thick): #34856 (.020 over)
      ---
    • HS40 piston/pin/rings assembly (thin rings): #35544
    • HS40 piston/pin/rings assembly (thin rings): #35545 (.010 over)
    • HS40 piston/pin/rings assembly (thin rings): #35546 (.020 over)

    • HS40 piston/pin (thin rings): #35541
    • HS40 piston/pin (thin rings): #35542 (.010 over)
    • HS40 piston/pin (thin rings): #35543 (.020 over)

    • HS40 piston rings (thin rings): #35547
    • HS40 piston rings (thin rings): #35548 (.010 over)
    • HS40 piston rings (thin rings): #35549 (.020 over)

    • HS40/HS50 side case oil seal PTO side (bushing style): #27897 (typical of a snowblower motor)
    • HS40/HS50 side case oil seal PTO side (ball bearing style ala Rupp): #28540
    • HS40/HS50 side case ball bearing (as used on Rupp Tecumseh motors): #28458
    • HS40/HS50 side case ball bearing clip: #28539
    • HS40/HS50 side case oil seal magneto side: #32600
    • H50 side case oil seal magneto side: #27876
    • H50 side case oil seal PTO side: #28427
    • H30/H35/HS40/HS50/HSSK50 side case (sump) gasket: #27677
    • Side case HS40/HS50 single PTO hole, bushing, low oil fill: #32700
    • Side case HS40/HS50 single PTO hole, bushing, high oil fill: #34674
    • Side case H35/HS40/HS50 single PTO hole, ball bearing, low oil fill: #30756
    • Oil fill plug no dip stick: #27625
    • Oil fill plug with dip stick, level motor mount: #34245
    • Oil fill plug with dip stick, 20 degree motor mount: #32969
    • Oil fill plug gasket: #36832
    • Oil fill plug and dipstick (for high fill location): #37884

    • HS40/HS50 steel flywheel lighted (CDI): #611083 (also #611093)
    • HS40/HS50 Lighting coil for CDI steel flywheel above: #611095
    • HS40/HS50 steel flywheel (CDI): #611081
    • HS40/HS50 steel flywheel (points): #611029 or #33701
    • HS40 alloy flywheel (points): #32517 or #610754
    • HS40 alloy flywheel (points) lighted: #610769
    • HS40 alloy flywheel (points) for electric start: #610845
    • H50 alloy flywheel (points) lighted: #30755
    • LAV35 alloy flywheel (points): #31331
    • H35 alloy flywheel (points): #31332
    • H30 alloy flywheel (points): #1267 (aka #30542 and #29165 and #30367)
    • H30 alloy flywheel (points): #610781
    • LAV35 (vertical) alloy flywheel (points): #610782
    • CDI alternator lighting coil: #34960
    • Alloy flywheel pull start cup: #590416
    • Flywheel screen: #33668
    • Flywheel key: #610961
    • Breather assembly: #31337 (best version to get has a rubber breather tube)
    • Breather assembly gasket: #31619

    • Condenser: #30548b
    • Points: #30547a
    • Points cover aluminum: #30550 (or #610947)
    • Points cover gasket (small hole): #610948
    • Points cover gasket (large hole): #32052
    • Points magneto stator assembly: #30561B
    • Points coil only: #30560
    • CDI magneto stator: #34443
    • Spark plug cover (aka boot): #610118
    • Pull start handle: #590387
    • Spark plug shorting clip: #30747

    • Air cleaner assembly: #730127 or #730164 (both mount plates)
    • Air cleaner mount plate: #31691 or #31914 (offset)
    • Air cleaner body: #31715
    • Air filter paper: #30727
    • Air filter foam: #31700
    • Air cleaner gasket: #27272A
    • Exhaust L shape: #35771
    • Exhaust L shape with pipe: #37684
    • HS40,H35,H30 Exhaust gasket: #33170
    • HS50 Exhaust gasket: #33670
    • HS40 Intake gasket: #32649
    • HS50 Intake gasket: #33673

    • Gasket set HS40: #33240
    • Gasket set HS50/HSSK50: #33683 or #36444

    • Diaphragm & Gasket carb rebuild kit 236B #630978
    • Diaphragm & Gasket carb rebuild kit complete #631893
    • Diaphragm Inlet Needle, Seat, Gasket, Spring carb rebuild kit 236A #630932a
    • Carb float: #632019 (metal) or #632802 (plastic)
    • Carb float repair kit: #31840

    The HS40 seems to be the most abused motor. You see a lot of 1970s HS40 motors with worn cylinders. The bore was 2.6250 and the piston size is 2.6215 with a .0035 piston-to-wall clearance (Tecumseh specs are .004 to .006 on inspection.) If the cylinder is actually .008 big/tapered/scratched, a +.010 piston won't fix the problems, you just can't get the issues fixed with that little material to work with. So the Tecumseh #35546 piston (.020 over) is the best choice to resize the HS40 cylinder. Tecumseh says to use a rigid hone (like Sunnen AN-112) to resize and 390 grit stones. Well 400s are what's available and they must be silicon carbide. The only lubricant to use is Goodson Honing Oil or automatic trans fluid.

    Head Gaskets on the HS40 and HS50 motors are different! You can't not mix them. Yes they are the same outside shape, but the HS50 head gasket is cut wider on the inside. This accomodates the larger bore on the HS50. If you put an HS40 head gasket on an HS50 engine, it will not run right!

Here's an HS40 head gasket on top of an HS50 head gasket. Notice the difference? You can not mix these up. The combustion chamber size is different.


10. Getting the Color Off and Back On.
    Let's say you're going the standard minibike color (white or black.) But the motor you have is not that color. How do you handle this? Well the motor can be repainted. Since you have the motor tore down to the long block, all the metal pieces can be easily painted. But keep in mind that you need to use a gasoline resistant paint! Otherwise just one gas spill, and the paoint comes off your motor!

    To get around this problem, I prefer to use powdercoat. As part of the process they will sandblast the parts. And the finished product looks great, and is completely gasoline resistant. On color, personally I don't use "pure white" or any glossy colors. Why? Because they are higher maintenance. Gloss colors are really hard to keep looking good, and show scratches and defects and dirt very clearly. For this reason I use an off-white matt color and medium black (not gloss black.) Lots of people do use the 'bright white' or gloss colors. But personally I just don't like that look. Also most paints are not gasoline resistant. If you're using something unknown, the SEM 1k clear in a can works well as a good gloss sealer.

These are the removable parts which were powdercoated.

    What about that Primer bulb on the blower housing (above the pull start)? For a minibike, the primer bulb goes away. It will pull off or unscrew. Discard it, we won't be using it. I guess you can leave the primer bulb hole(s), but personally I try and plug the hole(s). Using a TIG welder I can usually pull this off without distorting the blower housing metal (it can be a bit tough to do.) This is obviously optional, but is a nice touch. We won't be needing that primer bulb, so plugging the hole is ideal. After welding it shut, use a flap grinder and then sandpaper to make it smooth. Obviously do this before paint or powdercoat!

Rustoleum Appliance Epoxy spray paint. And SEM 1k clear #40903. If you don't use the Appliance Epoxy paint, the SEM 1k clear works really well as protection against gasoline.

    So fine, you have decided to powdercoat the removable metal. But what about the block? Obviously you can't powdercoat that! For me, I put the motor at top dead center and block the exhaust and intake ports (and breather) with rags. Then I sandblast the long block! This gets the old color off (important if you bought an orange motor!) When done, blow off the motor with compressed air. Then I use a Home Depot available spray can product to paint the long block. This paint is "Rust-Oleum Appliance Epoxy" paint. It's available in white, almond, black and silver (though silver I had to order online.) I like the almond color for my white engines (the white color is "too white" for me). If you let this spray paint dry for 48 hours, it's really gas resistant. More resistant than other paints I have found (short of spraying 2-part urethane.)

A sandblasted Tecumseh 3.5hp "side popper" long block, ready for new paint.

    After letting the long block paint dry, don't re-assemble the blower housing parts. Because the next thing we have to address is...


11. Replacing the Points and Condenser, kill switch.
    If you have an older motor, now is the time to deal with the points and condenser. If you have a newer motor with electronic ignition, skip this step. Note we are doing this after we blasted and painted the long block.

This is a CDI electronic ignition motor (flywheel removed.) No points to replace! If you want a remote kill switch, solder a wire to the solder lug shown with the blue arrow. Note this block can support either a CDI ignition *or* a points/condenser magneto. This can be seen by the mounting points for the points/condenser magneto (not used.)

Adjusting the CDI coil is pretty easy. Just loosen the two screws that hold it, put a business carb between the bottom of the coil and the top of the fly wheel magnets, and tighten the coil mounting screws. that's it!

    Since you already have the blower housing off, you can see the flywheel. This will need to be removed to access the points/condenser. Remove the center nut first. Some guys have luck using a rubber mallet and hitting the crankshaft end (where the nut was remove) to "pop" the flywheel. Personally I've never had luck doing that. For this reason I bought a 3 leg flywheel puller off ebay for $20. But be warned, a jaw type flywheel puller can crack the flywheel. This is not really an issue with a steel flywheel (as used on snowblowers and most Tecumseh engines after 1975). But if you are working on a minibike Tecumseh or very early Tecumseh with an alloy flywheel, you can crack a flywheel using a jaw type puller. Just keep that in mind. For that reason, a lot of guys use a stiff plastic mallet and hit the crankshaft (where the flywheel nut was removed), while using a screwdriver to push behind the flywheel.

Using a flywheel puller to remove the flywheel. Not recommended for an alloy flywheel.

Here's the recommended flywheel puller, Tecumseh #670215. There is also a couple other versions that are similar like #670306, #670218, #25183.

    With the flywheel off, you can see the coil and (covered) points and condenser. First off, just replace the consender #30548b. Don't mess around, it's a $5 part, just install a new one. I also take a Sharpie and write the date on the top of the condenser. It's easy to replace this part, it's one screw to hold it in place, and then the nut where the wires all connect (on the points.) Don't replace that nut just yet, we'll need it off to service the points.

The Tecumseh magneto (coil). The blue arrow shows the condenser #30548.

Working on a minibike Tecumseh with lights? Then the magneto is slightly different (to power the bike's lights.) Here's a Rupp style magneto (coil). It's really the same, but the condenser is in a slightly different location. The blue arrow shows the condenser #30548.

    Next remove the aluminum sheild that covers the points. It's just a spring clip that holds the cover in place, easy to remove with a small plyer. Then you can see the points. Rotate the engine's PTO shaft to TDC (top dead center). That's with the shaft's key straight up at 12 oclock. Also there's usually an arrow or other mark on the point riser colar showing TDC. When the PTO at TDC, the points should be gapped at .020". Don't worry about that right this second... Just use a flathead screwdriver and remove the screw that holds the points in place. Also pry the wire bolt out, and the points will be completely removed from the magneto.

Top blue arrow shows colar arrow which indicates the points adjustment location. It's not TDC but I will often call it that! You want the points at the highest point on the colar. Note if you have this colar installed backwards, with the flywheel key at 12 oclock, the points will not be open! So obviously don't do that. (Better yet, don't remove the points colar.) Bottom blue arrow shows .020" point gap at TDC.

    With the points removed, you can examine the actual points. They are probably pitted (unless they are new.) That's why we are replacing them. Note the pitting is exaggerated by a bad condenser. In fact, that's the condenser's only job, to save the points from pitting (the engine will work without the condenser, but the points will pit severely, and fail.) The condenser is essentially an electronic shock absorber, preventing arcing across the points (which causes the points to pit and lowers the voltage at the spark plug.) For this reason you should always replace the condenser when the flywheel is removed. Anyway, the points are part #30547a, and new ones can be purchased for about $10. Alternatively you can re-face the original points to remove the pitting. It's your choice, either way works fine. Personally I like to re-face the original points myself.

    Remember, when installing new points, you need to clean them before installing. There will be a protective coating over the face of the points. I use some 600 grit sand paper to do this. Just close the points with the sand paper between them, and pull the paper out. Make sure you get both sides of the points. This will make sure your new points work as they should.

Tecumseh points #30547a, removed. Note the pitting on the contact! That's a bad thing...

    Now install the new (or re-faced) points. A lot of companies (other than Tecumseh) made replacement points. My suggestion is to use 'real' Tecumseh brand points if you're replacing them. The off-brand (like say Sten) don't seem to be quite right. Since the engine is still at the high point on the colar, adjust the point gap at .020", and tighten the screw. When you do this the wire nut should be tight - point gap can change if that nut is not tight. Also as you tighten the flat head screw to set the points, sometimes they 'walk'. So re-check after you have the adjustment flat head screw tight, to make sure it's still at .020" gap.

    You may now install the aluminum cover #30550 (or #610947) and it's gasket #610948. Usually the cover and its gasket will be in fine condition and can be re-used.

    Now you may replace the wires on the points screw. The nut should have been tigthened when you adjusted the points, so remove it and install the wires (assuming that was not done before). I add a 3 foot length of wire (light blue wire in the pictures) to this screw/nut too. This is the kill switch wire! I like all my minibikes to have a handlebar mounted kill switch, and you'll need a wire to ground out the points. I use #18 or #20 gauge stranded wire for this. This is very important to do, since you have the entire motor already apart! Alternatively you can use a spark plug shorting clip #30747. But personally I hate those things.

Note how the wires are run around the magneto (and underneath the points retaining clip. Get this right, it is very important! Also note the flywheel key and the notched portion is towards the center of the engine (see blue arrow).

    How the wires are run around the magneto is very important! If you get this wrong, the flywheel can rip or fray the wires, killing the engine. The wires run under the point cover retaining clip. They also run underneath the condenser "leg". Get this right!! Finally if the flywheel key #610961 was removed, note it's orientation - the notch goes towards the center of the engine. With this finished, you can replace the flywheel and tighten the flywheel retaining nut. This nut must be pretty darn tight! Otherwise the flywheel can become loose, and shear the flywheel key. This will change the timing, and the motor will never run. If you have a torque wrench, tighten the flywheel nut to about 50 foot/pounds. Not a good idea to use an impact driver to tighten the flywheel, it can over-stresses the flywheel and crack it.

When you are finished, you work should look like the picture below. Note the orientation of the wires! This is how 1970s wiring is done. In the next picture though...

For the last few years of the points/condenser models, Tecumseh started to wire their points and condenser a bit differently. Below is the more "modern" version of how they ran the wiring from the condenser to the points' bolt. Either way works, but I stay with how the motor was originally wired. NOTE the red arrows in the picture below. DO NOT remove these bolts! They adjust the timing, and there's no reason to do this (assuming you do not remove those two bolts!)

    Setting the Timing is a trick on these engines. The CDI motors don't allow you to change the timing, so those are really easy! There's nothing to do, other than set the distance between the magnet and the coil (using a business card, picture seen above a few screens.) But on points motors, the timing can be set to .035� BTDC (before top dead center). My suggestion is this.... don't mess with it! There's NO reason to remove the magneto in the first place (it's held in place with two 1/4-20 bolts.) Hopefully you didn't remove these. Because adjusting the timing requires a dial indicator and an ohm meter, and some work.

To set the timing on a points/condenser engine, you need to use a dial indicator to find .035" before top dead center. Once this is done, the magneto bolts so the entire magneto frame can be turned. Using an ohm meter on low ohms (or buzz tone), position the magneto so the points open (buzz off) at .035" before TDC.

    Basically you set the points gap to .020", and then adjust the piston to .035� BTDC (for an HS50). Now adjust the magneto frame (stator plate) to just open the points there (using an ohm meter.) It's just not worth doing unless you removed the magneto - and even if you did do that, usually you can put the magneto back and adjust it to the old 'witness marks' from the bolts. That will get you pretty much perfect correct.

    Since you installed a kill switch wire on the points, I also suggest running a hard ground wire with the kill switch wire. Though technically the engine's frame is grounded through the entire bike, if you have rubber mounted handlebars, that ground may not work. So have a discrete hard ground is recommended. I mount that wire directly to an engine bolt (see picture below.) Then I used a two wire kill switch like the one used on the modern-ish DB30 (these are available on ebay for $5.)

Here's the (light blue) points wire and the green hard ground wire. I used these for a kill switch mounted on the handbars. Also note the new 1/4" fuel line is installed here.

    One important note... In the picture above you can see I used the bolt that holds the engine sheild as the ground point. This bolt is a 1/4-20 style bolt, and it's not very long (1/2" long). DO NOT use a longer bolt than 1/2"!! I can't stress this enough. If you use too long of a bolt, and can push a hole into the combustion chamber. That would be a bad thing...

The DB30 style handlebar mounted kill switch. Though way more 'modern' than a traditional kill switch, it provides a lot more safety for both the rider and the motor. I don't like the "push and hold" style kill switches... Having a hard 'on' and 'off' handlebar mounted kill switch is a much better system.

    After the flywheel is installed, you need to check to make sure you did everything right! Also remember to use a torque wrench to tighten the flywheel. A steel flywheel gets 550 inch/pounds (or 46 foot/pounds), and an alloy flywheel gets 450 inch/pounds (or 37 foot/pounds).

    The best way to check your work and check for spark is to remove the spark plug. Leave it connected and laying on the top of the head (as a ground connection), and quickly spin the flywheel (the flywheel should spin easily with the spark plug removed.) You should see a small spark across the plug. Having the shop lights off helps, but usually you can see the spark in a lit room too.

Testing the points/condenser installation with the spark plug removed.

    Another method is to use a spark testing gadget. This item is available online and plugs into the spark plug wire, and then to ground. There's a 240 volt light bulb inside the tester, and it will light up if there is indeed spark (obviously you need to spin the flywheel quickly.) Personally I like using the spark plug method better, as it's more realistic. But this works too.

Testing the points/condenser installation with the spark plug removed, using a spark testing gadget.

    Another easy way to test for spark is with the flywheel attached, but with the blower housing still removed. Handy to do before you re-assemble the blower housing. This requires having the spark plug removed too. Put a socket on your drill, put your drill in "forward", and spin the crankshaft using the flywheel nut and the drill. Since the plug is removed, you can easily see if there's spark at the plug.

Testing the points/condenser installation with the spark plug removed, using drill and socket on the flywheel nut.

    If you don't have spark, then obviously you did something wrong. One thing I do too often is to have the colar that moves the points installed backwards. If you do that the spark will never work (as the magnets on the flywheel won't line up with the points opening.) Remember with the flywheel key at 12 oclock, the points should be fully open.

    Lighting Coil or Charging Coil Wiring.
    If your Tecumseh engine originated on a Rupp, MTD, etc style minibike with a headlight, the magneto has what is known as a "lighting coil" or "charging coil." The flywheel is special too, with many more magnets. As the motor spins, it supplies 12 volts AC at 3 amps to the lighting system (headlight, tail light, etc.) These motors are considered "special", as putting a snow blower motor on a Rupp Roadster isn't going to make the headlight work (if that's important to you, which it may not.) It's easy to tell if your motor has a lighting coil. There's a 3 prong plug on the front side of the motor that is used for the lights.

The lighting coil plug used on Tecumseh motors with the ability to power light (headlight for example.)

    If you're wondering what the wiring is for this plug, as we're looking at it in the picture above, here's the layout (note that "ground" is just the engine casing):
    • Top lug (green wire) = ignition grounding (kill switch)
    • Left lug (black wire) = light power to headlight
    • Right lug (red wire) = light power to tail light

    Note that in the 1980s the wire color at the 3-prong plug changed. Green stayed the same, but the two power wires changed to yellow.

    You can check your lighting coil on a running engine. At lower RPM (say 2500) the voltage will be lower (around 8 volts.) But as the motor revs it goes up to as much as 14 volts. You can use a volt meter and check at the plug for AC voltage. Note on bigger motors (say 10hp), where there's a battery (and the battery is charged), there will be a diode or bridge rectifier to convert the AC generated by the magneto to DC voltage. But for smaller Tecumseh 4 and 5hp engines, this is not the case.

    As far as headlights go, Rupp for example in 1970-1975 used a sealed hi/lo beam lamp #4456 (hard to find and about $30). The 1969 Rupp headlight did not have hi/lo beam, and used a #4411 sealed lamp (much cheaper, about $12 and easier to find, even at your local auto parts store.)

Lighting coil which can be added to CDI solidstate ignition motors. But the down side is this only supplies 12volt at 350ma.

    On CDI (solidstate) engines, lights can also be added. The least expensive way is to get Tecumseh alternator coil #34960. It's a lighting coil that bolts on the side of the existing CDI ignition coil. It's not really that expensive, and it doesn't require a different flywheel either. That's the upside. But the downside is it only provides 350ma of power (compared to 3amps for the points lighting coil version.) Why is that? Because there's no additional magnets on the flywheel. Also the points lighting coil has more coils too. So what can you light with 350ma of power? Probably not a lot more than an LED headlight on your mini bike.

Lighting coil #34960 and how it fits on an existing CDI coil.


12. Piston Rings and Valves (Honing/Boring).
    Sometimes you will need to re-ring a motor. This is especially the case if you're working on a vintage 1970s motor that is original to your vintage 1970s minibike. If you're doing a snowblower to minibike conversion, you probably don't need this information, and you can dis-regard this section.

    If your HS40 or HS50 motor needs new rings, this section should help with that installation. So how do you know you need new rings? With the spark plug installed, how does it "pull" (with the pull starter)? Do you feel compression? You can use a compression check gauge too. But if the motor has a compression release (some do), this test won't work. Most HS40 and HS50 motors have a BCR (bump compression release). But you can still get some idea on compression by just pulling the starter cord.. The later HSSK50 motors do however have a more severe compression release, which helps with starting the engine. If that's the case, the slow pull start test won't tell you much. If the engine starts and burns oil, chances are good you have a ring problem.

    You can also pull the head and just look at the cylinder bore. If it's smooth that's good. If you can see the cross-hatch pattern, that's even better! A lot of snowblower engines will have very limited use. If you see cross hatches on the cylinder bore, you have a very light use engine. That's a great thing! You can also do the piston "wiggle" test, as seen in the video below.

Short movie checking the cylinder of an HS50/HSSK50 engine for wear.

    If you don't have any history with the motor (that is you've never seen it run), and you can remove the piston, and remove the top most ring, you can measure the gap of the top most piston ring inside the cylinder. This will tell you if there's ring wear, and if you'll need a new set of rings. Obviously you'll need to remove the side cover, the cam, and the two screws holding the bottom cap to the connecting rod (1/4" socket needed.) Then you can push the piston out of the top of the cylinder.

    If for some reason you can't push the piston out of the top of the cylinder, you have a bigger problem. That motor is worn, and you have a ridge ring around the top edge of the cylinder. Hopefully this is not the case, as this would indicate a really worn cylinder. And short of boring it to .020" over, this block will probably be beyond usage.

Measuring the top ring gap. This feeler gauge only goes to .035", and there's still probably twice the gap! This motor really needs new rings. And probably needs to be bored to use a .010 or .020 over piston size.

    When your thick ring Tecumseh engine was new, the ring gap was .007 to .010". The working specs are .007" to .017" ring gap on thick ring pistons. With a thin ring piston (after 1992), this working gap is .010" to .020" (with it at .010" when it was new.) Basically if you get above .020" ring gap (either engine type), it's probably time for new rings (either engine type). In the picture above, the feeler gauge only goes to .035", and the ring gap is clearly twice that (if not more.) I would say this engine needs new rings (or maybe an oversized piston too.)

    Because a cylinder wear more at the top and center of the stroke then the bottom, it's better to use a ring gap of .010" on thick ring pistons. Trying to get it to .007" is not a good idea (unless you just had the cylinder re-bored and honed.) Because as the ring goes down the cylinder, it will tighten the gap. If the gap "crashes", you'll be in a world of hurt! And that could happen on a used cylinder. So better to error on the side of caution.

    Also when checking ring gap on a used cylinder, you really need to do this at the center of the cylinder bore (I usually go down an inch or so). Why? Because the center of the cylinder wears more than the bottom or top. You could have a .010" ring gap on the top/center of the cylinder, but a .005 ring gap on the bottom. This WILL cause a problem! The motor will run great cold, but as it warms up in a few minutes, it will "soft seize" and stop running. So you must check the ring gap at the bottom of the cylinder too, making sure it's not too tight.

Same cylinder, but with a new .020-over ring installed. You can see from this that the cylinder is not big enough for an oversized piston. That's a good thing, meaning we can bore the cylinder to fit an oversized piston, if desired.

Same cylinder, after honing, but with a new .010-over ring installed. You can see the gap is almost enough to bring the ring together. But not quite!

Here's a brand new Tecumseh H35 short block (with thick rings.) This motor has never been run! Check the ring gap... it measure about .010". That's brand new, from the factory. This should give you some perspective on ring gap.

This HS50 motor had a "soft seize". This happened because the new rings were gapped to .007 at the top of the cylinder, but not checked at the *bottom* of the cylinder. The ring gap there was about .005... as the engine got warm, the expansion of the rings tightened the ring gap, and seized the motor. Let the motor cool, and it would start right back up (for 5 minutes.) Had to re-hone the cylinder and widen the ring gap, and now the motor works fine.

    Let's say you don't want to bore the cyliner oversized. It's understandable, you need to have a machine shop do that work (it costs money.) There is another way to handle this, and that's to use oversized rings, and grind one end of the ring to get a .010" gap. Though probably not the best approach, it does solve a problem. It's the cheap way to re-ring a motor that probably needs to be bored. Use the standard piston, and modified oversize rings. This does work!

    Egg Shaped Cylinder and Boring.
    The way these aluminum bores wear is egg shaped. That is, the very top and bottom of the cylinder stay pretty round. But the middle area wears unevenly. There's only one way to fix that, and it's boring the cylinder to oversize. You can not hone an out of round cylinder. You *will* make it worse, not better, trying to hone it to oversize. For example, I had a really worn HSSK50 engine (see video above.) I could almost drop a over-size .010 piston into the cylinder. For fun, i honed the cylinder to accept the .010 over piston. This took a decently running motor (with a lot of oil burning, but it ran!) to a completely non-running motor.

    If you decide to bore a cylinder, you will need an oversized piston. A machine shop should be able to do this on a Bridgeport Mill for you. Cost is usually about $40 to $50. Make sure you give them the specs and the new piston.

Boring a cylinder. If a cylinder is really wore and egg shaped, this is the only way to fix it.

    If the motor you are boring is a newer HSSK50 or a 1990s HS50 with the thin ring piston and smaller exhaust valve, there is a way to get an over-sized piston easily (sometimes over-sized pistons are hard to find.) When Tecumseh went to the small valve and thin-ring piston on the HS50/HSSK50, they decreased the bore size from 2.812" to 2.795". This means you can use an old stock size HS50 thick ring piston #33562, and get almost .020 over on the newer format HS50/HSSK50 engines. It's cheating, but this does work if you can't find a proper over-sized piston!

    Honing the cylinder.
    Before you install new rings (or any sort), you should hone the cylinder. It wants a cross-hatch pattern for the new rings to seat correctly. It will also remove light wear or scratches in the cylinder. Honing is not boring though! Meaning that honing will remove .0005 to .0020" of material (at most). It's not like boring where lots of material (.010 or .020") is removed. The purpose is to get that cross hatch pattern, and possibly clean up some light cylinder wall scratches. It won't remove serious issues. In fact, if your cylinder is out of round, honing will make it worse (not better.) But honing does help the new rings seat better and work better.

    Boring will make your cylinder perfectly round again - honing does not do this. As a cylinder wears, it tends to go out-of-round (egg shape). Especially at the middle of the bore, that's where you get the most wear. Only boring can return a cylinder back to perfectly round. Remember, the whole idea behind honing is to remove minor scratches and to get that cross hatch pattern. In fact, after a cylinder is bored, it needs to be honed before installing the new oversized piston.

Honing the cylinder to get a cross-hatch pattern. This is highly recommended when installing new rings.

    There are basically two different styles of hones: the 3-stone type, and the flexhone ball type. In the picture above, the ball type is used. The flexhone type is a bit easier to use (as it can be used in a hand drill, where the 3-stone type should be used in a drill press.) But either style is fine, just get a 320 or 400 grit variety (with 400 grit being ideal). I have found the ball type hone to be more agressive than the 3-stone type. It's also just seems to work better and is easier. For this reason I use the flexhone ball type. Remember hones come in different diameters (and grits) too. I use a 2 3/4" Flexhone, since it works with an HS40 (2 5/8") and HS50 (2.812"). Remember you MUST use some sort of oil when honing. Automatic transmission fluid works really well for this.

After honing the cylinder you will get a cross-hatch pattern. Now the new rings will seat properly in their new motor. You can see some minor scratches in the cylinder, that were too deep for the hone to remove.

    So how much honing do you do? Don't spend a lot of time doing this, that's not the point. When using the bead style hone, I usually move up and down the cylinder wall for about 10 to 20 seconds. Then wipe down the cylinder and check the cross hatching and the scratches. Don't go too wild! Remember you're honing, not boring!

    After doing the honing, you need to clean the cylinder really well. The aluminum that gets left behind is just like sand paper. It needs to be cleaned. The right way is to put the cylinder in a soapy water bath (kitchen sink). Then wipe the cylinder down with a white cloth. Any gray on the cloth, and you need to keep cleaning that cylinder. No gray on the cloth, and you've got it clean!

Wiping down the honed cylinder. Most people that do this work recommend washing the entire block in soapy water to remove the aluminum dust from the hone. Wiping the cylinder with a white cloth will tell you if you've been successful. There should be no gray on the cloth if you've done a proper job. In the picture below, there's still some work to do...

    Installing the New Rings.
    Getting the old rings off the existing piston is easy. Brute force works fine, you're throwing them out anyway. But getting the new ones on is much harder. Because you can easily break a new ring! The best way to do this is using a piston ring expander tool. You can do it without this tool, but it's not recommended.

Instead of using oil for assembly lubricant, you can buy assembly lube. It's designed for this, and probably a good idea, though certainly optional. You need to use something. Some people just use oil, some use grease. Assembly lube is kind of inbetween the two.

    The bottom most ring is the oil ring. I do that one first. Often it has a spring metal expander that goes on before the ring. That's easy to put on. But the oil ring itself I install from the bottom of the piston. Walk it up the piston skirt slowly, until you hit the bottom most ring gap.

Tecumseh form #690770 which explains the positioning of the thick style rings on the piston.

    The middle ring I put on from the top. Again walk it on slowly. Then do the same thing with the top most ring. Also note there is a "top" and "bottom" to the new rings. Usually there's a form #690770 in the ring box that explains position (it can vary from set to set.) For old school thick rings, the picture above shows the position of the angles on the top and middle rings.

    Ring Gap Positions.
    The position of the ring gap is really important. Basically you want the ring gaps to be spread 120 degrees away from each other. But there are some other things to consider. First never put a ring gap right next to the values. Also never put a ring gap over the wrist pin. So that kinds of leaves you with a certain position that works!

    Also many pistons have an arrow on the top of the piston. This should be oriented towards the values.

The blue circle in the picture shows the arrow on the piston, which should point towards the valves.

Here's where the ring gaps should go on these engines (positioning the ring gaps about 120 degrees apart.) Don't position the gaps right next to the values, and don't position the gaps right over the wrist pin.

    Now to slide the piston and new rings into the cylinder, you will need a ring compressor. You can probably do it without it, but it's not worth the risk (of breaking a ring.) Just buy one. Put oil on the cylinder wall, and on the wall of the ring compressor. Tighten the ring compressor as tight as you can, and put the piston in the cylinder (from the top.) Then using the wood handle of a hammer, you can knock the piston into the cylinder.

Getting the piston and new rings into the cylinder using a ring compressor. Using a hammer wood handle as the leverage.


13. Cams and Lapping the Valves.
    The cam used on HS40 and HS50 motors are the same. They are not high performance. You can however get high performance cams. But they mostly effect higher RPMs. If you go this route, you will need an ARC billet connecting rod, because all high performance cams add RPMs! Just keep that in mind. If you don't use a better connecting rod, you will "throw the rod" and probably ruin your engine.

    Dynocams.com in Delco, North Carolina does offer a "245", "255", and a "356" cam for the Tecumseh motors. They are specified for the HSSK50 motors, but will work in an HS40 or HS50 engine. In order to get these cams you have to send them a "core" and some money, and they will grind the cam desired. Here's the specs on the cams:

    • Stock Tecumseh HS40/HS50 cam lobe lift: 0.975"
    • Dynocam 245 HS40/HS50 cam lobe lift: 1.050". Kicks in at 3000 rpm.
    • Dynocam 255 HS40/HS50 cam lobe lift: 1.060". Kicks in at 3500 rpm.
    • Dynocam 356 HS40/HS50 cam lobe lift: 1.150". Kicks in at 5000 rpm.

    The Dynocam 245 is a nice cam. It is almost identical to 94-SS Briggs & Stratton flathead grind which has been the most popular DynoCam flathead cam for many years. The 245 doesn't seem really a lot different than the stock cam, except it allows the motor to accelerate noticeably faster. Also it will allow for higher RPMs (though probably not much more than 5000 RPMs, which is a bit more than the stock Tecumseh 3600 RPMs). You can use the existing valve springs and retainers too with this cam (they suggest not using stronger springs, because this cam is not welded or heat treated.) Pull starting does not seem any more difficult with this cam. It's a nice all around cam that gives a bit more performance.

    The Dynocam 255 is just slightly more aggressive than the 245, it is also a nice cam. The 255 is very close to the Predator/Honda clone "mod2" cam with a little more lift. It's probably the "sweet spot" in the Dynocam line. More performance than the mild 245, without being crazy. Again you can use the stock springs, though a slightly more aggressive spring would be better. Because this cam will allow for higher RPMs (probably into the 6000 range.)

Here's a stock cam versus the Dynocam 356. There's a lot of lift added on this cam!

    The Dynocam 356 is a beast. It has a lot higher lobes, and will probably make your low end suffer. But you can turn up to 8000+ RPM with this cam! It is a welded and heat treated cam, meaning that you can (and should) use stiffer valve springs (or the valves will "float" at higher RPM.) You should also have a ball bearing crank side cover too. I don't recommend this cam unless you are a racer. (And if you're a racer, you're probably not reading this document anyway.) Also the valve lift is so high I seriously doubt this cam would even work with a stock head (the valves would hit the head.)

Here's the Dynocam 245 versus the Dynocam 356.

    When using a Dynocam 245 or 255, carb jetting should be maintained. That is, a higher performance cam does not generally change carb jetting. Just keep that in mind.

Dynocam spec sheets.

    There's another company that does cam grinding for small motors called Small Engine Cams (Gardena CA). You can contact them for a higher performance cam too.

    All that aside, a stock cam should be checked. Mostly the issues I have seen are on the newer HS40/HS50/HSSK50 engines. Tecumseh added a slight "bump" to the exhaust lobe on these cams, known as the "bump compression release" (BCR). The bump is nearly oppose of the exhaust cam lobe. This bump comes into effect on the compression stroke, as a compression release, to make starting easier. Check your engine for this! It's easy to check. Just remove the head, and turn over the engine by hand. Look at the compression stroke, from the bottom of the stroke to about 1/2 the way up. If you see the exhaust valve "bump" up just a bit (probably about .010"), you have one of these cams. My suggestion is to modify this cam to remove the "bump". Otherwise your compression will be reduced quite a bit, compared to a non-release engine. It's actually pretty easy to do. With the cam removed, you can feel the "bump" (on the side of the lobe). I use a belt sander and gently grind the bump off the side of the exhaust lobe on these cams. Then re-install and check the exhaust valve (to make sure there's no compression bump on the exhaust valve.)

Cam from an HSSK50 that had a BCR compression release on the exhaust lobe. In this picture I have sanded off the compression release bump.

Here's a stock cam with the BCR compression release on the exhaust lobe (blue circle).

    So why did Tecumseh add this compression killer? Well it makes the motors start a lot easier (there's nearly no compression "kick" when pull starting.) But unfortunately, there's no way to turn off this compression release. Some early HS40 cams had a mechanical release. This is nice, because as the engine starts, the release disables, so you get full compression as soon as the motor starts. But with this new style BCR cam, the compression release is always present. It tones down the performance of these engines. It's a fine thing for a snowblower, but it sucks for a minibike!

Cam from an LH195 that had a mechanical compression release (MCR) on the exhaust lobe. This compression release is fine... As soon as the motor gets to idle, it swings out of the way. Early HS40 engines also had this style of compression release.

Some LH195 engines have a plastic cam (left)! Obviously you don't want that.

    Lapping the Valves.
    If you already have the motor apart, I would highly suggest lapping the valves. Basically this requires some valve lapping compound (cheap), and a wood lapping stick. This mates the valves to the valve seat, and will help increase compression. It only take a small bit of time to do this, and costs nearly nothing. It's a good idea to do.

    In order to lap the valves you'll need some valve lapping compound and a lapping tool (basically a wood rod with a suction cup.) I used the Permatex 80036 valve grinding compound. You can buy a kit with the compound and tool for about $10 on ebay.

    To do this you'll need to remove the valves. This is pretty easy. First make sure the head and the cam is removed. Then remove the side breather assembly #31337 and it's gasket #31619. Two 5/16" machine screws hold it in place. With it removed, the valve springs and retainer clips are visible. It's easiest to do this with the valve lifters removed (which means the side sump is removed, and the cam removed.) But it's not totally necessary to have the lifters removed (just easier.) The spring retainer clips are notched. Just move the retainers off their center, and this releases the valve spring. Then the valve can be lifted out.

    With the valves removed, you can clean up the valves and the ports. Using a Dremel wire brush works great for this. You can also put the valves themselves on the wire wheel and get all the carbon and other junk off.

Cleaning up the valve area and ports with a Dremel.

Cleaning up the valves on a wire wheel.

    Next thing is optional. If the valve seat are still crusty, you can use some emery cloth and clean them further. Just punch the valve through the emery cloth and use the valve to help clean the seat.

Cleaning up the valve seat with emery cloth.

    Now comes time to lap the valves. Put a thin layer of valve lapping compound on the valve seat. Then insert the valve, and attach the suction cup tool to the top of the valve. Then you can twist the tool between your hands to lap the valve. Pull up the valve after a few seconds, and repeat. This should give a nice lap of the valve to their seat.

Applying lapping compound to the valve.

Lapping the valve to the seat using the lapping suction tool.

How the valve and the seat should look after a sucessful lapping.

    With the valves lapped, now comes the time to re-install them with the valve springs. Again this is easier with the valve lifters removed. I used nylon ties to compress the valve springs. Once this is done, it's easy to insert the top cap, the spring, and the lower retainer onto the valve. Once in position, just cut the nylon ties and pull the nylon ties out with pliers.

Use nylon ties to compress the valve springs for easy installation.

The valve parts. Note on some motors (HS50), the exhaust top spring retainer is different than the intake spring top retainer. This is important! Don't mix them up. The exhaust spring top retainer has a bigger hole, because the exhaust valve seat extends into the valve spring area (see next picture.)

Exhaust seat on some motors (HS50) extends downward. That's why the top spring retainer has a bigger hole to accommodate the extended exhaust seat on HS50 engines.

After the valve spring and top and bottom retainers are installed, cut the nylon ties and pull them out with pliers. (This is an HS40 engine, and notice the exhaust seat does not extend into the spring area.)

    With the valve springs and retainers installed, the lifters and cam can be re-installed (assuming you removed them to lap the valves.)

    I would like to add that on some HS50 and HSSK50 and LH195 engines (basically 1990s and newer Tecumseh flathead 5hp motors), there is *no* top spring retainer! For some reason they removed these from these designs. I don't really like this, as the spring could wear across the top of the valve area (into the aluminum.) It also decreases the spring tension on the valves. Because of this, I install HS50 top spring retainers. This helps center the valve spring, decreases wear, and increases valve spring tension slightly. It's a good idea to add them!

    Checking the valve lash.
    I appreciate people wanting to do this, but frankly, since there's no easy way to adjust the valve lash, it's a bit pointless. The valve lash should be .006" (use a feeler gauge) with the piston at top dead center. And you can only measure it when the side case is installed (not necessarily bolted tight, but in place to hold the end of the cam in the proper position.)

Checking the valve lash with a feeler gauge. With the piston at top dead center, the gap should be .006" beween the bottom of the valves and the top of the lifters.

    If you have more than .006" in gap between the end of the valves and the top of the lifters, the only thing to do is to buy new valves (and hope the cam isn't the wear spot!) I guess you could TIG weld the end of the valve, and grind it down to get the proper lash. That would be another technique. But I have to be honest, I don't see out-of-wack valve lash much on these motors. I guess it can happen, but it seems rare.

    But before you go nuts with new valves or welding/grinding, try switching the two valve lifters. Though technically they are identical (most of the time, some engines with compression release cams they are different). This can often fix a cam lash issue.


14. The Crankshaft.
    The standard minibike crankshaft PTO diameter is 3/4 of an inch. With 3/4", you have the most versatility of clutch and torque converter option. But a lot of HS50 snowblower motors have 1" diameter PTO shafts. With a 1" PTO, the choice of clutches is not only more limited, but more expensive. For this reason, I always use a 3/4" PTO shaft. But with this in mind, don't discount an otherwise good HS50 engine with a 1" PTO!

    There's a couple ways to deal with this. Either just stay with the 1" PTO, or convert it. If you go on Ebay, you'll find used crankshafts for sale. Just make sure you get the correct one, and that the bearing journals are not worn (easy to see visually). Also between the horsepowers, the crankshafts are not inter-changable. That is, you can't use a HS40 crank in an HS50 engine.

    Finally your existing flywheel must mate to the crankshaft. Just look at the flywheel nut size to make this determination. If it uses a 5/8" socket for the nut, that's the smaller and older style crank with points and condenser. If it uses a 3/4" socket, that's the newer CDI electronic ignition crankshaft (though there is a bit of overlap in the 1982 time frame.)

    • HS50/HSSK50 crankshaft 3/4" PTO new style (no points): #34740
    • HS50 crankshaft 3/4" PTO old style (points): #33677 or #34728
    • HS50 crankshaft 3/4" PTO old style (points) ball bearing case: #33676 (1972 to 1980)
    • HS40 crankshaft 3/4" PTO new style CDI (no points): #34734
    • HS40 crankshaft 3/4" PTO old style (points): #32877
    • HS40 crankshaft 3/4" PTO, long, old style (points), ball bearing: #33080 (Rupp style)

A couple 1" crankshafts and a formerly 1" crank that was turned down to 3/4" and the 3/16" keyway milled back into the PTO.

    There is one more option too, and that's to put your existing 1" PTO crankshaft in a lathe, and turn it down to 3/4". After you do that, you'll also have to mill a 3/16" keyway into the crankshaft. Obviously this is work for someone with a lathe and a mill. Generally it's just easier to buy a new crankshaft off ebay. But i do this lathe/mill work, and find it quite satisfying.

Getting ready to cut a 1" PTO shaft down to 3/4" on a lathe. The distance from the cam gear to the start of the 3/4" cut is usually about 1.250". But I've seen 1.200" to 1.300" also. I scribe a line on the crank before removing the side cover to give a reference line.

    One advantage to cutting a 1" PTO down to 3/4" is you'll end up with a longer PTO. The stock Tecumseh 3/4" PTO is usually 2.25" in length. But most 1" PTOs are 3" (or close to it.) That's a nice thing to have if you are running a torque converter.

A crankshaft connecting rod journal that was melted over with aluminum from the rod. This aluminum can be removed with muriatic acid.

    If the crankshaft was used in an engine that over-rev'ed, there may be some aluminum on any of the three crankshaft journals. You don't want to sand the aluminum off. Instead use muriatic acid, which will basically melt the aluminum, and leave the steel journal intact. Just use a small paint brush to apply the acid, wait a few minutes, and it should wipe off. This may require a few attempts. After all the aluminum is removed, you can check the journal size with a dial calipher, to make sure it's still in-spec. Any scratches can be removed with very fine sandpaper (like 1500 or 2000 grit.) Don't over sand, as you can make the journal egg shaped.


15. Flywheels and Blower Housings (Torque Wench Numbers too).
    A lot of guys wants to re-create the Tecumseh look of thie 1970s minibikes, and really want to go that extra mile to do so. One of the complaints of using a snowblower motor is the look of the blower housing. Tecumseh H30 and H35 and even 1968 to about 1972/1973 HS40 motors have a more rounded blower housing. Where all HS50 (introduced about 1972) has a more square blower housing, as do 1973 and later HS40 motors. It's a subtle difference, but a lot of hardcore minibike guys can spot the newer square style blower housing from 50 feet. (Heck some guys can even spot the 1968 HS40 blower housing with no "line" below the three head bolt attachments, and no starter mount casting on the front of the case!)

Left: HS40 pre-1973 rounded blower housing.
Right: more square 1973 and later HS40 housing.

    I guess the question is... can you take an H30 or H35 or HS40 old round blower housing and put it on an HS50 motor (that had the larger square housing)? You could do this, but it would require replacment of the flywheel with a more compact alloy version (HS40 #32517). It does work (that is the motor runs), but frankly I'm still testing the concept - I don't know the long term issues that may arise. Such as the flywheel durability on an HS50 and if the round blower housing can keep the HS50 cool enough. Also remember the start cup will need to be replaced too with the thinner #590416.

Another look at the rounded blower housing verses the more square HS50 housing.

    Now to discuss flywheels. We touched on this earlier... The snowblower flywheel almost for certain will be steel, and often with "teeth" (to accomodate electric start.) Steel flywheels work great to maintain momentum, but since they weigh a lot, they don't let an engine rev quickly. For minibikes, rev'ing quickly is probably better than maintaining momentum. For this reason, stock HS40 minibike motors before 1975 almost always have an alloy flywheel (usually the #32517 alloy flywheel, as used by say Rupp.) On minibike H30 motors they used an alloy flywheel #31332 (will not fit on HS40 or HS50 motors, as the crankshaft taper is different.) But around 1975 Tecumseh pretty much started to use steel flywheels for all their motors.

Tecumseh alloy flywheel #32517 (and pull start cup #590416) as used on minibikes, versus the steel Tecumseh flywheel on the right (HS50). The steel flywheel is definitely larger (taller), and much heavier. Hence the more square and larger HS50 blower housing to accomodate this.

    In addition, the steel flywheel magnets which are glued into position on the flywheel, can come loose at higher RPM. If this happens, the magnets throw and can ruin the coil (and other stuff.) That's why guys that run at higher RPM will have an alloy flywheel (the magnets are secured differently.) Personally I'm not into high RPM motors, but it's something to keep in mind. Also it's not a bad idea to check the flywheel magnets to make sure they are secure. If loose, their spot can be sanded clean with 100 grit sandpaper, and the magnets glued and clamped (while drying) using JB weld. Note the magnet must be glued in exactly the same location. And also north needs to meet the stator plates first during rotation (opposed to south.)

Here's the backside of the same flywheels as above. See the magnets glued to the side on the steel flywheel? At excessive rpm these can come off, causing engine damange. So don't make a high rpm monster motor with a steel flywheel.

Here's a steel flywheel where the magnets have come unglued from the flywheel. They need to be re-epoxied back to the flywheel, in the correct position. I use JB Weld and a clamp for this task.

    With this in mind, can you put an alloy #32517 flywheel on an HS50 motor with the rounded blower housing? Well if the HS50 motor is points/condenser, yes you can. But it will require not only the #32517 flywheel and the slimmer rounded blower housing, but also a different starter cup #590416. This cup is about 1" tall, where the steel flywheel's cup is 1.75" tall (which is too tall and won't work with the rounded housing.) Remember to use a torque wrench and tighten the flywheel nut (specs below.) And yes all HS50 engines always had the squared blower housing, even the very first year (1972).

A couple other alloy points flywheels that I have yet to try. On the left is #1267 (aka 30542). On the right is #610781. Both are really light weight, much lighter than the standard minibike alloy flywheel #32517. They are cut to be used only on H30/H35 motors though (will not work on HS40/HS50 motors).

    There are other points/condenser alloy flywheels out there that will not work. For example Tecumseh #1267 (aka #30542). This flywheel is significantly lighter in weight than the #32517 - the #32517 (as used in HS40 minibike motors) has additional weights around the inside of the flywheel. The #1267 does not have this added weight, so it's really light. Likewise the #610781 alloy flywheel is like that too (even lighter than the #1267.) These flywheels are cut for an H30/H35 crank only (will not work on a HS40/HS50 motor.) There is also the Tecumseh #610769 points flywheel for the HS40 - this flywheel has many magnets and supports an alternator style magneto (used on say a Rupp Roadster.) Note there is a lighted Tecumseh H50 flywheel too #30755.

    Oh wait, there's more stuff to consider on rounded blower housings... That is if you are using a lighted Tecumseh engine (one with a magneto that supports lights.) The blower housing on these motors is slightly different. You can used a lighted blower housing on a non-lighted engine. But going the other way (non-lighted housing on a lighted engine) is a bit more tricky.

Here's a 1970 HS40 lighted flywheel rounded blower housing. Notice the blue circles, showing the 1/4-28 studs that hold the starter mech to the blower housing. Notice their low profile. This is the difference between a lighted blower housing and potentially a non-lighted blower housing. The low profile is needed to clear the flywheel fins on a lighted aluminum flywheel.

    The aluminum lighted flywheels (in particular HS40 models) are just slightly thicker than their non-lighted counterparts in the 1960s to about 1974, before Tecumseh changed to steel flywheels (about 1975). This is why all lighted HS40 blower housing, pre steel flywheels, have studs for the starter recoil instead of welded 1/4-28 nuts or rivets to hold the pull starter. This way nothing is sticking into the shroud. The riveted or nutted versions can be used on a lighted motors, but you have to thin to nuts to making sure there's clearance for the fins of the flywheel. And if the blower housing is dented or pushed in (a common thing on mini bike engines), the situation is even worse. These are minor details, but something that needs to be considered.

Starter mech positions: To add to the blower housing confusions, there's yet another thing to consider... the position of the starter mech. Notice these two rounded (old style) blower housings have the pull start mech in slightly different positions: 12 oclock (left) and 10 oclock (right). Technically speaking, the 12 oclock position is for flat motor mounted minibikes. The 10 oclock position is for 20 degree angled mounted motors. A small and probably meaningless detail, but one I thought we should probably cover here.

    Snowblower Flywheel Starter Gear.
    If you are using a snowblower motor 1975 and later, often they will have the option for an electric start on the cast iron flywheel. This is something not needed on a minibike. The starter gear on the cast flywheel is removable. Originally they heated up the gear and slid it onto the flywheel. It looks like it's cast onto the flywheel, but it is not.

A points flywheel and CDI flywheel. Both had a starter gear. I removed them to lighten the cast iron flywheels.

    I usually remove this starter gear as it just adds weight to the flywheel. There's a couple ways to get it off. I use a press. But you can also use a cutoff wheel on a hand grinder (Metabo), and cut the gear (don't cut into the flywheel!) Either way, the gear does come off. Having a lighter flywheel will give (in theory) better acceleration, that's why I remove it. On the other hand, if you're using a flywheel puller, with no starter gear it's hard to remove the flywheel with a puller.


16. Torque Wrench Numbers and Engine Specs.
    All this information came from the 1998 Tecumseh Technicians L-head Service Manual.

    Torque Wrench Specs:

    • Steel Flywheel nut: 550 inch/pounds or 46 foot/pounds
    • Alloy Flywheel nut: 450 inch/pounds or 37 foot/pounds
    • Cylinder head bolts: 200 inch/pounds or 16.5 foot/pounds
    • Spark plug: 250 inch/pounds or 21 foot/pounds
    • Engine side sump cover: 115 inch/pounds or 9.5 foot/pounds
    • Connecting rod bolts: 105 inch/pounds or 8.5 foot/pounds
    • Ring gap end: .007" to .017" (points/condenser motors)
    • Ring gap end: .010" to .020" (solid state motors)

    HS40 Points/Condenser Engine Specs.

    • Displacement (in�): 10.49
    • Stroke: 1.938
    • Bore: 2.625 to 2.626
    • Timing Dim. BTDC: .035
    • Valve Clearance: .004 to .008
    • Valve Seat Width: .035 to .045
    • Valve Guide Oversize Dim.: .2807 to .2817
    • Crankshaft End Play: .005 to .027
    • Crankpin Journal Dia.: .9995 to 1.0000
    • Crankshaft Mag. Main Brg. Dia.: .9985 to .9990
    • Crankshaft PTO Main Brg. Dia.: .9985 to .9990
    • Camshaft Bearing: .4975 to .4980
    • Conn. Rod Dia. Crank Brg.: 1.0005 to 1.0010
    • Piston Diameter Bottom of Skirt: 2.6202 to 2.6210
    • Ring Groove Side Clearance (1st/2nd Comp.): .002 to .005
    • Ring Groove Side Clearance (Bottom Oil): .001 to .004
    • Piston Skirt Clearance: .0040 to .0058
    • Thick Ring End Gap: .007 to .017
    • Cylinder Main Brg.: 1.0005 to 1.0010
    • Cylinder Cover/Flange Main Bearing Dia.: 1.0005 to 1.0010

    HS40 CDI Solid State Engine Specs.

    • Displacement (in�): 10.49
    • Stroke: 1.938
    • Bore: 2.625 to 2.626
    • Valve Clearance: .004 to .008
    • Valve Seat Width: .035 to .045
    • Valve Guide Oversize Dim.: .2807 to .2817
    • Crankshaft End Play: .005 to .027
    • Crankpin Journal Dia.: .9995 to 1.0000
    • Crankshaft Mag. Main Brg. Dia.: .9985 to .9990
    • Crankshaft PTO Main Brg. Dia.: .9985 to .9990
    • Camshaft Bearing: .4975 to .4980
    • Conn. Rod Dia. Crank Brg.: 1.0005 to 1.0010
    • Piston Diameter Bottom of Skirt: 2.6202 to 2.6210
    • Ring Groove Side Clearance (1st/2nd Comp.): .002 to .005
    • Ring Groove Side Clearance (Bottom Oil): .001 to .004
    • Piston Skirt Clearance: .0040 to .0058
    • Thin Ring End Gap: .010 to .020
    • Cylinder Main Brg.: 1.0005 to 1.0010
    • Cylinder Cover/Flange Main Bearing Dia.: 1.0005 to 1.0010

    HS50 Points/Condenser Engine Specs.

    • Displacement (in�): 12.04
    • Stroke: 1.938
    • Bore: 2.812 to 2.813
    • Timing Dim. BTDC: .035
    • Valve Clearance: .004 to .010
    • Valve Seat Width: .035 to .045
    • Valve Guide Oversize Dim.: .2807 to .2817
    • Crankshaft End Play: .005 to .027
    • Crankpin Journal Dia.: .9995 to 1.0000
    • Crankshaft Mag. Main Brg. Dia.: .9985 to .9990
    • Crankshaft PTO Main Brg. Dia.: .9985 to .9990
    • Camshaft Bearing: .4975 to .4980
    • Conn. Rod Dia. Crank Brg.: 1.0005 to 1.0010
    • Piston Diameter Bottom of Skirt: 2.8072 to 2.8080
    • Ring Groove Side Clearance (1st/2nd Comp.): .002 to .005
    • Ring Groove Side Clearance (Bottom Oil): .001 to .004
    • Piston Skirt Clearance: .0040 to .0058
    • Thick Ring End Gap: .007 to .017
    • Cylinder Main Brg.: 1.0005 to 1.0010
    • Cylinder Cover/Flange Main Bearing Dia.: 1.0005 to 1.0010

    HS50 (G and earlier) and HSSK50 (M and earlier) CDI Solid State Engine Specs.

    • Displacement (in�): 12.04
    • Stroke: 1.938
    • Bore: 2.812 to 2.813
    • Valve Clearance: .004 to .008
    • Valve Seat Width: .035 to .045
    • Valve Guide Oversize Dim.: .2807 to .2817
    • Crankshaft End Play: .005 to .027
    • Crankpin Journal Dia.: .9995 to 1.0000
    • Crankshaft Mag. Main Brg. Dia.: .9985 to .9990
    • Crankshaft PTO Main Brg. Dia.: .9985 to .9990
    • Camshaft Bearing: .4975 to .4980
    • Conn. Rod Dia. Crank Brg.: 1.0005 to 1.0010
    • Piston Diameter Bottom of Skirt: 2.8072 to 2.8080
    • Ring Groove Side Clearance (1st/2nd Comp.): .002 to .005
    • Ring Groove Side Clearance (Bottom Oil): .001 to .004
    • Piston Skirt Clearance: .0040 to .0058
    • Ring End Gap: .010 to .020
    • Cylinder Main Brg.: 1.0005 to 1.0010
    • Cylinder Cover/Flange Main Bearing Dia.: 1.0005 to 1.0010

    HS50 (H and later) and HSSK50 (N and later) CDI Solid State Engine Specs.
    Note this is when Tecumseh went to a slighter smaller bore on the HS50 and used thin rings on the piston. Also the exhaust valve was reduced in size. This happened in the early to mid 1990s. This is a big deal as you can't use an earlier piston or rings in these newer HS50/HSSK50 motors. The piston skirt size was reduced from 2.807" (old) to 2.790" (new). And the cylinder bore was reduced from 2.812" (old) to 2.795" (new).

    • Displacement (in�): 11.09
    • Stroke: 1.938
    • Bore: 2.795 to 2.796
    • Valve Clearance: .004 to .008
    • Valve Seat Width: .035 to .045
    • Valve Guide Oversize Dim.: .2807 to .2817
    • Crankshaft End Play: .005 to .027
    • Crankpin Journal Dia.: .9995 to 1.0000
    • Crankshaft Mag. Main Brg. Dia.: .9985 to .9990
    • Crankshaft PTO Main Brg. Dia.: .9985 to .9990
    • Camshaft Bearing: .4975 to .4980
    • Conn. Rod Dia. Crank Brg.: 1.0005 to 1.0010
    • Piston Diameter Bottom of Skirt: 2.790 to 2.791
    • Ring Groove Side Clearance (1st/2nd Comp.): .002 to .005
    • Ring Groove Side Clearance (Bottom Oil): .001 to .004
    • Piston Skirt Clearance: .0040 to .0058
    • Thin Ring End Gap: .010 to .020
    • Cylinder Main Brg.: 1.0005 to 1.0010
    • Cylinder Cover/Flange Main Bearing Dia.: 1.0005 to 1.0010


17. Dipstick, Oil Plug, Spark Plug.
    Often the oil dip stick used on snowblowers is a high and tall version, which screws into the top of the oil sump side case. This just won't work for a minibike. But the good new is, very easy situation to fix. Just order a new Tecumseh dipstick #37884, and use that. It's not perfect, but it's the closest dipstick I've found for this application. Unscrew the existing tall dipstick assembly and discard. Screw in the new #37884. If you need a new gasket, that's part number #29673.

The super long dip stick assembly on the snow blower motor has to go. It just unscrews, and a new dipstick #37884 (or equivalent) used as a replacement.

    The only downside to dip stick #37884 is it's oil range is slightly different than the original stick. With oil in the engine you can fill the crankcase with the old dip stick to 'top', and then compare with the new dip stick. You can scribe a new 'top' line on the new dip stick if you desire.

Comparison of the old/new dip sticks. Don't be fooled. The best way is to have the oil to the top (using the original dip stick), and THEN compare the level on the new dip stick! Use a dremel and put a mark into the new dip stick to mark the level permanently.

New dip stick is marked and ready for use!

Proper oil fill "full" mark is two inches from the bottom edge of the side case.

    You can also measure two inches from the bottom of the side case to get the new oil fill mark. The new dip stick is obviously a lot more elegant than the old set up.

    Next up is the oil plug. Some snow blowers have extra long oil plugs/drains. This is great for a snow blower, but not so great for a minibike. But the good news is, it's easy to convert this to minibike style.

Replacing the snowblower oil drain with the smaller mini bike style.

    Any hardware store in their plumbing department will have these oil plugs. They are 1/4" NPT plug. Just buy one and replace the extended snow blower drain plug with the new NPT plug.

    The spark plug is obviously important on these motors too. Generally Tecumseh motors came with a Champion sparkplug. You have a choice of three Champion spark plugs to use:

    • J19LM (Heat range 9)
    • J8C (Heat range 8)
    • J17LM (Heat range 7)

    Listed these from high to low temperature. The Champion J17LM is typically the plug for a Tecumseh HS40. But most people seem to use the Champion J8C. Another plug I really like is the Autolite 255. The gap should be .030" on any spark plug used. Note that plugs come in different lengths. For mini bike usage, I like the 'short' plugs (like the Autolite 255), as it is less likely to interfere with the top tube of the frame.

    After running the engine for a bit, you can remove the current spark plug and check it's color. It should be a nice gray. If it's white, that's too hot. If it's black, that's too cold. You can adjust the plug (or jetting) used to compensate for this.


18. Installing the Minibike Throttle Assembly and Gas line.
    The snowblower throttle assembly will not work for a minibike application. Sure you can probably hack the original snowblower throttle to work, but I really do not suggest this. Especially since a new minibike throttle assembly is only about $20, part #730136a. Lots of places sell this online, just buy one.

The original snowblower throttle assembly has had its rivets drill out, and we're ready to install the new minibike throttle assembly #730136a. Also it's time to run a new 1/4" fuel line too.

    First you'll need to remove the old snowblower throttle assembly. I use a 3/16" drill bit, and drill out the two rivets holding the old assembly in place. If you still have the flywheel removed (from the points/condenser servicing), this is perhaps a bit easier. But if the flywheel is in place, that's fine too.

    Again you can do all this work with the flywheel installed, but if the flywheel is removed, it is a bit easier. (It is a bit easier to install the new fuel line too when the flywheel is removed.) Install the new throttle assembly using two #8-32 bolts with nylon lock washers. The kit comes with bolts, but personally I like nylon lock washers better than what they supply.

New throttle assembly and fuel line is installed.

    Also if you are running a engine mounted gas tank, install a new 1/4" fuel line. Don't be cheap!! Buy brand new fuel line. It's not worth messing around with, just get new fuel line. I prefer the original style black double walled variety myself.

    Now is the time to re-install the blower housing. There are two bolts on either side, plus the three head bolts that hold the blower housing in place.


19. Pull Start Mechanism Maintanence.
    If you're doing powder coating, you will have to take the pull start mech completely apart (plastic will melt otherwise!) That's a ballsy move, and one I generally don't take. Instead I usually paint the pull start mech (assuming it doesn't need a new spring.) I just thought I would mention that...

    Since the blower housing is installed now, it's time to address the pull starter mech. If it works fine, then I guess you're done. But in my experience, this assembly always needs some work.

    First thing is to make sure you have a proper pull start handle. A lot of snowblowers use a big plastic handle, and frankly this just isn't going to work on a minibike. The correct Tecumseh rubber pull start handle is part number 590387. These are available at many online sources and frankly are pretty cheap. It's nice to have a new one.

The stock 11/64" pull cord likes to get wedged between the moving plastic wheel and the stationary housing, jamming the pull starter.

    Next up is to address the pull cord. Originally the Tecumseh pull cord is 11/64" nylon cord on older pull starts. Personally I use slightly smaller 5/32" nylon cord. Why? because I find that the larger cord size can get wedged between the moving plastic wheel and the stationary metal housing. This makes the pull starter "stick", which is really irritating. Because of this I replace the pull rope with 5 feet of 5/32" nylon cord. It sucks when the pull cord is too short, and jams your shoulder when you pull one with a short rope. Tecumseh recommends 54" for the pull length on the old 4 leg style pull starters. I generally go with 5 foot (60 inches) because I always end up with extra on the ends after I make the knots.

    To replace the pull rope is pretty easy. Just pull the handle until all the cord is out of the housing. Then jam a flat head screwdriver between the plastic wheel and the stationary housing. Now you can cut off the old rope and install the new rope section. Don't forget to melt the ends of the new nylon rope so it does not fray. I use three knots at both ends.

Installing the new rope.

    After the new rope is installed, there may not be enough spring in the pull start to suck all the rope into the housing. This is easy to deal with too. Just pull out about 1/4 of the rope length, then wind the extended rope around the plastic wheel. That will take up the slack.

Taking up the extra rope.

    With the new rope length installed, you can install the pull start mech onto the blower housing. Note on old pull start handles, the hole in the metal U channel is sometimes hogged out. Another good reason to replace the pull handle with a new #590387 assembly. But you can take a hammer is make the hole in the metal U channel smaller, if needed.


20. Starting the Engine for the First Time.
    Now you have your basic "long block" all ready. You have done some modifications perhaps (governor, etc.), and maybe new rings and/or lapped the valves. You have the magneto all set with good points and a new condenser. The flywheel is sorted out and installed too. The dipstick (and oil!) are installed, as is the blower housing and pull start. The only thing left to do is the carburetor and the exhaust, right?

    Well now is the time to make sure you have a good viable engine. It's time for the first run, to make sure you've done everything right up to this point.

    You can use a compression tester to test for compression. I bought one on ebay for about $30 and it seems to work well. Screw in the brass fitting to the spark plug, and pull the cord. It will record the highest compression the motor can generate. It seems like anything over '50' is a good viable engine.

Using a compression tester on an HSSK50 engine. This is too low, the compression should be nearly twice this.

Using a compression tester on an HS50 engine. This is where it should be!

    In order to test the engine, you will need some starter fluid, aka ether. This can be purchased at nearly any hardware store or even many gas stations. Auto parts stores should have it too. It comes in a spray can.

    There's a couple ways to do this. Have the motor on the ground, on a firm surface. Spray the ether into the intake port. A good one or two second squirt. Then put your foot on the top of the blower housing, and pull the starter cord. The engine should fire up for about one second.

    Another way, my personal favorite, is to remove the spark plug, and then spray the ether into the head through the removed spark plug. Then quickly replace the spark plug (finger tight is fine) and spark plug wire, and with your foot on the top of the blower housing, pull the starter cord.

Short 20 second movie of starting a Tecumseh HS50 engine using Ether
(starting fluid), which was sprayed into the spark plug hole. You will only
get about one second of running time like this. But it demonstrates that
the engine has spark and good compression.

    In either case, the motor should start for about one second, in a full throttle type rumble. This is enough to tell that you have: 1) proper compression, 2) spark. With this confidence builder, the only thing left to figure out is the carburetor!


21. Installing a New Carburetor.
    With the blower housing installed and everything tidy, it's time to deal with the carburetor. Forget the original carb that came with the snowblower, it's not a good fit here. Just throw it away and forget it.

The new carb #632230, before any of the required modifications.

    I've experimented with many different carbs on Tecumseh HS40 and HS50 engines. What I have found is the Tecumseh replacement carb #632230/632272 availabe on ebay work great. They are really inexpensive (about $10 or so), and they work well. The choke lever is a nice fit for a minibike application. And the general openings of this carb work really well for a minibike. But there are a couple modifications needed to make this carb work best for a minibike application. Note another carb people like to use is #631918. Personally I do not like this carb as much, as the choke lever is rather long, and it has smaller internal orifices (less performance.) But of course you can bend that choke lever to fit your application. The 631918 carb does have a nice 90 degree gas intake though.

The new #632230/632272 carb, air filter, and stock intake manifold.

    You will also need an air filter for the new carb. The snowblower did not originally have any air cleaner (why would you need one in the snow?) Luckily a new Tecumseh minibike style air cleaner is available. It consists of three parts: The mounting plate (#31691 or #31914 offset version), the body (#31715), the air filter itself (#30727), and gasket (#27272A). Or you can buy the entire assembly (#730127), or even the expensive version entire assembly (#730164, comes with both mounting plates and the filter). Note when doing your powder coating it's best to have the air filter body and mounting plate powder coated too!

Modification one to the #632230 ebay carb. Open up the breather hole with a 1/16" drill bit.

    With the ebay Tecumseh carb #632230, the breather hole on the side of the carb needs to be opened up. This hole allows the fuel bowl to breath. Use a 1/16" drill bit and widen the hole. The bit only need to go in about 1/4" (you will feel it go through.)

Modification two to the #632230 ebay carb. Add a 1/16" hole to the throttle butterfly.

    Second modification is to add a 1/16" hole to the butterfly throttle control. This is needed for the throttle arm. It makes the throttle work better than the existing factory drilled holes.

New carb and control arms all attached, ready for an air filter assembly. The lower control arm replaces the stock spring. Do this if you removed the governor internally. Note this is also the "cheaters" way to remove the governor, if you didn't remove the internal governor parts. Not really recommeneded as it puts added stress on the internal governor parts, if still installed..

    With these modifications you can bolt the carb/air filter in place and connect the throttle control wires. I use Piano wire and make new control wires. But usually you can use the stock existing ones. From the new throttle control assembly we installed in the prior steps, there should be a spring to the throttle control arm. If you have deleted the governor, this should now be a straigth wire. Even if you didn't delete the governor you can still use a straight wire here. That will essentially delete the governor too (though all the internal governor parts will be working against the wire!)

A fuel shut off value is highly recomended to be installed at the gas tank. Trust me on this, you need it! Also I add a two pin connector to the kill switch wires. This is a handy addition too.

    Now is a good time to install the gas tank (I assume you are using a motor mounted gas tank.) I get a fuel shut off value for the gas tank, as you will probably need that at some time, if you ever have to work on the carb. Highly recommended.

Identifying the carb manufacturing number, so it can be cross referenced to get the actual Tecumseh part number. Tecumseh carburetors have a manufacturing number stamped on the mounting flange or the body followed by a date code. The manufacturing number on recent models is four digits and is followed by three characters for year, month and day of manufacture. Older carbs have shorter manufacturing numbers.

    Original Carbs.
    If you are not using a snowblower motor, and are rebuilding an original minibike motor, some people may want to use the original carb. Of course it will need to be rebuilt (which may end up costing more than just buying a brand new carb!) But for those that this matters (or if your motor uses a diaphragm carb, for which there are not cheap replacements), a rebuild may be in order. To do such a thing you will need the carb part number. You may think Tecumseh just put the part number on the carb, but you would be wrong. It's a coded system. There's a short 1 to 4 digit number which transcribes to a longer 5 to 7 digit actual part number. Then with the actual part number, you can buy the appropriate rebuild kit. Once you have the carb number identified, you can reference the Carb Id Document and get the actual carb part number. Another nice document shows how to rebuild a carb.

    If you have a minibike motor from 1969 to early 1971 that was originally on a slanted 20 degree motor mount, originally it had a diaphragm carb. For a Tecumseh H30 or H35 motor the diaphragm carb is #631595, with the stamp number on that body being #379. The Tecumseh HS40 diaphragm carb is #631588, with the stamp number on its body being #356.

    Recreational Gasoline.
    In my area, "recreational" gasoline is available. I highly suggest using this gas. It contains no ethanol. Why is this a big deal? Because most regular pump gas has about 10% ethanol. That's probably fine for your car, but these older engines don't like it. Mostly it has to do with rubber parts. For example, the brand new gas line you installed, and brand new carb (or rebuilt carb) rubber parts hate ethanol. It makes the rubber hard, or expands the rubber. It's not a good thing. Use gasoline with no ethanol if you want your carb system to last longer, and run with fewer problems.


22. Slant Carb Intake.
    Around 1970 a lot of minibike makers starting using a 20 degree slant on their Tecumseh HS40 motor mounts. Rupp and Fox and MTD all did this, but a lot of other makers did too. What was the thinking with this move? Probably had to do with torque converter mounting and improved case oil distribution. And it looks cool too!

    The problem with this is the carburetor. In 1970 all the bike makers used a diaphragm carb. Why? Because with the motor at a 20 degree forward slant, a float bowl carb does not work well (as the float bowl will be at an angle.) The diaphragm carbs, which doesn't have a float bowl and hence doesn't care about angles, are hard to find. And hard to get parts for (unlike the float bowl carb which you can buy brand new on ebay for $10!) So to implement a standard float bowl carb, in 1971 Tecumseh offered their slant mount motors with a slanted intake. This put the float bowl level when the motor is mounted at 20 degrees forward pitch.

1981 Tecumseh ad for the HS40 motor, designed for a slant application, and a slant intake manifold.

    With any snow blower motor, there will *not* be a slant intake manifold. If your minibike application has a 20 degree motor mount, you will have to deal with this. Ideally, just buy a 20 degree slant intake manifold #33301 (for the HS40). They are available from BlackWidowMotorSports.net: Tecumseh HS40 slant intake manifold. Note there are separate slant intake manifolds for the Tecumseh H35 and H50 motors, which are different (the ports have slightly different shapes.) Unfortunately BlackWidowMS is the only source for a new slanted intake manifold, and it's only the #33301 part, designed for the HS40 intake port. But you can use the #33301 slant intake on the HS50 motor, but you need to take some things into consideration.

This picture shows the differences in the three styles of slant carb intake manifolds for the Tecumseh H30/H35, HS40 and HS50 engines.

    The Tecumseh slant intake #33301 is designed for the HS40 motor. If you're using an HS50, the intake port is not round like the HS40 port, it's a "D" shape. The #33301 will work, but you need to use a different gasket. Why? Because if you don't, there will be an opening at the motor/intake mate bottom right, and it will suck in air.

The HS50 intake port (left). After cleaning up the lower right corner of the intake port with a wire brush/sandpaper, I put a small dap of JB Weld there. This blocks the 'hole' created when using an HS40 slanted intake manifold.

    There is another approach to fix the HS40/HS50 intake issue. Personally when I re-do an HS50 motor, I put a small dap of JB Weld on the bottom right corner of the intake port. This way if I need the motor for a slant application, I can just use the (readily available) BlackWidowMS HS40 intake #33301 without any issue or special gasket. That said, BlackWidowMS does offer a special gasket to cover the differences between HS40 and HS50 intake port. This works too. You just need the Tecumseh #33670 Exhaust/intake mainifold gasket, and it covers the bottom right "hole", allowing the HS40 slant intake #33301 to be used on an HS50 engine. Personally though I like the JB Weld idea... Or heck do both JB Weld and the #33670 gasket!

A gentlemen on the minibikes forums is offering a CNC made aluminum slant intake adaptor. This is a really nice part that utilizes the stock intake manifold (for any generation Tecumseh motor), and gives the 20 degree carb slant.

    Another alternative is something I found on one of the minibike forums sold by Jamie Nice. It's an aluminum adaptor that goes between the intake manifold and the carburetor. This adaptor gives the desired 20 degree slant. It's a less expensive alternative to the Tecumseh/BlackWidow #33301 slanted intake manifold, and it works very well. It does require another gasket though, Tecumseh #33515.


23. Alternative Carbs.
    One thing I never really liked about Tecumseh carbs when used on minibikes, is the lack of throttle control and travel. It's very short travel, pretty much idle or wide open. There is middle throttle, but it's pretty small on a minibike throttle assembly. Also I never really liked how the Tecumseh carb hooks up to the governor. Nor do I like how I have to fiddle with the high speed adjustment, depending on outside temperature (I like in a four season climate.) These things always bothered me. Is there a solution to these problems? Also can we get a bit more performance out of the Tecumseh H35, HS40, HS50 motor too?

Using a silde carb on a Tecumseh. Is this the answer I'm looking for? Yes!

    I've experimented with several different slide carbs on the Tecumseh HS40 and HS50, and have come up with a nice and inexpensive option for these motors. Did I say inexpensive? Almost cheaper than a standard Tecumseh carb (if you include the air cleaner assembly in the formula.) For about $15 to $25 (including shipping) off ebay, you can buy a a Honda XR80 Carburetor PZ16 and air filter. These work great on a Tecumseh. And they look cool too. This is a 16mm slide carb.

The 16mm slide carb PZ16 installed on a Fox minibike.

    I should also mention that you will probably need a new throttle and cable to work with the slide carb too. But fortunately, that is only about $10 to $15 too. (And your minibike probably needed that anyway.) Personally I used the DB30 Baja Doodlebug style throttle and cable as sold on ebay. It works great with a slide carb, and is inexpensive. The DB30 throttle assembly comes with a dual ball cable, which is needed on a slide carb.

Another bike (Gemini) with the 16mm Honda PZ16 slide carb installed on a Tecumseh HS50 motor.

    The only downside to using the slide carb is that it really needs a spacer of some sort to move it away from the motor (and more important, away from the exhaust.) Without the spacer, the throttle cable or the gas line is a bit close to the motor and/or exhaust. Now if your application is a slant 20 degree motor mount, the aluminum spacer/angler mentioned above is an excellent answer to this. Otherwise, you need to make some spacer from aluminum (which is what I did in the pictures above.)

Modified mount holes on 16mm PZ16 carb to fit the tecumseh intake manifold.

    Also the 16mm Honda PZ16 carb isn't exactly a perfect fit to the existing Tecumseh intake manifold. It is off by a millimeter or two. That is easy to deal with though using a drill to make it line up. You have to make the carb holes a bit wider for it to bolt onto the Tecumseh intake manifold. Not a hard thing to do. I use a 1/4" drill bit to do this, moving it at an angle to make the carb mount hole oblong.

    But overall I am really happy with the 16mm Honda PZ16 slide carb on the Tecumseh motors. I find it gives a really nice throttle range and a nice performance boast too. They also make this same carb in 20mm and 22mm format. It is basically identical to the 16mm version, except the "throat" is larger. Unfotunately it does not run correctly on the Tecumseh engines out of the box (idles but will not accelerate, unless the choke is on about 3/4 way.) It will need a different main jet to work right. For this reason, if you're going the slide carb route and don't want to mess around with jetting, stay with the 16mm version.

PZ22 slide carb 22mm with a custom intake manifold. Great carb with even better performance, but the main jet will need to be changed to #94.

An M4 jet kit off ebay with 85 to 95 main jets. I used #94 jet out of this kit on the PZ22 and it worked great. These are M4 size jets.

    Now if you want more performance from a larger carb, the PZ22 carb 22mm with the metal choke is a good carb. Note there are different versions of this carb - do *not* get the one with the round float bowl and right side plastic choke lever. Get the one in the picture above with the metal left side choke lever. Now you will have to change the main jet! This carb is more expensive ($20 versus $12) than the PZ16, and you have to buy a new main jet. This involves a couple dollars, and float bowl removal to change the stock main jet. The stock PZ22 main jet is not marked, but my guess is it's about #75 main jet. If change the PZ22 carb main jet to #92 or #94 (M4 size), your Tecumseh HS50 will run great and give even more performance than the 16mm PZ16 cousin. It's a great option. Personally I have found the #94 jet to be slightly better than the #92 (assuming an open exhaust). A #95 jet is too much though, you will get massive backfires when you let off the game. Note all these carb from 16mm to 22mm use "M4" style jets. This means the thread size is #4 metric threads.

    So why use a 22mm carb over a 16mm carb? Well if you go to a performance cam, the 16mm carb isn't big enough to support it. Therefore the 22mm carb is just more versatile, in case you decide to install a performance cam. p>

Using a right side choke round float bowl 19mm PZ19 carb on a Tecumseh H35 engine. Main jet changed to #94. This setup worked great, and was $15 on ebay, including the air filter! Note the custom intake manifold, and the crazy custom exhaust!

The above Tecumseh H35 motor.

    Note the round float bowl slide carbs also do work. I just don't personally like the plastic choke assembly. But on some mini bikes, you may need this, because the choke is on the right side. I have used the 19mm version of this carb on a Tecumseh H35 (side popper), and the right side choke was needed (because the exhaust is on the left side!) It works fine, but again, you need to change the main jet. The stock jet on the round float bowl PZ19 carbs is a #73, which is just too small. On the H35 motor (with open exhaust) I found, again, a #94 main jet M4 size to work great.

Drilling the main jet on a PZ22 slide carb. I bought a small drill bit set to experiment. Actually don't bother doing that... All you really need is a #64 or #63 drill bit to drill the stock main jet on the PZ22 carb.

    Of course there is another way to re-jet a carb. In the case of the PZ22 carb, we want a #94 jet. You can drill the original jet (which seems to be a #75) to the larger #92 jet. All you need is a #64 drill bit (and a drill that will chuck a bit that small.) Just drill straight down the original jet that came with the PZ22, and you'll have a #92 or #94 jet. By the way, the jet numbers are metric sizes. That is a #92 jet means the jet hole is .92mm in diameter. The SAE (American) size equivalent is a #64 drill bit. If you want to go as a #94 jet, use a #63 drill bit which is pretty much exactly .094mm (aka a #94 jet). The problem is, it's hard to drill without any wobble. So using a #64 bit probably get you more like a #94 jet.

    Also I should mention on the PZ22 there are two adjustment screws. One is for the idle - it just moves the slide up and down to increase or decrease the idle speed. The other screw is the idle air intake adjustment. On this screw, I have found it needs to be in more or less all the way (or close to all the way), and then backed out about 1/2 a turn. That seems to make the PZ22 work best with the #92 or #94 jet on a Tecumseh HS50 motor.

A Dellorto UB22 (22mm) carb on a Tecumseh HS50 motor.

    Dell'Orto carbs also work nicely on Tecumseh motors. These are Italian made carbs, from a company that's been around since 1933. The Dellorto Ua19s is probably the most famous of the Dellorto carb types. This was used as a standard carborator on the 1971 Rupp Black Widow. It was also an option for the Roadster and Hustler. It did not come with an air filter (!), which is really strange. As it would take about one hour of dusty riding to ruin the engine. Instead there was a "velocity stack" or trumpet for air intake. Not a good idea, but that's how they installed them on the Black Widow. I have found that using a women's sock or a short panty hose on the megaphone air intake works as a cheap air filter (use a 1 1/4" pinball rubber to secure it.)

A Dellorto UA18s (18mm) carb on a Tecumseh HS40 Rupp Roadster 1970. Note the use of a Dellorto air filter. The intake manifold is custom for this bike, made from 7/8" stainless steel tubing.

    Other Dellortos work well on HS50 motors too, if you have the correct intake manifold (they are available for HS40 and HS50 engines from www.BlackWidowMotorsports.net). The BlackWidow intake manifolds will work for 16-19mm Dellorto carbs. I've used UA16s, UA18s, UA19s and UB22 Dellorto carbs on HS50 motors, and they all give very nice performance increase over a stock carb. Is there any difference in the 16mm versus 19mm carbs? Not that I can tell, they all work pretty well (assuming you have the jetting correct.) The 1968-1969 Rupp catalog shows the Dellorto UB22 carb available as an option for the large frame H50 motor mini bikes. Likewise the Dellorto UA19s was an option for Rupp Roadster bikes from 1971 to 1975. Pretty cool option for those bikes.

A Dellorto Ua19s (19mm) carb as used on the 1971 Rupp Black Widow. The high speed adjustment screw can be seen at the right edge of the picture below. The huge megaphone (trumpet) needs some sort of air filter. I use a women's sock or a short panty hose as an air filter (and a 1 1/4" pinball rubber to hold it in place.)

    For reference, Dellorto used a lot of abbreviations in their model numbers. The number itself is the MM size of the carb. But here's some of the abbreviations:
    • U - Carb type, first letter, removable float bowl.
    • M - Carb type, first letter, fixed float bowl (not removable).
    • A - Generally clamp type fitting.
    • B - Generally rubber sleeve "elastic" type fitting.
    • F - Fixed mount (UBF22 for example).
    • S - "Sinistra" or "left" in Italian. Left hand idle mixture and idle speed screws.
    • D - "Destra" or "right" in Italian. Right hand idle mixture and idle speed screws.

    Using this info, the UA19s is "U" for removable float bowl, "A" for clamp fitting, 19mm (size toward engine), "S" for left side idle adjustment. Some other suffix letters may have no specific meaning, they have certain original applications.

    Generally speaking, the UA and UB Dellorto carbs are great for Tecumseh engines. But the MA Dellorto carbs also work well. On the UA/UB Dellorto carbs though you have to make sure the float bowl angle is workable. They come everything from level to 12 to 30 degrees. For a tecumseh engine, 12 degrees is probably OK, but you really want a level float bowl. The really angled float bowls won't work (anything above 12 degrees.) Note the UA19s has a level float bowl (as a comparision.)

Dellorto Ua19s (19mm) carb parts list from Rupp.

    Note there was a high speed adjustment jet screw on the Rupp installed Ua19s carbs. If this is missing (most Dellorto carbs don't have this, but use a standard "banjo" bolt instead to hold the float bowl in place), it's expensive to get the parts to install it. It's not a required thing if you're jetting is correct. But people look for it if the carb is going on say a Rupp Black Widow. Here's the parts needed, available from www.BlackWidowMotorSports.net:
    • #17651 Hi Speed jet screw, UA type (part #39 above) $10.00
    • #17652 HiSpeed Jet Housing, UA type (part #36 above) $40.00
    • #17653 Hex nut for HiSpeed jet housing, UA type (part #38 above) $2.50
    • #18177 O-Ring for HiSpeed Jet housing, UA type (part #37 above) $1.00
    • #18473 0-Ring for HiSpeed Jet housing, UA type (part #31 above): $1.00
    • #17654 Main Jet for ajustable set up (fits in atomizer, part #35 above) $15.00 REQUIRED
    • #18475 Needle Jet (type 1485) Atomizer #260, UA/UB type (part #33 above) $26.00 (probably not needed)
    • #18474 Pilot Jet #38, UA/UB/MA/MB type (part #32 above) $12.00 (probably not needed)

    Total cost for conversion is $79.50 ($54.50 plus $15 for the main jet receiver, which is *only* available from BlackWidowMotorSports.net and is required to work with the high speed jet.) A rather expensive conversion. There is another source for the adjustment UA/UB Dellorto main jet at www.guzzino.com/deubadmaje20.html, but you still need the $15 main jet receiver #17654. Don't forget with the above adjustable receiver jet. You also need a #260 needle jet and a #38 pilot jet (which your Dellorto should already have, so you probably don't need to buy those.) Other Dellorto parts are available at www.guzzino.com/deub202224.html

    On the adjustable high speed jet... This can be added to any Dellorto carb from UA 14 to 19mm and UB 20 to 24mm. But you do need the high speed jet receiver ($15), which is only available from BlackWidowMotorSports.net. You can not use the adjustable jet with a standard Dellorto main jet - you have to use the special high speed main jet receiver (which is a straight hole 3/32" or #41 bit .096" in diameter.) You can however made this jet receiver. I've done it before in a pinch. Get a 1/4-20 brass bolt about 1" long, and center drill it with a 3/32" or #41 drill bit. Then grind off the threads, and run a 5mm .80 thread die down the shaft. Then cut the length to be 1/2" in total length. It's kind of a lot of work, but if you're in a pinch, it does work (it's a lot easier to just buy the $15 jet receiver from Black Widow though.) Note you can't take a standard jet and just drill it with a 3/32" drill bit - the head of the receiver main jet has to be higher than a standard Dellorto main jet. Now if you could make the head taller, maybe that would work...

    When adjusting the high speed jet, I found that it is generally just a 1/4 to 1/2 turn out from, "all the way in". A little turn does a lot on the adjustment. But start with a 1/2 turn out and dial it in from there.

Dellorto special high speed jet (left) versus a #75 standard 5mm Dellorto jet (right). Notice the difference... you can't use an adjustable high speed jet with a standard 5mm Dellorto jet. You must have the special receiver jet, which has a 3/32" straight hole drilled through it (and it's taller in profile.) The adjustable main jet receiver is a straight hole. Opposed to a standard Dellorto main jet, which has a conical shape to the hole.

    Here's a rundown on the Dellorto carbs I've tried on HS40 and HS50 motors. Jetting is important on these. Remember the pilot jet controls the idle. The needle jet (atomizer) is what the needle moves in and out of, and controls 15% to 60% percent of throttle. The main jet screws into the needle jet and controls 50% to 100% of the power range.

    • UA16s. Usually comes with pilot jet 45, needle jet (type 1485) 260, 5mm main jet 80. In this format runs great on a stock HS50. Really nice, good power band.
    • UA18s. Usually comes with pilot jet 38, needle jet (type 1485) 260, 5mm main jet 82. In this format sputtered. Changed to pilot 40 and main jet 85, now works great. This is an awesome carb on a HS40 or HS50 motor.
    • UA19s. I've bought a number of these and they are all over the place with jetting. Really needs to be pilot jet 38, needle jet (type 1485) 260, 5mm main jet 98 or 100. In this format it runs great on a HS50 motors. The high speed adjuster is helpful to have too, lets you dial in the carb nicely. But a special high speed jet receiver (which fits into the atomizer needle jet) is required. Note the adjustable receiver jet is basically a regular jet drill with a #41 (.096") drill bit. If you don't have an adjustable high speed, a #98 or #100 main jet should work fine instead.
    • UB22bs. This is a pretty large carb for an HS50 engine, but it does work. Usual jetting that I've seen is pilot jet 45, needle jet (type 1485) 260, 5mm main jet 95.

    Note that Dellorto also had different float bowl weights available. On the UA19s the weight is 7.5 grams. But I've used 6.5 grams with no problem. I'm not sure the float weight is that big of a factor.


24. Drilling the PTO.
    If your Tecumseh motor is older, there's a good chance it is not drilled for a bolt at the end of the PTO. Why do you need the bolt? Well we don't use set screws to hold clutches any more! (They just loosen, fall out, and your clutch flys off.) The set screw idea went away with the 1980s. Now we use a 5/16-24 or 3/8-24 bolt at the end of the PTO to hold the clutch in place. In fact, if you're using a torque converter, that's the only way you can run.

    So how do we deal with a non-drilled PTO? Well it's a lot easier than you may think. It's a cast steel material, and it's actually pretty easy to drill with a standard bit and hand drill. The only problem is, how do you drill it centered?

Using an old centrifical clutch and a 1/4" drill bushing to make easy work of drilling the PTO shaft.

    To do the job you'll need an old centrifical clutch and a 1/4" drill bushing. The drill bushing is availiable from McMaster Carr, part #8493a074 (drill bushing 3/4" OD and 1/4" ID). With the drill bushing installed in the clutch, and the clutch installed on the PTO, it's an easy job to use a 1/4" drill bit and drill into the PTO shaft about 3/4 of an inch.

Using a 1/4" drill bit and a drill bushing, drilling the PTO shaft dead center is easy work. Drill about 3/4" deep.

    After the 1/4" hole is drilled into the PTO shaft, remove the drill bushing and clutch. Now re-drill the hole with a letter bit "I" or 17/64" or even a 9/32" drill bit. Then run a 5/16-24 tap into the hole. Remember to use oil on the tap. Go in a full turn, then back a 1/2 turn (this helps clear the flutes.) Do this until your tap does the 3/4" hole depth (or close to 3/4".)

The 5/16-24 tap.

    Note we are using the "fine thread" 5/16-24 version of the 5/16 bolt. This is important. Do not use the more common 5/16-18 tap/bolt. Fine threads are required for this application.


25. What about the Exhaust?
    Snowblowers generally come with a usable but not optimal exhaust, the Tecumseh #35771 or #32648 or #33697 muffler and gasket #35865. There's also another exhaust that is a bit more minibike-ish, Tecumseh #37684 which has an extended pipe.

The stock Tecumseh 35771 muffler, wire wheeled and painted.

    Really the 'standard' minibike exhaust was the Taylor Products model 100 (Tecumseh parts #33222, 33223, 33224), but they are no longer available (NLA). Taylor went out of business long ago, but sometimes you see them on ebay. Note there are several different Taylor styles (bends) and exhaust port styles. So if you're going that route, make sure you get the 'right' Taylor with the correct bend and correct exhaust flange (HS40 and HS50 have slightly different exhaust ports, as does the H30/H35.) But be prepared to pay for this exhaust. Frankly I don't think they are worth the price.

The Tecumseh 37684 muffler with the extended pipe, as used on the Tecumseh HSSK55 engine. This is actually a pretty nice exhaust for a lot of mini bikes. But not as cheap to buy as the 35771.

    Personally the path of least resistance is to just use the Tecumseh 35771 which was stock on most snowblowers. I take the exhaust to a wire wheel to knock off all the loose particles and rust, and then paint it with a high temperature black paint (rattle can). Short of buying a new 35771 exhaust (they are actually pretty cheap), this makes the original exhaust look somewhat decent. Alternatively you can look for the Taylor Products minibike muffler on ebay, but it won't be cheap. If you have a tubing bender you can buy 7/8" tubing and bend your own exhaust (flanges are available on ebay.) But most people don't have that capability.

The Taylor Products model 100, Tecumseh part #33224. This one is chrome plated, which is pretty rare, most are just painted black with no plating. Very cool, but old and hard to find. Also came in several different bends and flange configurations, depending on the exact Tecumseh motor application.

    I guess we could re-make the Taylor Products model 100, but the issue is all the different motor configurations. The HS40 and HS50 have slightly different exhaust ports. And then you have the H30/H35 variants too. It would be a fair amount of work, for a pretty limited market. Hence I haven't seen anyone re-issue these. You could also make your own using the Tecumseh #730165 exhaust adapter kit and some pipe.


26. Decals for the Blower Housing.
    As a finishing touch, you'll need a few decals for the motor. There's people on ebay that sell decals. These are good, but they are water-slide style decals. Which means you have to put some sort of durable clearcoat over the decal, or it just won't last.

All sorts of decals that can be used on your fresh motor!

    So what I do is to print the decals on sticky-back vinyl. This is much more durable. And if the decal becomes damaged, I can just peel off the old decal, and peel and stick another! It's way easier to deal with than water slide decals.

    So how do you make vinyl decals? I have all the graphics available in Photoshop format for download on my minibike webpage. For example:

    The "balloon" decal was used on the 1968-1970 Tecumseh HS40. All Tecumseh HS40 and HS50 motors from 1971 and up should used the "flags" decals. Since the HS50 wasn't introduced until 1972, it will always have a "flag" decal. The air filter/oil decals can go on any motor if you like.


27. Conclusion.
    As you can see, a decent snowblower Tecumseh HS style engine can be converted to mini bike format. Is it easy? Yes and no. I guess it depends on your skill set. But I have found following this document a great way to get a vintage style motor (like I remember from my youth) inexpensively for a 1970s minibike.

The finished product! A nice Tecumseh HS40 ready for installation on a vintage mini bike.

    Note you don't have to go nuts when re-doing a snowblower motor. Below is a picture of a dark grey snow blower motor. All I did was powdercoat the removable metal pieces to black, and clean the long block with a degreaser and some rags. I did *not* repaint or blast the long block (it is still sporting its original dark grey color.) This worked out really well too, and was a lot less paint/prep work!

Here's a Tecumseh HS50 motor "before" our conversion, just after it was removed from the snowblower.

Here's the same motor "after". From dark grey to black on this HS50 motor, it's now ready for a minibike!