Converting snowblower type carburetors to mini bike application (HS series)

markus

Well-Known Member
#1
I bought some factory NOS Tecumseh carbs for HS40 sized snowblowers and one HS50 sized "service" carb to convert over to recreational/mini bike application. There is some work involved to get them ready for MB app but its worth it over using the aftermarket Chinese made replacement carbs. I've done this in the past with original carbs I got on snowblower donor engines, whenever the check/metering rod still rattles in the idle circuit I salvage the carb. Finding a carb for mini bike application NOS is a rarity so sometimes you gotta improvise.

These NOS carbs usually are priced high, I started to research and learn how to better figure out the HP size service and replacement carb numbers anyway though as sometimes they will pop up cheap..Actually got these 3 new in box ones cheaper than what 3 china paperweights would have cost me, and even though there is still work that has to be done to convert, there is actually less to do than on a china carb.

Left is a replacement "service" carb, these come as seen, they are intended to fit a few different setups and you use your old levers or order new, this particular one is for an HS50 5hp engine size. On the right is a pair of snowblower type replacements and are HS40 4hp size specific. I like the older ones in the red/white box, but these later ones are still made in the USA Tecumseh carbs with the proper needles and seats still and actual Tec parts... they just have a plastic float in them unlike the brass in the older ones:


Main things that need to be addressed to make the conversion are the choke lever change out and the removal of the primer nipple, couple other things thrown in as well mostly to make it more legit looking for an early/mid 70's engine.

First clamp them nipples then twist an pull.....hurts so good!!!


Next fill the hole. I just mix up a little JB weld (24 hour) for this. Pulling the welch plug out that sits just above the float inside the float chamber is a good idea here. Your able to get to the backside of the hole you are filling easy so you can make sure you splooged enough JB through and got it fully filled in, plus it makes giving it a good cleaning before reassembly and easier for the something else that needs to be done when you do this...so leave it out for now.


If your doing an older one your rebuild kit will have the replacement welch... I had some extras in my parts bins for the New carbs thankfully. They don't have to be there in a pinch but without you may see some leakage/spiting out the breather hole on a bike with all the bouncing and vibrations since the vent chamber would be more open to the float chamber.

Holes filled (I did a 4th good used carb body I had on hand at the same time as well)


Next thing that HAS to be done if you removed the nipple (or even just cap it off) is to make the upper atmospheric vent in the chamber larger.
standard size for carb body that does not have a primer nipple or other type ventilation tube is 1/16"....verified in this old OG carb


Just simply drill it out, If you aren't going pull the welch plug or do this such as an "on the bike" senario, put some grease on the bit and turn it very slow and hopefully any shavings will not go in the chamber. can see the size difference in the vent hole sizes in this shot:


If this is not done, there is a good chance you will have some flooding issues. This is VERY easy to do and you just drilling into a large open hole in the carb so don't worry about going through something.

Next up, choke lever....
 

markus

Well-Known Member
#2
Next thing to work on is the choke levers, In this shot the old one on the bottom is what the the HS mini bike and kart engines got starting right about 1971. These were made to clear the headers. they, like the carbs, are also NLA parts so I have been collecting them whenever I see them come available or just taking them off bad carb bodies if they are still useable.


Big problem is the snowblower type lever has the detent spring in the wrong location. But as you can see from above most real Tecumseh carbs made for the HS engines will in fact have the hole pre drilled for the detent location closer to the shaft that this style lever uses, note the "service" carb has a spring in each hole and you pull the one your not gonna use out. This is a problem on the Chinese aftermarket carbs in the close to correct size carb body, They only have the one detent that's in the wrong spot. I did convert one of those before and put that info in another thread a few years ago and that can be found here: Tecumseh aftermarket OEM carb choke lever modification for the HS40

For these carbs thankfully its just a matter of removing the spring detent and putting in the correct spot. Good pliers close to the body and twist usually gets them out, Putting them back in the same way.....sometimes its smooth sailing and threads right in/out, sometimes its a fight....was about 50/50 with these 4 carbs anyway.



something I noticed on these newer carbs when I pulled the snowblower choke levers out were the screws that hold the throttle and choke plates onto the shafts are torx head and almost twice as long as they need to be so they stick way out of the shaft. The early ones barely stick out of the shaft. So in an effort to get as much restriction out of the picture I threaded in the screws (along with a choke plate) onto one of the snowblower shafts while out of the carb body and ground the ends of all the screws down to the depth of the shaft....The snowblower choke shaft was headed into the garbage can anyway so I was not worried about marring it up while doing this. I did this to the screws for the throttle plates as well. Technically Tecumseh suggests not reusing the screws if I am not mistaken. I wont lie, I reuse them. They do appear to have a sort of locktite or something like that sometimes, these new ones looked like maybe something blue on them. I use a little tiny dab of nail polish I stole from my wife.



The Idle adjustment screw on the newest carbs in the batch had the same size torx head on it as well, I opted to swap that out with one off a bad carb so it looks right on a 1970's and when making adjustments you will only need one tool for the needles and idle.



some other things was of course manipulating the throttle stop tab so it just about goes full open at WOT on the plate, they don't open past like 80% about stock. And on one of them for now I pulled the plastic end 90 off and installed a steel 90 on.....that's just nit picky earlier 70's correct, but honestly once its painted I don't think using the plastic 90 is very noticeable, not like a choke lever bent all over place to clear the header and air cleaner anyway that you usually see when someone uses a Chinese carb ;)

 
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