Adapting slide valve carb to B&S, Tecumseh, etc to mini bike or go kart.

Right now I have a go-kart with an 8 HP B&S engine and a mini bike with a Tecumseh 3.5 HP engine. As usual both engines won't idle/run correctly due carburetor issues. I'd like to get these things running this summer.

Last time I fooled with the 8 HP B&S engine, I came to the conclusion that the reason that engine won't idle is throttle shaft is worn out. I sprayed WD-40 on the carburetor one day while trying to adjust the idle mixture and stalled the engine. I replaced the throttle shaft, rebuilt the carburetor. It will run but won't idle. I guess it's getting too much air. A new updraft carburetor for this B&S engine costs around $125. I can go to Harbor Freight and buy a new Chinese engine for that! I bought a used carburetor at flea market for $10 but have not installed it yet to see if it's any good.

With the Tecumseh engine, I once had this bike running great. But after it sat over one winter, the E10 must have messed the carburetor up. It wouldn't idle in the spring. I tried cleaning the carburetor but to no avail. I've found cleaning/rebuilding carbs is hit or miss.

I found a new carburetor off Ebay for $25. Now the problem is the throttle will only open 1/4 way when the engine is running. With the engine not running, the throttle will open all the way. I know that makes no sense so what does the maximum throttle position have to do with the engine running or not. It has something to do with the governor. On both of these engines, the throttle cable connects to a spring mechanism which connects to the governor arm then a rod connects to the throttle plate on the carburetor.

I have fooled and fooled with this Tecumseh engine to the point I'm blue in the face and still can't get it right. ARRGGHH!!!

I'd like to know if it's possible to install a slide valve carburetor like what's on my Tomos TX-50 and my 110cc ATV. Like the one below.



If it's possible, then how to match the carburetor. An 8 HP engine is supposed to be around 300 ccs and a 3.5 hp engine would be around 150 ccs. So would I need to go by CCs or the venturi size on the original carburetor?

I realize I'd need to fabricate an adapter plate and throttle and choke cables but that shouldn't be a big deal. Is it possible to use a carburetor like this on an engine with a governor?

What would I have to do, wire the governor closed or wire it all the way open?

I can see the purpose of a governor on a lawnmower where you hit tall grass and the engine speeds up automatically so you don't have to constantly adjust the throttle but on a go-kart or mini-bike the engine speed is controlled by the throttle or accelerator. Why is the throttle cable connected to a spring mechanism then to the governor arm to a rod then back to the throttle plate on the carburetor in mini-bike and go-kart applications? Can someone please explain this Rube Goldberg setup?

The governor is there to prevent the engine from overspeeding. If you open the throttle all the way with no load, your engine will rev too high and explode.

primerbulb120 said:

The governor is there to prevent the engine from overspeeding. If you open the throttle all the way with no load, your engine will rev too high and explode.

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So is there anyway to "bypass" the governor so I can use a slide valve carburetor? Wire it wide open?

You might be able to do what you want to do, but you will be putting the driver of the unit in a very dangerous and rediculous position. If you understood how a governor works and what can happen if you override it, you would not even think about attempting this.

We are talking about a cycle and go kart not a lawn mower. Yes you have some very good ideas. Do away with the governor all together you shouldn't have any problems. I have a friend that races go-karts and the very first thing he dose is removes the governor. Sounds like you are going to have a very good set up. :thumbsup: Just have fun with it. I run ATV's and motorcycles all the time and NO GOVENORS on any of them.

reynoldston said:

We are talking about a cycle and go kart not a lawn mower. Yes you have some very good ideas. Do away with the governor all together you shouldn't have any problems. I have a friend that races go-karts and the very first thing he dose is removes the governor. Sounds like you are going to have a very good set up. :thumbsup: Just have fun with it. I run ATV's and motorcycles all the time and NO GOVENORS on any of them.

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Thanks for the reply. Do I need to physically remove the governor from the engine or just remove the arm or run a piece of wire through one of the holes in the arm and tie it off to something?

Concerning carburetor size. It seems these Chinese made carbs area one size fits all application. What's the best way to pick one of these slide valve carbs? Match the venturi or based the carburetor on whatever size engine it was designed to go on?

The one on my 110cc ATV is 19mm. A B&S 8HP is roughly 100cc but I'm not sure if the venturi size is the same.

You could just wire off the governor, but it would probably come apart inside the engine and cause damage to the other internal parts. I would recommend removing it completely.

Your 110CC ATV/motorcycle engine is 8.8 HP. However, the displacement of a Briggs and Stratton 8 HP engine would be much higher since it is a utility engine instead of a motorcycle engine.

The Chinese slide valve carbs are used on a wide range of different engines, but the jets are different. You can't run a 3 HP engine with the same jets as an 8 HP would use. I replaced the carb on my ATV and I had to install the original main jet on the new carb because the engine wouldn't run right with the new one.

turbofiat124 said:

Thanks for the reply. Do I need to physically remove the governor from the engine or just remove the arm or run a piece of wire through one of the holes in the arm and tie it off to something?

Concerning carburetor size. It seems these Chinese made carbs area one size fits all application. What's the best way to pick one of these slide valve carbs? Match the venturi or based the carburetor on whatever size engine it was designed to go on?

The one on my 110cc ATV is 19mm. A B&S 8HP is roughly 100cc but I'm not sure if the venturi size is the same.

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I would say yes match the size venture or base to the engine. I really can't give you a answer seeing its something I have never did. The engines I have worked on with the slide valve style carburetor have a rubber intake manifold that matches the engine and carburetor. It may have to be something you will have to make fit? I would think its something that might fit a scooter engine. Have you looked on any of the go kart or scooter forums. Let us know how you make out with this. As far as removing the governor I think all you need is to seal the governor shaft hole or as you said tie off the arm, or just remove the governor link because it can't go anywhere.

primerbulb120 said:

You could just wire off the governor, but it would probably come apart inside the engine and cause damage to the other internal parts. I would recommend removing it completely.

Your 110CC ATV/motorcycle engine is 8.8 HP. However, the displacement of a Briggs and Stratton 8 HP engine would be much higher since it is a utility engine instead of a motorcycle engine.

The Chinese slide valve carbs are used on a wide range of different engines, but the jets are different. You can't run a 3 HP engine with the same jets as an 8 HP would use. I replaced the carb on my ATV and I had to install the original main jet on the new carb because the engine wouldn't run right with the new one.

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What I meant by "one size fits all" I mean this:

http://www.ebay.com/itm/262175108685?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

This makes no sense to me. How the same carb used on a 50cc bike would also work on a 125cc with the same jetting.

When I was tuning my Yugo turbo, I used the same Weber used on the 1100cc engine. I used a 1500 Fiat X 1/9 engine (same engine just bigger). I had to step up from a 100 to a 125 main primary jet. In 95F weather, the car ran fine under cruise but when the temperature started to drop below 60F, the car chugged and went on under cruise. Below freezing the car would backfire due to the lean condition. Now it runs fine in any kind of weather with the 125 main primary jet.

This was what I was trying to use as a guide:

https://www.jackssmallengines.com/bs8intekpro.html

https://www.jackssmallengines.com/bs35horizontal.html

There are dozens of different sized Mikuni and Keihin knockoff carbs on Ebay. I figured I'd try to match up the displacement of the B&S engine to the same carburetor used on something the same displacement. I figured that would be a good start. Trouble is I don't know if you can buy replacement jets for these things. They seem like throwaway carburetors.

The one I have on my ATV might be suited for my 3.5 HP Tecumseh bike since it's about the same displacement. The go-kart being 8 HP, might need something like a 28 mm carb.

One issue I'm wondering about is the throttle cable on the go-kart. The B&S engine uses an updraft carburetor. I can't remember what series it is but it's been in production from the 70s up through the 90s. The throttle cable goes horizontally to the throttle mechanism on the back of the engine. If I use the slide valve carburetor, then the throttle cable is going to go vertically.

On my Manco Thunderbird, this should't be an issue because the motor is located under the seat. But on the go-kart, I'm not sure how to prevent the cable from getting put in a bind since it's going to have to make a 90 and possibly another 90. I thought maybe bending some copper tubing, running the cable through it then to the linkage that the accelerator hooks to.

There's gotta be some way to do it.