What Is Causing My Run Away Engine?

[tsr22b]

Hi, when you say the throttle opens and closes as normal when the engine is running wild, can you actually see this? i.e. by looking into the carb mouth when it happens? Is the slide actually going down when the throttle is closed off, or is it sticking? Also check that the needle clip in the slide is in place properly and the needle is not lifting on its own. The needle should be fixed to the slide or held down by the throttle return spring. I doubt if you would have been able to set the carb up properly if there was a big air leak anywhere. When it goes nuts does it correct itself almost immediately or do you have to wait a while for everything to cool down? If it cures immediately then the slide is sticking or the needle is lifting on its own. If it takes a while then something is contracting when it cools and stopping the air leak. hope this helps.

[jse]

Ok, I followed everyone's advice, replaced crank seals, new carb to engine boot. Now it just runs away when the engine rpms get high. For example, if I am decending a hill and this causes the engine rpms to increase, when I get the bottom of the hill the engine runs wild unless I slow the rpms with the rear brake on the decent. Or if I am riding between sections and get the rpms up in 4th or 5th gear, the engine starts to run away. This makes for pretty exciting sections...

At lower rpms, it runs fine. When it runs wild, the throttle opens and closes fine, no hang ups there. As before the only way to stop the run away is turn off the gas, pull the plug wire and wait for it to run out of gas.

Do I need to call a priest?

Sounds like you've covered most of the bases and matbe it's time to call a priest who can do a "leak-down" test on the engine. That would involve plugging the exhaust port, plugging the intake manifold and then pressurizing the crankcase with about 5-7 psi and checking all the sealing junctions (and looking for casting flaws) with soapy water in a spray bottle and look for bubbles. If you had a very accurate air pressure gauge, you could monitor the pressure drop, but the soapy water routine usually works fine.

Any good two-stroke engine builder will have a leak-down kit available and/or you can make one yourself using various "freeze plugs" from an auto parts store (they are used to seal casting holes in engine blocks and the rubber expanding ones work well for exhaust ports and the steel ones fit intake manifolds). Compressed air and a small pressure regulator attached to a fitting in the manifold plug and you're set.

Some engines are prone to air leaks in certain areas, for instance, the early KX80 Kawasaki's had a casting flaw involving three bosses behind the stator area and a tuner would routinely seal and test these when modifying the engine. Newer case reed Trials engines are sometimes prone to air leaks under the manifold area case sealing surfaces and you want to spend a little extra time making sure that area seals well.

For most riders who work on their own engines, this seems like overkill, but your problem seems to be out of the normal range of symptoms and a test would eliminate sealing issues as a cause.

Jon

[wherry]

Also check that the needle clip in the slide is in place properly and the needle is not lifting on its own. The needle should be fixed to the slide or held down by the throttle return spring.

I like the sound of this as a possible cause. Have you got the little plate between the needle circlip and the throttle spring? The plate holds down the needle onto the slide using the tension of the spring.

[woody]

Haven't you got another carb you can try from another bike as that is where I'd start. Doesn't matter if it isn't jetted correctly and the bike runs rough, as long as it runs and you can ride through the rev range you can see if the same thing happens with another carb.

I'd be surprised if air leaks could cause what is happening, they make an engine run on a little when you shut off but not usually to rev on at full chat. It's a remote possibility that when the engine revs high, intake pressure is holding the slide open if the return spring is too weak to close it. This sort of thing used to happen on early 290 Shercos. I had a few moments on one I borrowed once and someone else I know broke his wrist when the same thing happened to him. These weren't one-off moments, they happened to a few Sherco owners round here, when you shut off the slide wouldn't close and the motor carried on racing for a short while until the spring pressure finally overcame the intake pressure.

Try and borrow another carb and see if that stops it - or the one off your GasGas should do

Edited December 2, 2008 by Woody

[tony283]

A very annoying and frustrating experience but often a cheap fix. With older machines there are often a lot of "shade tree mechanic's" work which take time to rectify. Most problems of this kind are invariably carb related, or a problem in the inlet tract. I have had a couple of bikes in where "shade tree" has used the dreaded SILICONE on the faces of the carb mounting or on the barrel itself. This stuff will heat up and melt and then when you are downhill on idle it will blow back on the leading edge of the slide and then hold the slide in that position until it cools. The other likely problem is a very small deposit of corrosion on the fuel inlet. As the engine starts to run out of fuel it sucks in more air and the engine speeds up, this will then vibrate the sticking float mechanism allowing some more fuel in and the cycle continues.

First I would use the soapy water technique and look for the tell tale bubbles, head, base of barrel, and all of the inlet joints. Next take the carb off and remove the float bowl and then holding upright see if the fuel inlet opens when you move the float up and down. If it sticks at all give the plunger a light polish with a fine emery. Check the slide for smoothness and clean well. Blow through ALL orifices in the carb and check all the inlet joints for that dreaded silicone...... last resort change the carb.

Tony