Yamaha runaway engine problem

[markproy]

I have a 1979 Yamaha DT 100, and it is pretty much new, im only the 3rd owner. When i ride it, no matter what gear im in, if i go above 1/2 throttle, the engine revs up to full rpm on its own, and then the kill switch nor the key will shut it off. I have to lower the rpms by the brake, or hit the choke. i dont know if it is an electrical or mechanincal problem, maby both. Any help is appreciated.

 

[sharky243]

Ya I had an old 77' DT 250 that would do that once in awhile on a real hot day. There are acouple things to look at that will cause that. What is happening is that the fuel is igniting from heat rather than spark, much like a diesel engine does. Start by cleaning out the carb., It maybe dirty and causing a lean condition. Then check for air leaks (ie. a bad crank seal), this will also lean out the engine. Next, check the ignition timing, make sure noone has messed with the stator plate and retarded the timing. One other thing to look at is carbon build up in the combustion chamber. Carbon will hold heat and ignite the fuel before the spark will, also check the spark plug for the proper heat range. Make sure the cooling fins are clean. Those bikes had points and condenser if i remember correctly, so if someone has tried to eliminate the points by installing a CDI modual, this could be a cause. ( I tried it on my 77' and it didn't work well LOL !! )

 

[TRexRacing]

My YZ465 did that.Kinda scary.Cranks seals were the issue.

 

[markproy]

Thanks, ill try that. I'm new to dirt bikes, so i don't know all of that stuff. Thanks for your help!

 

[freakystyley]

start with easy stuff as mentioned, air leaks, clean the carb, etc. and check the throttle cable for kinks and the return spring on the carb

 

[bullz-i]

I had a old Maico that did that and it was the Manifold that goes between the Carb and the Reed block. It was made of rubber and had cracked in a place I couldnt see. Thank God for Duct Tape it saved the weekend.