My Engine Revs nonstop on 1983 yz 125

Question

Ok i just bought a bike, an 83 yz 125 and at first i couldnt get it started, after about 2 hours of kiking and toying with it it started and worked fine. next day same thing, day after that i started it and it revd the engine nonstop nomatter what i did (yes i let go of the gas) so finally the only way to stop it is to unplug the spark plug wire, everytime i start it thats what it does now. 1 more thing when i give it gas it works fine but the engine sounds different, when i let go of the gas it revs like crazy till i kill the spark.

Answer 1

sounds like a stuck carby float

Answer 2

The hard starting makes me think the crankshaft seal is leaking.  That'll make them hard to start and will allow air past, allowing them to rev uncontrolled.

Answer 3

maybe you need a new fuel pump... not really sure but sounds like theres a problem with the fuel

also what kind of gas are you putting in it older engines sometimes need premium to work properly

Answer 4

If it's not cable related it sounds like you have a stuck float. You can try tapping on the side of the carb.  

Answer 5

Not sure if I understand your problem the way you described it, but I know for sure there is no fuel pump. It's gravity that gets the gas down into the carb.

It sounds like maybe you have a problem with the throttle cable and the slide in the carb. If the motor revs when you start it, the slide may be stuck in the wide open position.

Unscrew the top of the carb and pull the slide out and have a look at it and check if it moves OK inside the carb. be careful putting it back in, as there is a notch that you have to line up.

I suspect you need a new throttle cable. Good luck.