I put a new top end in my son's 90 about 2 months ago. We went riding for an entire day and at the end of the day the quad started to rev up by itself and finally quit. I just checked the compression and it reads about 90 psi. Is this enough? Should it be higher? I don't know how I would already have a compression problem, but somthing's up.

 

If running wild,then possible lean condition. Check base gasket ,head gasket,also possible crank seal leaking which can do the same thing. If plug looks white then is lean! 90 psi a little low,but acceptable. Did you just put a piston and rings in or did you have it bored out to the next oversize? Have had a couple that the cylinder studs would back out,loosing compression also.

 

I replaced the piston, rings, and put on a new can. I also just checked the spark and noticed it is very inconsistant. I thought this is a bit strange and found out the stator was bad again. Sent back the stator for a new one and put it on and the spark is still very weak. So, I just ordered a CDI in hopes this will clear up the spark issue.