My buddy's running a 91 Yamaha yz250 motor, it's powervalved. It runs fine on lower rpm's but woln't run at about anything past 1/2 throttle, just stalls and sputters. First, the bolt allowing the powervalve to open as the motor reved was loose so it wouldn't open, we tightened it up and it still woln't run for anything. We just put new crank seals in it, air filter is clean, jetting is on, and we have eliminated just about everything we know of? he's running golden spectro at 32:1.

Thanks for anyhelp

 

First off, Spectro SX (synthetic) is a lot better than golden, but this shouldn't be your problem.

I take it the powervalve is working? If you don't think it is, simply unhook it. Wire the powervalve open. The engine should run fine but it will have a little less low end. If this takes care of it then you might make sure the centrifugal assembly is working.

I had a 91 RM125 and had the pipe laying on the ground while I sent my cylinder to have it renickisiled. A couple of weeks later I put my bike back together. It wouldn't run at anything over 1/4 throttle. After many hours screwing with the powervalve and even pulling the engine back apart twice, I found the problem. A bird started a nest inside my pipe and a small amount of air could flow so it ran perfect at lower rpms but couldn't flow the air above about 3000rpm.

Also, If I can remember right, that motor had an adjustable timing plate. Since you had the crank out, you had to removed the plate. You might check the timing.

Please let me know if any of this helps.