2 Stroke Engine cutting out in midrange

[Meacham]

I have a 99 YZ 125 that I think may need crank seals. I've checked the carb, reeds, cylinder torque, head torque. The plug is super lean. I went to a bigger main jet and raised the needle and it was still lean. I run Amsoil Series 2000 @ 36:1 with 50/50, 94 pump/110 race fuel. The bike starts perfect, will sit at idle, rev with the motor under no load and it sounds fine. When I drop it into gear and give it throttle the motor cuts out violently. I can lug it or run at WFO and it runs good, but when I try to accelerate from the bottom end it's not good! I don't have a leak down tester so I'm hoping someone may know what's up.

 

[billtx]

I have the same bike. Sounds like the left side crank seal. I'm having a problem with the right side seal..I think. Stopped by the Yamaha dealer today, he said the left side seal going is more common on that bike than the right. It will make it run lean.

The good news is the you can change the seal without to much effort. You can leave the engine on the bike, you just need a flywheel puller. It probably won't cost that much at the Yamaha shop either.

 

[Meacham]

Figured out what was up. I replaced the left crank seal and it made no difference. I decided to tear down my top end and there it was, a broken spring in the power valve. Only one valve was working.

 

[overbore]

can you tell me why a powervalve would make it run lean.
I am a pretty good mechanic,but dont quite understand how
this would do the simptines.