1993 YZ250 tuning, I'm stumped!

Bought a 1993 YZ250 recently and it's tuning issues are confusing the hell out of us. I'm used to 80cc 2 strokes and not the big ones w/powervalves and such.

Bog off idle sounds chunky and rich, smoking a ton. Sounds like it gets on the pipe, a little burst of power, then it falls on its face and has no more go up to WOT. It doesn't sound like its bogging and doesn't loose power really, just doesn't really develop any more power with more RPMs, definitely not revving to max RPMs. 310 Main jet, all clean.

Powervalve issues? Rich on bottom then lean on top? Something else?

Edited by chixwithtrix


The powervalve is actuating as the rpms go up. Does that mean it is fine or could something else be wrong with it?

The spark plug is black and wet after running it. That suggests it is rich, but it has a 310 sized main jet when the manual says the 350 is stock. Needle is in the middle position.

Check the slide and carb cavity for excessive wear because this would cause an air leak directly into the cylinder. You also may have a leaking crank seal which is also often times directly related to those exact symptoms

May also want to pull the jug and check the bore/piston specs, if its too far gone it wont want to rev out.

Its a bike im selling, was hoping not to tear into it. Ill just sell it as is if I can't figure out something not internal

did you try moving the needle down one clip? this will make a change for better or worse, then go from there. jet each circuit by itself. try a smaller pilot, then try off idle response while turning the air screw in or out.. once that is smooth, move to the needle, then the main. easiest way to see where its at is to move the needle one clip. check air screw also, these bikes are very sensitive to temps and jetting changes.....there is a good thread on jetting in the stickys...

the strange thing is that we did try moving the needle down one clip to lean it out...didn't seem to change anything power wise. It doesn't pop on decel with the 310 main jet and leaned out one clip. I'm starting to think the crank seal is leaking the clutch oil into the crankcase like someone suggested. The engine doesn't act lean, smokes a lot, and won't rev out. When I looked into the powervalve it had black nasty oil coating everything in that compartment, is that normal or is it like that since the engine could have been burning normal tranny oil?

I don't have a compression tester at the moment, but I took off the exhaust and looked at the piston/cylinder from what I could see. The piston skirt looked great with no black marks and I could see the crosshatch on the cylinder walls with no scoring or chips around the ports.

The reeds visually looked to be ok, not sure if there is a better test for them. The expansion pipe had a lot of carbon buildup on the walls, maybe about 1-2mm thick all around the neck and into the chamber. I didn't think that would effect the power that much since its an aftermarket pipe anyway. The silencer looked to have a clear passageway.

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