yz250

I have a 2000 yz250. Just recently the engine races at a high rpm went
in neutral and even higher with the choke on. I replaced the throttle
cable and showed little if any improvement. Does this sound like a carb
issue?

 

Do you have any free play in the throttle cable at all? You must have
about a millimeter or more of free play to avoid raising the throttle
slide from the fully closed position.

Have you been messing around with the pilot screw? That's item # 15 on
the CARBURETOR diagram at www.partsfish.com

You can register at www.partsfish.com and look at parts diagrams for
free.

The pilot mixture screw on this carburetor is not like the pilot air
screw on older slide valve carburetors.

Turning the pilot mixture screw counterclockwise does NOT lean out the
mixture, it makes it RICHER.

Then the amateur mechanic notices that the idle RPM is LOWER than it
should be, so he adjusts the idle speed screw higher to keep the engine
from stalling.

Then the engine gets too much air and gasoline when the engine is
idling in neutral and the idle speed runs away.