2000 YZ 125 Breaks up at High RPM. We think its the power valve. any tips would be great

most likly needs new rings-or bad plug if you are lucky

http://www.bikebandit.com/houseofmotorcycles/2000-yamaha-motorcycle-yz125m/o/m8835#sch111222


Please see this schematic.

Pull started? on a YZ 125?
The power valve will not allow high RPM power if it is gummed up.
Low compression is due to bad rings, a different problem.
You may have both problems.

There's no extra valve that kicks in to give the bike extra power. The term "power band" is simply referring to the RPM range of the motor that produces the most torque. Make sure the carb is clean and check the spark plug gap and fire. You need a hot blue,purplish fire not a weak white looking fire. If the plug itself is whitish in color you need to richen up the carb if it's black you'll need to lean it out. You should have a chocolate almond color. I'd bet the bike is getting too much air.

You don't have to do anything It does it on its own as long as the pieces are all there. The power valve is the thing that opens up a larger exhaust hole for the two stroke engine. It is shaped like a spool and it twists as pressure builds due to high rpm. On the 1984 it is on the left side of the bike right up front behind the clutch cover.

hi this may help,it sounds to me like the carb is running way to lean,this may cause the bike to miss at high rpm,also check that the spark plug cap is in good condition as it may be breaking down under high load conditions...cheers