I have a 2001 Yz125. Bike is completely stock with the exception of a FmF Turbine core silencer. Bike has been fouling plugs every 10 minutes of riding. Jetting is all stock. I just got done rebuilding the entire engine. New piston set, gaskets, crank seals and all other engine seals were replaced as I thought the engine was consuming the cank oil. Just started the bike up and took it for the first ride after the rebuild. Fouled the plug after about 10 minutes. Install a new plug and it will start right back up and run for another 10 and die??!! I've checked to see if it's loosing spark right after it diesand it has spark but only on a new plug soit is definetly fouling it out. Any ideas??

Did you do a crank case leak test? Did you use a good crank case sealant?

Raise the needle clip a notch and install a smaller main jet.....it worked on my '01 YZ 125 with a similar problem.

I know this is obvious, but the air filter is clean and your pre-mix isn't too rich, right?

Good Money says is it is the Jetting, way too rich, I had the same problem with my RM125

KEEP IT IN THE POWER BAND, dont granny it around the yard....lol :}}}}

In my opinion, I'll bet you have a vacuum leak somewhere. Maybe in the intake manifold somewhere, Check the gaskets or check or cracks in manifold itself. Check cylinder base gasket too. You really need to do a crank case leak test too.

What kind of condition is the carb in? You can try and lower the needle, readjust the float, and readjust the idle air screw. You also may want to open up the carb and check the condition of the metering valve connected to the float. If the meter valve is all metal check for any groves, if it's rubber tips then you should replace it ASAP anyway.

Once you've check these things above,, get three new spark plugs,, then start the bike up, ride it around at low speed under half throttle,,, take that plug out and mark it low speed,,, then mid throttle,,,,,,, then full throttle. Now after that compare the three plus and see which are fouled.

If the low range is fouling worse that the top then most likely it's a carb, and maybe a vacuum leak still. (check to make sure the vent lines on the carb are in good shape and are lower that the carb it's self, also check the top of the carb there can't be any leaks up there either.

If the mid speed plus is fouling worst then your carb is out of adjustment, reeds maybe bad or carb it's self maybe warn out. (check all the seating surfaces in the carb. take out the idle air screw and check for wear as well as the idle screw, and the meter valve surfaces.)

If the high speed is fouling worst then your carb my still be out of adjustment, worn-out or reeds are bad, You may also have a ignition problem too, if your not getting good spark then it'll foul plugs in all ranges. check the coil and stator.

Quote:: It just goes from fine to dead that fast??

This might indicate a bad ignition, check that the coil is rust free and sand off and rust on the frame, check the mounting surfaces of the stator for a good ground