HAVE YOU TRIED JUST REPLACING THE SPARK PLUG, I HAVE AN 05 YZ125 AND MY BUDDY HAS A 02 YZ 250 AND APPARENTLY THEY ARE ALL KNOW TO FOUL UP THE SPARK PLUGS, IF YOU HAVE ALREADY TRIED THAT THEN IT MUST BE SOMETHING WITH THE CARBURATOUR, DID YOU GET THE OWNERS MANUAL WITH YOUR BIKE, IF NOT IT WOULD TELL YOU EXACTLY HOW TO DO IT.