Today when my brother went to start his 01 YZ426, it had absolutely no compression (he still has the original exhaust cam in it as well). The bike ran fine when he parked it. It uses no oil and starts first kick every time. The bike has been running VERY rich for the entire time he has owned it, approx 1 year. We could hear the pressure from the cylinder excaping through the carb, so we figured the valves seats might be worn out, but couldn't understand why it would be running so strong one day and then just go out without any warning or symptoms. We took the head and carb off and did the carb cleaner in the intake port test and found that 2 of them were leaking. What we also found was an ENORMOUS amount of carbon build up all over EVERYTHING. We cleaned up the carbon and opened each valve and cleaned the carbon build up off of the seats and tried the test again. This time no leaking. I could not believe what was happening here so we put the head back on with the old head gasket (just to test, I am waiting for my new one to get here for final assembly), and kicked it over, the compression was just like it had been the day before when it was running perfectly. we checked the valve clearances and the .005 gauge fit perfectly (the manual said the service limit was .0039). Anyone heard of anything like this before?