My '03 YZ125 has been running fine. Yesterday after riding for about an hour and stopping for about 10 min, it wouldn't start up. Double checked for fuel flow, ok, checked spark, ok; plug not loaded up, clean. Several hours later at home, bike started right up. I killed the engine. Would not start. Bike is stock except for an FMF Short silencer. Original top end. I'm doing a top end shortly, but need to find out what is causing this start/running problem before doing the top end. Any suggestions on diagnosing the problem?