My son's2005 RM85Lstarted running poorly without warning last time it was ridden. It misses like it's an electrical problem in the entire RPM range except at idle and wide open. I have replaced the CDI,coil and spark plug. The stator checks out OK. I have also cleaned the carb but no change. Is itpossible that a dirtyexhaust valve can cause this? I know the ownersmanual says to cleanthe valveevery 6 hours of run time, it has a whole lot more time on it than that. Thanks.