2005 RM85L running bad
 

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.