i had the same problem with my 2000 300exc.the previous owner had no idea how to setup a dirtbike i had to reset everything. i buy bikes from guys like him because most times they havent been raced i run a good synthetic around 40to1 cleanout the carb leanout the setting donĀ“t run to lean try a few settings.i run a little rich, the engine will last longer.use the right plug check the filter and so on.