CRF250R 2006. New piston rings cylinder. The head was checked by race shop. Valves are shimmed correctly. New CDI, new spark plug and new coil. After several kicks when it starts it runs strong for about 3 or 4 minutes after that it starts skipping and exhaust backfire. The only thing I haven’t changed is the stator and trigger coil. The carb has also been cleaned. I’m lost now for what the problem could be.
