Has anyone else noticed what looks like cabon backfiring into the air filter. This engine for me has always left black residue on the inside of the air filter. Right now I am running a P4 plug about sea-level and I haven't raced a day that was over 85 degrees yet. Wondering if possibly I am running to hot a plug or something messed up with my tune? I have not seen this on any of my other motors (i used to run nova and OS). This motor preforms great just wondering if possibly I am doing something wrong.