I have a 1993 Bayou 300 4x4. I can't keep the plug free of carbon. After only a short time of riding it, it starts to not want to speed up, it backfires, runs real bad. I change or clean the plug, then it will run fine for a short while. I put a new carb but nothing changed. I adjusted the fuel/air screw, still the same. Could it be that the valves need adjusting? I did notice a hole in the airbox. would this cause any problems? All help would be appreciated.