My banshee just started sputtering and no power at all when winding it up. It drives normal and sounds normal when just driving around, but it really sputters and runs like **** when you get into high rpms. This doesn't happen when ideling and revving it though. I replaced the spark plugs, and tried messing with the air screws but i still can't get it. I turn them a 1/4 of a turn each time and nothing. What else could it be?