We're trying to figure out why the bike won't idle and every time you let off the throttle all the way the bike just shuts of. The bike also tends to backfire, not all the time but we were thinking that maybe that has something to do with it. perhabs messing with the air/ fuel mixture? The bike also used to have a Dr.D exhaust on it and was jetted for that but it was broken in a crash, so maybe the jetting has something to do with it because it wasn't rejected after the stock exhaust was put back on?? If anyone has any ideas as to what it might be please help. Also, is the black knob on the left side of the carb the air/fuel mixture or is it the idle screw?