i recently rode my dirt bike for the first time after the winter and had no problems. i accidently put bad gas in it and it started sputtering and stalling so i soon realized the problem and drained the gas and then the plug fouled. i replaced the spark plug and it ran fine. i bought an aftermarket exhaust system and the power now system. so today i changed the motor oil cause it needed it and put the after market mods on. if you're not familiar with the power now system, its a carburator mod that streamlines the air for increased air flow through the carb. so putting those two mods on in one day i was somewhat expecting to rejet the carb to make it richer, but after putting them on i only adjusted the "idle nob" i think its called, its under the fuel on/off switch, anyway the bike seems to run fine to me but when the bike backfires it has a tiny burst of flame out the tailpipe. also when i shut the bike down and only when i do that the tailpipe smokes. does anyone think i need to adust the carb needle or is the idle nob good enough.