only when u shut it off when the motor stops then it back fires thanks aceOnly back fires when you shut it off or when also you let off the throttle on a hard acceleration? Check the obvious things like maybe a hairline crack in the header, valve lash, new fuel filter. Maybe RuttingRhino will chime in. He's really good at determining if it a fuel problem.
u my be right because it does sound like a poofThis doesn't really help you, but my wife's 450 does something similar when shut off.....not really a backfire, but more of a "poof" sound. Ours will backfire on a hard decel, but I know these machines are jetted lean from the factory. I bet going one step up in size on the pilot jet would resolve alot of this.