hi, I have a sears 10hp snowblower and it use to backfirers when I would shot it off quickly with the throtle handle. my engine was running too riche, so I adjusted the main jet screw on the crarburator, now there's no backfirer (flame) but you can hear a small <pow> when the engine it suddenlt shot off.

If I read you correctly,what you're doing is quickly idling it down and shutting it off at the same time. I think you'll find it will stop doing that if you idle it down,let it run at low rpm for 10 seconds or so,THEN shut it off. What you're doing is shutting it off with too much fuel in the combustion chamber then it can handle,causing post detonation. Try it!!