Quote Originally Posted by amerkarim View Post
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I think the point about switching off with the choke out is a valid point and is something he will try today after his flight to see if the mixture is too lean for any reason...
Amer,

My understanding is that backfiring is caused by excessive un-burned fuel in the exhaust system that ignites. I would think that “too lean” wouldn’t be the problem. Maybe excessive carbon build-up in the exhaust system. That could be the result of a too-rich mixture. I agree with the spark plug change for some reason also being a possibility.

Thanks, Jim & Dave for the feedback... I’ve been doing it (shutting off the fuel valve after shut-down) as you suggested Dave, & that is how I was analyzing it also. For shut-down I use the “2500 rpm, then retard throttle while simultaneously turning off both ignitions” technique. It seems to work very well.