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.