Backfiring thru muffler might mean your running rich and/or need a valve job especially on the exhaust valve.
Running it hard for a period of time gets the muffler really good and hot.
Along with too much unburnt fuel and kaboom.
Mine does it almost everytime I shutdown after mowing the yard.
Not even at full throttle, maybe 3/4ths to 7/8ths.
I hope yours is not backfiring out the carb.
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