Does your engine have the anti-back fire carb solenoid? If so sounds like yours is not working. Engine OFF, turn the key and listen for a small click around the carb area. Turning key on should make a click again. Or remove the solenoid and do the same. Better this way as you can watch the plunger do it's thing with the key cycling.
Idling down a mower engine is not recommended per Briggs. Briggs says run at full beans for max longevity all the time. These are not car engines.
At low revs there is insufficient cooling taking place. These are air cooled and reach full temp in 1-2 minutes.
Also clean your cylinder cooling fins yearly per your engine manual. Neglect this and you are asking for engine damage.
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