Stop in for a cup of coffee

zkx14 said
OK, so I determined that it is actually in the bowl vent passage. Apparently it opens when mower is shut off.

Anti-run on. Some engines would backfire when hot engines are shut off. Perhaps adding air to idle circuit prevents that.
Many small engines use a fuel solenoid,i guess to prevent leaks after engine is off. But they fail.
Side cutter fixes that.