The owner still had the sack from the auto parts store with the sales slip. There was an engine compartment explosion and ensuing fire. A little girl, who was asleep on cushions under the fore deck, never made it out. They didn't get a dime. Backfire flame arrestors serve a very important safety function.
All gasoline engines with carburetors tend to vent fuel vapor after shutdown.
If shut down from high speed, they are hot and will boil off some of the fuel in the float bowl. This vapor goes into the engine compartment and will linger for a while unless taken out by the ventilation blower. The vapor itself does not harm, as long as there is no source of ignition present. Sources of ignition can be electrical or from a backfire in the engine intake system when it is restarted. Most of us have heard a backfire.