In my younger day if you did not idle down and cool off before shutting off the switch it would backfire and break the connecting rod. Carbon builds up on the head and glows red from heat, when you got up the safety switch for the seat shut it off . The red hot carbon on the head lit the fuel with the piston in who knows what position. If you can remove the spark plug and put your thumb over the spark plug hole and crank it over, should be compression and blow some air around your thumb. You might of got luck and blowed out the head gasket, check that out and come back.
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In my younger day if you did not idle down and cool off before shutting off the switch it would backfire and break the connecting rod. Carbon builds up on the head and glows red from heat, when you got up the safety switch for the seat shut it off . The red hot carbon on the head lit the fuel with the piston in who knows what position. If you can remove the spark plug and put your thumb over the spark plug hole and crank it over, should be compression and blow some air around your thumb. You might of got luck and blowed out the head gasket, check that out and come back.