in largewr engines it is but i have never had a model engine who's head gasket was not that think plastic unless it is copper that appears and feels that way. the head gasket missing will not account for the idling with the carburator closed. in order to run you have to get air so if the carburator barrel is closed and it is still getting air, that it has to be behind the carburator. that's the carburator's seating. it neds a new o-ring.