a control system for a dual fuel engine using primary diesel fuel and supplemental gas fuel such as propane or natural gas includes a control valve that controls supplemental fuel mixing with intake air by using an air deflectable member that controls the position of a valve controlling the admission of supplemental gas to the engine intake airstream . a damper is provided on the air flow responsive member to improve valve operation and a lost motion connection is provided between the air flow responsive member and the control valve element to permit independent movement of the elements . normal flow of supplemental fuel to the control valve is further modulated in response to certain engine operating conditions using an engine condition comparator that generates a control signal used by the supplemental fuel pressure regulator to further modulate the supply of supplemental fuel to the intake airstream . the system includes a supplemental fuel shut - off system that restores full primary diesel fuel flow to the engine when the supply of supplemental fuel approaches depletion .