Abstract: A gaseous fuel delivery system for a gasoline engine having means for gaseous fuel delivery comprising idle fuel delivery means and acceleration fuel delivery means. Electrically operable means responsive to ported vacuum in the carburetor throat controls operation of the idle fuel delivery means. Electrically operable means responsive to intake manifold vacuum controls operation of the acceleration fuel delivery means. An idle switch responsive to idle position of the throttle prevents opening of the idle fuel delivery means except when the throttle is in the idle position.The engine carburetor is arranged such that no gasoline is delivered for idle operation. Idle needle valves are closed, or eliminated. Transition slots are sized to be operational only as the throttle moves from the idle position and supply no fuel when the throttle is at idle.