Abstract: The invention comprises a system for introducing gaseous or vaporized fuels into a carburetor from a storage tank including a regulator connected between the tank and the carburetor for providing a supply of hydrocarbon fuel in a gaseous state at a regulated vapor pressure to a mixing chamber having air and fuel intake ports to blend said hydrocarbon fuel with intake air for injection into the carburetor. In a typical installation, the pressure change of the vacuum manifold alters the pressure within the mixing chamber to draw in fuel and air in proportion to a near constant equivalence ratio, due to the respective areas of the intake ports, over the entire operating range of the engine.