Abstract: In addition to the gasoline-air mixing primary carburetor on an internal combustion engine, an auxiliary carburetor is provided for mixing air and alcohol, to thereby provide a substantial portion of the fuel-air mixture used by the engine during idle conditions and light-load conditions, while the primary carburetor provides an increasingly greater portion of the total fuel-air mixture during high-load conditions. The alcohol fuel-air mixture flows directly to the intake manifold, thus by-passing the throttle valve which controls the fuel-air mixture from the primary carburetor. The auxiliary carburetor is vacuum controlled.