Abstract: A combustible mixture of air and minute fuel droplets is produced for supply to the cylinders of an internal combustion engine. The engine includes a carburetor having a main air passageway in it, and an intake manifold interposed between the engine and the carburetor. The intake manifold includes at least one wall defining a plurality of fluid passageways operatively connected to the carburetor and to the combustion chambers of the engine. A throttle structure is mounted in the intake manifold. The combustible mixture is formed by introducing liquid fuel into a stream of intake air and uniformly distributing the fuel in the air by passing the air and fuel mixture through a constriction to increase the velocity of the mixture to sonic speed. The constriction occurs at the throttle structure in the manifold. The area of constriction and the amount of the fuel entering the air stream are adjustable for correlation with engine demand.