Abstract: An air-fuel mixing device adapted to be disposed between the carburetor throttle valve and the intake valve of an internal combustion engine comprising a housing, which is larger in diameter than the diameter of either the inlet or the outlet thereto, or therefrom, which inlet communicates with the air-fuel outlet passage of a carburetor and which outlet opening communicates with the intake opening of an intake manifold. Because of the size and shape of the mixing chamber, the air-fuel mixture therein is mixed by divergence and convergence of the air-fuel flow and since the chamber is substantially devoid of any baffles or protuberances, a whirlpool effect of the air-fuel flow is present because of the natural forces of gravity.