Abstract: A variable venturi down-draft carburetor comprises a one-piece unitary casting that defines an induction passage and upwardly open cavities constituting a float chamber and a recess in which a movable venturi member is housed. A throttle valve, main fuel jet, and venturi member are all housed wholly within the confines of the casting, and the casting is covered by a single plate that closes the upwardly open cavities and defines an air inlet orifice to the induction passage. By housing the throttle valve, main jet and venturi in one unit, operation of the carburetor is unaffected by variations in the cover plate produced by manufacturing tolerances. Access to the interior of the carburetor is achieved simply by removal of the plate, which does not, in itself, affect the adjustment of the carburetor. The casting preferably includes integral mountings for a vacuum motor, and/or acceleration pump, and/or an automatic choke.

Abstract: A downdraft type carburetor having a conventional idle speed air/fuel mixture channel has an air/fuel mixing chamber that mixes the idle channel air/fuel mixture with idle bypass air from the main induction passage, the mixing chamber being connected to discharge into the induction passage below the throttle valve through a nozzle containing an orifice sized with respect to a larger orifice in the air bypass passage to provide a pressure differential creating sonic flow, the nozzle extending so that the sonic flow is followed by a shock wave, the turbulence created atomizing the fuel.

Abstract: A variable area venturi carburetor has a movable venturi wall with a fuel metering rod attached for cooperation with aligned main and auxiliary fuel jets located in an opening in the fixed venturi wall, a bypass passage connected between the jets siphoning or bleeding off fuel at closed throttle positions into a passage connected to the induction passage just below the closed position of the throttle valve, to prevent stalling of the engine by fuel flow into the main venturi at this time.