Fuel from the fuel bowl is metered through the main jet (7) and is mixed with air admitted through the air intake channel (G) and well vent (8) to the lower section of the discharge jet (5). This mixture is then discharged into the air stream at the point of greatest suction in the secondary venturi (4) by the discharge jet (5). From here the fuel-air mixture passes through the main venturi where it is mixed with additional air before being discharged into the engine.