Abstract: A carburetor assembly includes a base plate (12) having an air bypass passage (30) extending there through, with air channels (71-74) formed in the surface of the base plate intersecting the air bypass passage and each of the butterfly bores (25-28) of the base plate. An adjustable air bypass valve (31) is mounted on the base plate and controls the movement of air through the bypass passage. A valve needle (50) is adjustably mounted to the valve body (48) for adjustably constricting the flow of fluid through the radially extending air inlet ports (60) and through the base plate air bypass passage, to allow for the adjustment of the volume of the air delivered to the engine at idle speed, independently of the movements of the butterfly valves.