At a point of sufficient throttle opening manifold vacuum or suction is applied to the nozzles, which then take over the fuel delivery to the engine. This point is termed the transfer point as the fuel delivery is transferred from the idle system to the main metering system. To a large degree the calibrated main metering jets govern the fuel mixtures throughout this range of 30-60mph, assisted by the air bleeds in the main well tubes admitting varying amounts of air from within the outer bowl to mix with fuel passing from the nozzles according to engine demans. As shown in the image, the throttle valves “F”, are opened sufficiently so that the manifold suction is now greater upon the nozzles “G” than at the idle discharge holes “E” in the throttle body; as a consequence, fuel continues to pass through the calibrated main metering jets “A” to the main wells. However, the suction being now greater upon the nozzles, the fuel passes upward along the main well tubes “H” rather than through the idle tubes and out the discharge nozzles “G”.

The main well tubes “H” have four calibrated holes which admit air from the main well bleed “C” in the cover. As the throttle valves are opened to a greater degree the suction is increased upon the nozzles causing them to deliver more fuel to meet the engine demand. This increase in fuel delivery lowers the fuel level in the main wells; consequently, the holes in the main well tubes are exposed in turn by the lowering of the fuel level to admit more air to joing with the fuel being delivered by the nozles. Thus, although the nozzles suction is increased by throttle opening, the fuel/air mixture to the engine remains constant throughout the part throttle range.