The main fuel metering sytsem is used for most of the part-throttle operation, and it is the primary source of the full-throttle operation. The fuel flow in this system is regulated by manifold vacuum acting on a diaphragm that controls the movement of the metering rod in the mtering jet. As shown in view A, high engine vacuum raises the metering rod and overcomes downward spring force, lifting the metering rod, and producing restriction in the main metering jet. With the metering rod in this position, the flow of fuel through the main metering jet is reduced. With a decrease in vacuum, see View B, spring force will move the metering rod down. Thus, the small diameter of the metering rod will expose a larger portion of the main metering jet orifice, allowing an increased flow of fuel through the main metering jet.

As you will notice in View C, fuel is pulled up through a main well bleed tube that has calibrated holes in the sides. As the fuel passes through the tube, metered air from the high-speed bleed (located in the float chamber) passes through the side holes and mixes with the fuel. Then the aerated fuel flows on to the main discharge nozzle. Additional air from the main air bleed is mixed with the aerated fuel before it is finally emitted from the tip of the nozzle into the booster venturi.