The main metering system (Figure 13) supplies fuel to the engine from off-idle to wide-open throttle.  The primary bores (two smaller bores) supply fuel and air during this range through plain tube nozzles and the venturi principle.

The multiple venturi in each primary bore produce excellent fuel metering control due to their sensitivity to air flow.

The main metering system begins to operate as air flow increases through the venturi system.  It supplies additional fuel to each bore to maintain the required air/fuel mixture of the engine.  Fuel from the idle system gradually diminishes as the lower pressures are now in the venturi system.

The main metering system consists of main metering jets, vacuum operated power piston and hanger assembly, primary metering rods, main fuel wells, main well bleeds, main discharge nozzles, triple venturi, fuel pull-over enrichment (some applications), and adjustable part throttle (A.P.T.) mixture control (on some applications).

The main metering system operates as follows:

As the primary throttle valves are opened beyond the off-idle discharge ports allowing more air to enter the intake manifold and engine speed to increase, air velocity increases in the carburetor venturi.  This creates a drop in pressure in the large venturi and a still greater drop in the boost venturi.  With atmospheric pressure being greater on the fuel in the fuel chamber, fuel is forced to flow out of the main discharge nozzles and into the main air stream.

Fuel from the fuel chamber flows through the primary metering jets into the main fuel wells.  It passes upward in the main wells where air is introduced from the air bleeds, or air bleed tubes, located at the top of the wells.  The air/fuel mixture then passes from the main wells through the main discharge nozzles into the boost venturi.  At the boost venturi, the air/fuel mixture then combines with the air entering the engine through the carburetor bores creating a combustible mixture that passes through the intake manifold and on into the engine cylinders.

The main metering system is calibrated by tapered and stepped metering rods operating in the main metering jets and by variations in the power piston spring(s) and main well air bleeds.

During cruising speeds and light engine loads, manifold vacuum is high.  In this mode of operation, the engine will run on leaner mixtures than required during heavy loads. The primary main metering rods are connected to a vacuum responsive piston which operates against calibrated spring force. Vacuum is supplied to the power piston through a vacuum channel.  When the vacuum is high, the piston is held downward against the spring load and the larger metering diameters of the metering rods are positioned in the metering jets.  This results in leaner air/fuel mixtures for economy operation.  As engine load increases and manifold vacuum drops, the spring force acting on the power piston overcomes the vacuum pull and gradually lifts the metering rods to position the smaller diameter of the rods in the metering jets.  This enriches the fuel mixture enough to give the desired power required to overcome the added load.