The main metering system (Figure 9) 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 venturis 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 and additional fuel is needed to supply the correct air/fuel mixture to 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 primary metering rods, main fuel wells, main well air bleeds, main discharge nozzles, triple venturi, fuel pull-over enrichment (some applications), adjustable part throttle (some applications).
As the primary throttle valves are opened beyond the off-idle range allowing more air to enter the engine intake manifold, air velocity increases in the carburetor venturi. This causes a drop in pressure in the large venturi which increases many times in the boost venturi. Since the low pressure (vacuum) is now in the smallest boost venturi, fuel flows from the main discharge nozzle as follows:
Fuel from the float bowl flows through the main metering jets into the main fuel wells. It passes upward in the main well and is bled with air by an air bleed located at the top of well. The fuel is further bled air through calibrated air bleeds located near the top of the well in the carburetor bores. The fuel mixture then passes from the main well through the main discharge nozzles into the boost venturi. At the boost venturi, the fuel mixture then combines with the air entering the engine through the carburetor bores. It then passes as a combustible mixture 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 jet and also through the main well air bleeds.
During cruising speeds and light engine loads, manifold vacuum is high. In this period, 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 spring tension. Engine manifold vacuum is supplied to a power piston through a vacuum channel. When the vacuum is high, the piston is held downward against spring tension and the larger diameter of the metering rod is in the main metering jet orifice. This results in leaner fuel mixtures for economy operation. As engine load increases and engine manifold vacuum drops, spring pressure acting on the power piston overcomes the vacuum pull and gradually lifts the metering rods partially out of the main metering jets. This enriches the fuel mixture enough to give the desired power required to overcome the added load.
There is a primary metering rod adjustment on a Rochester Quadrajet carburetor — but it’s a bit specific.
On the Quadrajet, the primary metering rods are controlled by a power piston that moves up and down based on engine vacuum. Under normal operation, there's not a lot of "adjustment" you do directly to the rods themselves — they’re sized (tapered) for a certain fuel curve. What you can adjust is:
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The tension of the power piston spring (indirectly by changing it).
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The height of the rods at idle (but this usually requires bending the small linkage that connects the rods to the piston — not typically recommended unless you really know what you're doing).
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Changing the metering rods and jets to different sizes to tune for richer or leaner fuel mixtures.
In short:
There’s no traditional "set screw" for adjusting the primary rods. Instead, adjustment involves selecting different rods and/or jets, or modifying the power piston spring tension (heavier spring = richer mixture sooner, lighter spring = leaner).
