The power system in the Quadrajet carburetor provides extra mixture enrichment to meet power requirements under heavy engine loads and high speed operation. The richer mixtures are supplied through the main metering systems in the primary and secondary sides of the carburetor.

The fuel mixture is enrichened in the two primary bores through the power system. This consists of a vacuum operated power piston and a spring located in a cylinder contacted by a passage to intake manifold vacuum. The spring under the power piston pushes the piston upward against manifold vacuum.

During part throttle and cruising ranges, manifold vacuums are sufficient to hold the power piston down against spring tension to that the larger diameter of the metering rod tip is held in the main metering jet orifice. However, as engine load is increased to a point where extra mixture exrichment is required, the power piston spring overcomes the vacuum pull on the power piston and the tapered tip of the metering rods moves upward in the main metering jet orifice.

The smaller diameter of the metering rod tip allows more fuel to pass through the main metering jet and enrichen the fuel mixture to meet the added power requirements. As engine load decreases, the manifold vacuum rises and extra mixture enrichment is no longer needed. The higher vacuum pulls downward on the power piston against spring tension, which moves the larger diameter of the metering rod into the metering jet orifice returning the fuel mixture to normal economy ranges.

The primary side of the carburetor provides adequate air and fuel for low speed operation. However, at high speed more air and fuel are needed to meet engine demands. The secondary side of the carburetor is used to provide extra air and fuel through the secondary throttle bores.

The secondary section of the Quadrajet has a separate and independent metering system. It consists of two large throttle valves connected by a shaft and linkage to the primary throttle shaft. Fuel metering is controlled by spring loaded air valves, metering orifice plates, secondary metering rods, main fuel wells with air bleed tubes, fuel discharge nozzles, accelerating wells and tubes. The secondary metering system supplements fuel flow from the primary side and operates as follows:

SECONDARY METERING SYSTEM

When the engine reaches a point where the primary bores cannot meet engine air and fuel demands, a lever on the primary throttle shaft through a connecting link to the seconday throttle shaft, begins to open the secondary throttle valves. As the secondary valves are opened, engine manifold vacuum (low pressure) is applied directly beneath the air valves. Atmospheric pressure on top of the air valves force the air valves open against sprng tension and allows metered air to pass through the secondary bores of the carburetor.

On most models accelerating wells are used to supply fuel immediately to the secondary bores. This prvents a momentary leannes until fuel begins to feed from the secondary dishcharge nozzles.

When the air valves begin to open, the upper edge of each valve passes the accelerating well ports (one for each bore). As the edge of the air valves pass the ports, they are exposed to manifold vacuum and immediately feed fuel from the accelerating wells located on each side of the float bowl chamber. The acceleration ports will feed until the fuel in the accelerating well is depleted. Each accelerating well has a calibrated orifice when meters the fuel supplied to the well from the float chamber. Some models have the accelerating well ports located beneath the front edge of the air valve instead of above. These begin to feed fuel to the secondary bores almost instantly after the secondary throttle valves open and before the air valves begin to open. This type porting is used on some models where added enrichment is needed during cold operation when the air valve is locked closed and also provides an earlier cut in of fuel from the ports than the models which have the port located above the valves. The use of either type of porting is dependent upon engine fuel demands.

The secondary main discharge nozzles (one for each bore) are located just below the center of the air valves, above the secondary throttle valves. The nozzles being located in a low pressure area feed fuel as follows:

As the secondary throttle valves are opened, atmospheric pressure opens the air valves. This rotates a plastic cam attached to the center of the air valve shaft. As the cam rotates it lifts the secondary metering rods out of the secondary orifice plates through the metering rod lever which follows rotation of the eccentric cam.

Fuel flows from the float chamber through the secondary metering orifice plates into the secondary main well tubes. The air emulsified fuel mixture travels from the main wells through the secondary discharge nozzles where it sprays into the secondary bores. Here the fuel is mixed with air traveling through the secondary bores to supplement the air/fuel mixture delivered from the primary bores and goes on into the engine as a combustible mixture.

As the throttle valves are opened further and engine speeds increase, air flow through the secondary side increases and opens the air valve to a greater degree which in turn lifts the secondary metering rods further out of the orifice plates. The metering rods are tapered so that fuel flow through the secondary metering orifice plates is directly proportional to air flow through the secondary carburetor bores. In this manner, correct air-fuel mixtures through the secondary bores are controlled by the depth of the metering rods in the orifice plates.

The depth of the metering rods in the orifice plates in relation to air valve position are factory adjusted to meet air-fuel requirements for each specific engine model. A service setting is released should field adjustment become necessary due to parts replacement or malfunction in the field.

There are other features incorporated in the secondary metering system which are as follows:

1. The secondary main well air bleed tubes extend downward into the main fuel well below normal fuel level. These bleed air into the fuel in the secondary wells to quickly emulsify the fuel with air for good atomization and improve fuel flow from the secondary nozzles.
2. The secondary metering rods have a milled slot at the larger diameter of the metering tip. The purpose of the slots is to ensure an adequate supply of fuel in the secondary main wells when the air valves are in the closed position. At this point, the metering rods are nearly seated against the metering orifice plates. The slot in the rod is adjacent to the orifice plate and allows a small amount of fuel to pass between the metering rod and metering disc. During extreme hot engine idle or hot soak the fuel could boil out of the secondary fuel wells. The milled slots allows enough fuel to by-pass the orifice plate and keep the main fuel wells fuel of fuel. This insures adequate supply in the main wells at all times to give immediate fuel delivery from the secondary dicharge nozzles.
3. There are two baffle plates in each secondary bore located just below the air valves. They extend up and around the secondary discharge nozzles. Their purpose is to provide good fuel distribution at lower air flows by providing equal fuel distribution to all engine cylinders.
4. An air horn baffle is used on some models to prevent incoming air from the air cleaner reacting on the secondary main well bleed tubes.

The baffle is located adjacent to the secondary well bleed tubes and extends above the air horn between the primary and secondary bores.

This prevents incoming air from forcing fuel level down in the secondary wells through the bleed tubes and prevents secondary nozzle lag on heavy acceleration.