In some “modified� Quadrajet carburetors, a multiple stage power enrichment system, consisting of two power pistons (Figure 21), is used for more sensitive control of air/fuel ratios during light duty engine power requirements while providing for richer mixtures during moderate to heavy engine loads.

AUXILIARY POWER PISTON

An auxiliary power piston and single metering rod assembly, located in front of the man (primary) power piston, is used for light duty power requirements. On light throttle opening when manifold vacuum drops to a predetermined point, the spring force under the auxiliary piston overcomes the vacuum pull and raises the piston which lifts the single metering rod out of a fixed metering jet. This provides partial fuel enrichment for light duty engine loads.

MAIN (PRIMARY) POWER PISTON

During moderate to heavy engine loads when a further drop in manifold vacuum occurs with increased throttle opening, the main (rear) piston spring force overcomes the vacuum pull and raises the piston which lifts the two metering rods out of the metering jets for additional fuel enrichment for heavy duty power requirements.

The multiple stage (two piston) power enrichment system is specifically calibrated for the power requirements of each engine by controlling spring rates of each piston. The system requires no adjustment in the field; however, the main (rear) power piston and metering rod assemblies and the auxiliary (front) power piston and metering rod assembly are removable for normal cleaning and service replacement as needed.

         NOTICE: The main (rear) and auxiliary (front) power piston springs must NOT be interchanged.  To prevent mixing of power piston springs at time of carburetor disassembly, lightly wrap a piece of masking tape around the auxiliary power piston spring for identification.  Then, on reassembly, remove the tape and install the spring in the front location beneath the auxiliary power piston with single metering rod.

POWER SYSTEM – TURBOCHARGER APPLICATIONS

Some modified Quadrajet models are designed for Turbocharger applications.  The power system in these models operates in the same manner as previously described except for one important difference.  The vacuum supply, directed to the underside of the power piston, is controlled externally by a Turbocharger Vacuum Bleed Valve.  The vacuum signal is routed to the carburetor through a hose from the bleed valve to an external tube located on the side of the carburetor.

The power system operates as follows:

During part throttle and cruising ranges, engine load is light and vacuum, from the Turbocharger Vacuum Bleed Valve, is sufficient to hold the power piston down against spring force and the larger diameter of the metering rod tip is held in the metering jet for leaner mixtures.

As engine load is increased to a point where extra fuel enrichment is required and the intake manifold is pressurized by the exhaust gas driven Turbocharger, the vacuum bleed valve “switchesâ€� and reduces vacuum to the power piston to zero.  At this point, spring force operating on the power piston lifts the main metering rods for increased fuel delivery.

The remote power enrichment feature, through the vacuum bleed valve, provides richer mixtures during heavy engine loads and wide-open throttle power requirements when the intake manifold is pressurized by the Turbocharger at a time when manifold vacuum is high enough tending to operate the power piston in the normally “leanâ€� position.  In this way, the power system controls fuel metering during light and heavy power requirements.

SECONDARY SYSTEM

The primary stage of the Quadrajet carburetor provides adequate air and fuel for low speed operation.  However, at higher speed, more air and fuel are needed to meet engine demands.  The secondary stage of the carburetor provides the additional air and fuel through the secondary throttle bores for power and performance requirements.

The secondary stage has a separate and independent metering system (Figure 22).  It consists of two large throttle valves connected by a shaft and linkage to the primary throttle shaft.  Fuel metering is controlled by a spring-loaded air valve, secondary metering orifice plates, secondary metering rods, fuel wells with bleed tubes, fuel discharge nozzles, accelerating wells and tubes.  These are used to modify fuel flow characteristics for exact air/fuel calibration.

The secondary metering system supplements fuel flow from the primary stage and operates as follows:

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 pick-up lever on the secondary throttle shaft, begins to open the secondary throttle valves.  This occurs only if the choke has warmed the thermostatic coil sufficiently to release the secondary throttle valve lockout lever (if used).

As the secondary throttle valves open, engine manifold vacuum (low pressure) is applied directly beneath the air valves.  Atmospheric pressure on the top of the air valves forces the air valves open against spring and air valve dashpot forces, provided the choke coil has warmed sufficiently to release the air valve lockout lever, if used.  This allows 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 prevents a momentary leanness until fuel begins to feed from the secondary discharge 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 edges 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 fuel chamber.  Each accelerating well has a calibrated orifice which meters the fuel supplied to the well from the fuel 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 of 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 enter of the air valves and above the secondary throttle valves. The nozzles, being located in a low pressure area, feed fuel 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 improved fuel flow from the secondary nozzles.
2. The secondary metering rods may 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 slot allows enough fuel to bypass the orifice plate and keep the main fuel wells full of fuel.  This ensures adequate fuel supply in the main wells at all times to give immediate fuel delivery from the secondary discharge nozzles.
3. Some applications use secondary discharge nozzles that incorporate a vertically drilled cross hole located about half way down the length of the nozzle.  The hole serves as a distribution passing through the secondary discharge nozzle.
4. A baffle plate, extending into each secondary bore, is located just below the air valves on all models.  The baffle extends up and around the secondary discharge nozzles to provide equal fuel distribution, as near as possible, to all engine cylinders at lower air flows.
5. On some models, an integral baffle is added to the bottom side of the secondary air valve.  The baffle improves mixture distribution from the secondary side at higher air flows.
6. 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 the fuel level down in the secondary wells through the bleed tubes and prevents secondary nozzle lag on heavy acceleration.
7. Some models use notched secondary air valves to reduce the vacuum signal at the nozzles for leaner air/fuel mixture ratios during initial air valve opening.  The leaner mixtures assist in meeting emission requirements and also improve throttle response when operating at high altitudes.