On “modified” Quadrajet carburetors that use separated main wells, the A.P.T. adjustment consists of a pin pressed in the side of the power piston which extends through a slot in the side of the piston well.  When the power piston is down (economy position), the pin stops on top of a flat surface of the adjustment screw located in a cavity next to the power piston (See Figure 18).  The adjustment screw is held from turning by a tension spring beneath the head of the screw.

During production flow test, the adjustment screws is turned up or down which, in turn, places the tapered metering rod at the exact point in the metering jet orifice to obtain the desired air/fuel mixture ratio to meet exhaust emission requirements.

         NOTICE: The A.P.T. screw is preset at the factory and no attempt should be made to change this adjustment in the field.  If a float bowl replacement is required during carburetor service, the new service float bowl assembly will be supplied with the adjustment screw preset as required.

POWER SYSTEM

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 system in the primary and secondary sides of the carburetor (Figure 19).

The fuel mixture is enriched in the two primary bores through the power system.  This consists of a vacuum operated power position and a spring(s) located in a cylinder connected by a passage to intake manifold vacuum.  The spring(s) under the power piston apply an upward force against manifold vacuum force tending to pull the piston downward.

During part throttle and cruising ranges, manifold vacuum is sufficient to hold the power piston down against spring force so that the larger diameter of the primary metering rod tip is held in the main metering jet orifice to provide leaner mixtures during these periods of engine operation.  However, as the engine load is increased to a point where extra mixture enrichment is required, the power piston spring force overcomes the vacuum pull on the power piston and the tapered tip of the primary metering rod 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 enrich 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 force which moves the larger diameter of the metering rod into the metering jet orifice returning the fuel mixture to normal economy ranges.

Dual power piston springs are used beneath the power piston in the piston bore of some 4MV-4MC Quadrajet models (Figure 19).  A smaller diameter power piston spring seats in the center of the piston and bottoms on the float bowl casting.  The spring is used to control power enrichment during light engine loads.  A larger diameter spring surrounds the smaller inner spring and exerts additional pressure on the bottom of the power piston to provide efficient mixture ratios at heavier engine load conditions.  The dual power piston spring feature, on models so equipped, assists in providing improved fuel control of air/fuel mixture ratios to meet emission and power requirements of the engine.

POWER SYSTEM – MECHANICAL OPERATION

Some Quadrajets have a mechanical power enrichment system in addition to the vacuum enrichment feature.  This provides accurate control of fuel mixtures at high engine speeds and load and yet allows the use of vacuum enrichment for improved fuel control during medium engine loads.

The mechanical enrichment is controlled by a stem pressed into the base of the power piston which extends into the throttle body.  The stem is operated by a lever which is hinged to the throttle body casting and a cam on the center of the throttle shaft.  When the throttle valves are opened to a pre-determined point, the cam on the throttle shaft forces the lever upward until it contacts the stem on the power piston and pushes the complete piston assembly upward against engine vacuum (Figure 20).  This, in turn, lifts the metering rods, placing the smaller diameter of the metering rods in the main metering jets for positive mixture enrichment at greater throttle valve openings.

The power piston has a “trapped” spring which limits the travel of the piston during vacuum operation.  The spring is retained between the piston and a fluted washer on the power piston stem.  The washer is retained with a “C” clip located in a groove on the power piston stem.

During high engine vacuum, the power piston spring is compressed and the fluted washer and piston are seated at the bottom of the power piston cavity.  As engine load increases, vacuum drops and the power piston moves upward against spring tension until the “C” clip seats against the fluted washer.  No more enrichment will take place until the pin in the power piston is contacted by the mechanical enrichment lever.  As the throttle valves are opened further, the complete power piston assembly is forced upward placing the smaller diameter of the metering rods in the jets for maximum enrichment at higher engine speeds and loads.

When engine load and speed is decreased, the power piston will return to the down position, seating the fluted washer and piston in the bottom of the power piston cavity as high engine vacuum compresses the power piston spring.  This returns the metering system to leaner fuel mixtures for light engine loads.