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The secondary air valves use an air valve dashpot feature to control opening rate of the air valve. This prevents an uncontrolled air valve opening rate which results in an instantaneous air rate change and a “lagging” fuel rate change as the secondary throttle valves are opened. The dashpot, through linkage, controls opening of the air valves to provide a smooth transition to secondary system operation.
There are two different types of air valve dashpots used in the Quadrajet carburetor:
The early type dashpot (Top of picture, Figure 23) consists of a piston which operates in a fuel well adjacent to the float bowl. The piston stem is connected to the air valve through a link and lever assembly. As the air valves open, the dashpot piston is pulled upward forcing fuel to flow between the side of the piston and fuel well which retards the air valve opening. A rubber washer attached to the piston stem acts as a check valve. During upward movement of the piston, the rubber washer seats and forces all fuel to flow around the piston. When the air valve closes, the check valve unseats and allows fuel to also pass through the center of the piston allowing the air valves to return closed rapidly.
The late type air valve dashpot (Lower picture, Figure 23) operates off of the front choke vacuum break diaphragm unit. The secondary air valve is connected to the choke vacuum break unit by a rod, to control the opening rate of the air valve. Whenever manifold vacuum is sufficiently high, the vacuum diaphragms seated, plunger retracted, against spring load. At this point, the vacuum diaphragm link is in the forward end of the slot in the air valve lever, or in the rear of the slot in the vacuum diaphragms plunger, and the air valves are closed.
During acceleration or heavy engine loads when the secondary throttle valves are opened, the manifold vacuum drops. The spring located in the vacuum diaphragm unit overcomes the vacuum force and moves the plunger stem outward. This action allows the air valves to open. The opening rate of the air valves is controlled by the calibrated restriction in the vacuum inlet of the diaphragm cover and the valve closing spring. The dashpot action, due to this restricted vacuum flow and spring force, provides the required delay in air valve opening needed until sufficient fuel flows from the secondary discharge nozzles.
ACCELERATING PUMP SYSTEM
During quick accelerations when the throttle is opened rapidly, air flow through the carburetor bores and intake manifold vacuum change almost instantaneously. However, the fuel which is heavier tends to lag behind causing a momentary leanness. To prevent this, the accelerator pump system is used to provide the extra fuel necessary for a smooth transition in engine operation during this period.
The accelerating pump system is located in the primary stage of the carburetor. It consists of a spring-loaded pump plunger and pump return spring (operating in fuel well), fuel passage, discharge check ball, retainer, and pump jets, one in each bore (Figure 24).
On most late Quadrajet models, an expander (garter) spring is used in the pump cup for constant pump cup to pump wall contact. The pump cup is of the floating design; i.e., the up and down movement of the cup on the plunger head either “seats” to provide a solid charge of fuel on the down-stroke, or “unseats” on the filling of the pump well (up-stroke). The cup remains unseated when there is no pump plunger movement which allows vapor to vent from the pump well.
The pump plunger is operated by a pump lever on the air horn which is connected directly to the throttle lever by a rod.
When the pump plunger moves upward in the pump well as happens during throttle closing, fuel from the fuel chamber enters the ump well through a vertical slot located near the top of the pump well. It flows past the “unseated” pump cup to fill the bottom of the pump well and pump discharge passage.
When the primary throttle valves are opened, the pump rod and lever forces the pump plunger downward. The pump cup seats instantly and fuel is forced through the pump discharge passage where it unseats the pump discharge check ball and passes on through the passage to the pump jets where it sprays into the venturi area of each primary bore.
It should be noted the pump plunger is spring loaded. The upper duration spring is balanced with the bottom pump return spring so that a smooth, sustained charge of fuel is delivered during acceleration. Selection of the duration spring by the factory is used to control the differences in rate of movement between the pump linkage and the plunger head for correct pump fuel delivery.
The pump discharge check ball seats in the pump discharge passage during upward motion of the pump plunger so that air will not be drawn into the passage and prevent proper pump fill.
During higher air flows through the primary bores, a vacuum exists at the pump jets. A passage, located just behind the pump jets, leads to the top of the air horn to vent the pump fuel circuit outside the carburetor bores. This acts as a suction breaker so that when the pump is not in operation; fuel will not be pulled out of the pump jets into the vernturi area. This insures a full pump stream when needed and prevents any fuel “pull-over” from the pump discharge passage.
In order to keep evaporative emission levels to a minimum, later model M4ME-M4MC models have a pump plunger stem seal and retainer located in the air horn. In the event of service repair or cleaning of the carburetor, a new seal and retainer should be installed (See Figure 25).
CHOKE SYSTEM
The Quadrajet choke valve is mounted in the air horn located above the carburetor primary venturi. A closed choke valve provides the correct air/fuel mixture enrichment to the engine for good cold engine starting and when partially open smooth running during the warm-up period. The secondary throttle valves, or air valve on some models, are locked closed until the engine is thoroughly warm and the choke valve is wide open.
The Model 4MV choke system (Figure 26) consists of a choke valve, vacuum break diaphragm assembly, thermostatic coil-mounted on the intake or exhaust manifold, fast idle cam, and connecting linkage.
Heat from exhaust gases is used for control of thermostatic coil temperature. Choke operation is controlled by a combination of intake manifold vacuum, the offset choke valve, temperature, and throttle position.
The thermostatic coil is calibrated to hold the choke valve closed when the engine is cold.
The choke system operates as follows:
When the engine is cold, prior to starting, depressing the accelerator pedal to the floor opens the carburetor throttle valves. This allows the fast idle cam follower lever to clear the steps on the fast idle cam. At this point, torque from the thermostatic coil closes the choke valve and rotates the fast idle cam so that the cam follower lever comes to rest on the highest step of the fast idle cam. (This opening of the throttle valves also pumps a priming mist of fuel through the pump jets into the primary throttle bores to aid starting). During cranking, engine vacuum below the choke valve pulls fuel from the idle system and main discharge nozzles. This provides adequate enrichment for good cold starts.
Some Quadrajet models use a choke enrichment fuel system to supplement fuel feed from the primary main discharge nozzles for good cold engine starting.
Two calibrated holes, one in each primary bore, are located in the air horn just BELOW the choke valve to supply added fuel for cold enrichment during the cranking period. The extra fuel is supplied through channels which lead to the secondary accelerating well pickup tubes to allow fuel at closed choke to be drawn from the secondary accelerating wells located in the float bowl chamber, (As shown in Figure 26). Also, during warm engine operation, the two calibrated holes in the air horn feed a small metered amount of fuel at higher air flows to supplement fuel flow in the primary bores to provide the extra fuel needed at higher engine speeds.
As mentioned earlier (See Main Metering System), other Quadrajet models use the fuel pull-over enrichment system. This system is similar to the choke enrichment fuel system except that two calibrated holes, one in each primary bore, are located in the air horn just ABOVE the choke valve to supply added fuel during higher carburetor air flows. The calibrated holes, located above the choke valve, do not feed fuel at closed choke during the engine cranking period.
When the engine starts and is running, manifold vacuum is applied to the vacuum break diaphragm unit mounted on the side of the float bowl. This moves the diaphragm plunger in until it strikes the rear cover, thereby opening the choke valve to a point where the engine will run without loading or stalling. (This is called the “vacuum break” position). At the same time, the fast idle cam follower lever on the end of the primary throttle shaft will drop from the highest step on the fast idle cam to a lower step when the throttle is opened. This gives the engine sufficient fast idle and correct fuel mixture for running until the engine begins to warm up and heat the thermostatic coil.
At the engine continues to warm up, heat gradually relaxes tension of the thermostatic coil to allow the choke valve to continue opening through air pressure pushing on the offset choke valve and the weight of the linkage pulling the choke valve fully open at which point the engine can run at normal air/fuel mixtures.
The fast idle cam has graduated steps so that the fast idle speed is lowered gradually during the engine warm-up period. The fast idle cam movement (and step position) is a function of choke valve position. When the engine is warm and the choke valve is completely open, the fast idle cam follower will be off the steps of the fast idle cam. At this point, the idle speed screw or solenoid controls normal (warm) engine idle speeds.
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