MODELS 4M-4MC-4MV PAGE 14
pump is used to provide the extra fuel necessary for smooth operation during this time.
The accelerating pump system is located in the primary side of the carburetor (Figure 14). It consists of a spring-loaded pump plunger and pump return spring, operating in a fuel well. The pump plunger is operated by a pump lever on the air horn which is connected directly to the throttle lever by a pump rod.
When the pump plunger moves upward in the pump well, as happens during throttle closing, fuel from the float bowl enters the pump well through a slot in the top of the pump well. It flows past the synthetic pump cup seal into the bottom of the pump well. The pump cup is a floating type. (The cup moves up and down on the pump plunger head.) When the pump plunger is moved upward, the flat on the top of the cup unseats from the flat on the plunger head and allows free movement of fuel through the inside of the cup into the bottom of the pump well. This also vents any vapors which may be in the bottom of the pump well so that a solid charge of fuel can be maintained in the fuel well beneath the plunger head.
When the primary throttle valves are opened, the connecting linkage 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 located in the air horn 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.
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; otherwise, a momentary lag in acceleration could result.
During high-speed operation, a vacuum exists at the pump jets. A cavity just beyond the pump jets is vented to the top of the air horn, 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 venturi area. This insures a full pump stream when needed and prevents any fuel "pull-over" from the pump discharge passage.
CHOKE SYSTEM-4MV
The Quadrajet choke valve is located in the primary side of the carburetor (Figure 15). The closed choke valve provides the correct air/fuel mixture enrichment to the engine for quick cold engine starting and (partially open) during the warm-up period. The air valve or secondary throttle valves (on some models) are locked closed until the engine is thoroughly warm and choke valve is wide open.
CHOKE SYSTEM - 4MV (Fig. 15)
The thermostatic coil is located in the engine manifold and is connected by a rod to the intermediate choke shaft and lever assembly. Choke operation is controlled by the combination of intake manifold vacuum, the offset choke valve, temperature, and throttle position.
The thermostatic coil located in the engine manifold is calibrated to hold the choke valve closed when the engine is cold.
NOTE: To close the choke valve, the primary throttle valves have to be opened to allow the fast idle cam follower to by-pass the steps on the fast idle cam and come to rest on the highest step of the fast idle cam.
When the choke valve is closed, on models using an air valve lockout lever, the lever is weighted so that a tang on the lever catches the upper edge of the air valve and keeps the air valve closed.
During engine cranking, the choke valve is held closed by the tension of the thermostatic coil. This restricts air flow through the carburetor to provide a richer starting mixture.
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. Also, during warm engine operation, the two calibrated holes in the air horn feed a small metered amount of fuel at higher
