During quick accelerations when the throttle is opened rapidly, air flow through the carburetor bores and intake manifold 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 smooth acceleration.
The accelerating pump system consists of a spring loaded pump plunger and pump return spring (operating in a fuel well), fuel passage, discharge check ball, retainer, and pump jets (one in each bore).
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 downstroke, or "unseats" on the filling of the pump well (upstroke). The cup remains unseated when there is no pump plunger movement which allows vapor to vent from the pump well.
The pump system operates as follows:
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 or link.
During throttle closing, the pump plunger is forced upward in the pump well and fuel flows through a vertical slot located in the side 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 throttle valves are opened, the pump rod and lever forces the pump plunger downward. The pump cup seats against the pump plunger head forcing fuel through the pump jets where it sprays into the venturi area of the carburetor bores.
The pump plunger duration spring is balanced against the pump well return spring so that a smooth, sustained charge of fuel is delivered during acceleration. The duration spring is selected by the factory 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 so that air will not be drawn into the pump passage during upward movement of the accelerator pump and prevent proper pump fill.
Movement of the pump lever also controls the position of the throttle position sensor plunger in the air horn (See description TPS under Main Metering System).
During higher air flow through the carburetor bores, a vacuum exists at the pump jets. A passage which is 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 venturi area. This insures a full pump stream when needed and prevents any fuel "pull-over" from the pump discharge passage.
A pump plunger stem seal and TPS plunger seal are included in the air horn assembly to reduce the possibility of fuel vapor losses to meet evaporative emission requirements. The retainers and seals must be removed from the air horn assembly whenever the air horn is to be submerged in carburetor cleaner.
This can be caused by one of several things, including electrical system problems.
Polish the pump well with crocus cloth, or 2000+ sand paper. Apply a couple of dabs of white lithium grease to the cup when installing. Today's gas has little lubrication. A little grease will help.