The purpose of the accelerator pump system is to provide the momentary additional fuel needed to sustain smooth engine operation under a rapid acceleration. This extra fuel is instantly needed due to the fact that as the throttle is suddenly opened wide, the air flow and manifold vacuum being light change almost instantaneously while the heavier fuel tends to lag behind.

When the operator of the motor vehicle presses down on the acceler­ator pedal, the pump rod which connects the throttle lever to the accelerator pump lever is forced upward causing the accelerator pump plunger to be moved downward into the pump well. The fuel which is under the pump plunger is in turn forced downward. The pump inlet and vapor vent check ball located inside the plunger body is forced up­ward by the resistance of the fuel, closing off the inlet passage preventing any fuel from being forced back into the fuel bowl.

After engine speed has increased and the operator of the vehicle releases the accelerator pedal, the pump plunger assembly will be returned to its upward position. As the pump plunger is pulled upward, the pump discharge spring held in place by the pump dis­charge guide will force the pump discharge ball into its seat. This action prevents any fuel from being drawn back into the pump well causing a partial void in the pump discharge circuit.

As the pump plunger is returned to its upward position, the pump inlet check ball located in the plunger body will drop, allowing more fuel to enter the pump well from the float bowl chamber in preparation for the next time it is needed. At the same time, on those models containing a separate pump inlet check ball, the partial vacuum created as the DLLA138S1191 plunger is moved upward will lift the pump inlet check ball allowing fuel to enter through the inlet filter screen and into the pump circuit.

Because the carburetor is subjected to high under-hood temperatures, it is possible for the fuel in the pump circuit to become vaporized. If the vapor is not disposed of, the pressure built up will force fuel from the pump circuit into the carburetor resulting in a rich mixture. To prevent the possibility of fuel vapors causing this to happen, when the pump plunger is in a ready or cocked position, the 8N7005 pump inlet and vapor vent check ball will drop to the bottom of its travel leaving the inlet opening uncovered. Any vapor that has accumulated under the pump plunger head is free to be dissipated into the float chamber past this open pump inlet and vapor vent check ball.