When the throttle is opened rapidly, the air flow and manifold vacuum change almost instantaneously, while the heavier fuel tends to lag behind causing a momentary leanness. The accelerator pump is used to provide the fuel necessary for smooth operation during this time.

Fuel for acceleration is supplied by a double spring loaded pump plunger. The top (delayer) and bottom (return) springs combine to move the plunger so that a smooth sustained charge of fuel is delivered for acceleration. When the pump plunger moves upward as happens during throttle closing, fuel enters the slotted pump well, flows by the checkball in the plunger head (some models), and also around the side of the pump plunger and into the bottom of the pump well. Downward motion of the plunger seats the checkball in the plunger head. Fuel is forced through the pump discharge passage where it unseats the pump discharge checkball and passes on through the passage to the pump jets in the cluster, where it sprays into the venturi area.

The checkball in the pump plunger head also serves as a vapor vent from the pump well. When the pump plunger is not in operation, the checkball drops off its seat and vents any vapors which may form in the bottom of the pump well into the float bowl area. Without this vapor vent checkball, vapor pressure in the pump well might force fuel from the pump system into the engine manifold causing hard starting and pump slugging conditions under extreme heat.

On some model 2 Jet carburetors, a fuel inlet checkball and channel is used in the bottom of the pump well. In this type unit, fuel is drawn through an inlet screen in the bottom of the fuel bowl, through the inlet channel, past the checkball to the bottom of the pump well. Downward motion of the pump plunger seats the inlet checkball forcing fuel out of the pump jets in the normal manner described previously.