The vacuum pump piston is controlled by engine vacuum through opening “B” and the vacuum passage. At idle and low engine speeds (with steady throttle), manifold vacuum is sufficient to hold the piston at the top of its stroke. A sudden opening of the throttle will cause the manifold vacuum to drop, allowing the accelerating pump spring to force the piston downward in the cylinder.

The downward travel of the piston increases the pressure on the fuel below the piston to force fuel past the power jet valve and through the accelerating jet to be discharged into the air stream through the center discharge passage. The accelerating jet meters the rate at which the fuel is discharged into the air stream. Fuel is supplied to the pump cylinder through a check valve located in the bottom of the fuel bowl.

This check valve serves two functions. It permits a supply of fuel to reach the pump cylinder, but closes on the down stroke of the piston to prevent the fuel from being forced back into the fuel bowl.