The accelerating pump piston is controlled by engine vacuum. At idle and low engine speeds, with the throttle held in a steady position, the vacuum in the manifold is usually above 10 inches of mercury. Under these conditions, manifold vacuum is strong enough to overrule the pump spring and hold the pump at the top of its stroke. A sudden opening of the throttle causes the manifold vacuum to drop and the accelerating pump spring to force the piston downward in the cylinder. The downward movement of hte piston creates enough pressure in the pump cylinder to close the pump check valve and force fuel through the accelerating jet which meters the rate at which the fuel is discharged into the air stream. The check valve located at the bottom of the pump cylinder serves two purposes. It permits a supply of fuel to reach the pump cylinder, but closes on the downward stroke of the pump piston, thus preventing fuel from being pushed back into the fuel bowl. The pump rod has three grooves at the lower end of the rod which permit adjusting the pump for short, medium, and long stroke.