The power valve vacuum piston. (18) and the spring (19) are actuated by the vacuum below the throttle plate (5) which communicates with the top of the piston (18) through passage (20).

At idle the vacuum is the highest, and it decreases as the load of the engine increases. The piston (18) (actuated by vacuum) and the spring (19) are held in the “up” position which allows the valve (17) to remain closed until the vacuum drops to approximately 7.5 to 6.5 inches of mercury. Below this vacuum the piston force is not great enough to resist the compressed load of the spring (19) and thus opens the power valve (17).

Under load, as in climbing hills, etc., the vacuum drops because it becomes necessary to open the throttle wider in order to maintain speed. When. the vacuum drops below 7.5 inches of mercury, the power valve is opened by the piston stem. The fuel then flows into the power valve chamber and through the restriction (21) into the main well (6), and is discharged together with fuel from the main metering system. This gives the additional fuel required for high speeds for heavy loads and low speeds at full throttle.