How Does the Holley 94 Power Valve Work?
During high power operation, the carburetor must provide a mixture richer than is needed when the engine is running at cruising speed under no great power requirements. The added fuel for power operation is supplied by the power enrichment system, sometimes referred to as the economizer system.
This system is controlled by manifold vacuum which gives an accurate indication of the power demands placed upon the engine. Manifold vacuum is strongest at idle and decreases as the load on the engine increases. As the load on the engine is increased, the throttle plate must be opened wider to maintain a given speed. Manifold vacuum is thus reduced because the opened throttle plate offers less restriction to air entering the intake manifold.
A vacuum passage in the throttle body transmits manifold vacuum to the power valve diaphragm in the vacuum chamber. The manifold vacuum, acting on the diaphragm at idle or normal load conditions, is strong enough to hold the diaphragm down, overcoming the tension of the power valve 11 spring on the stem. This holds the valve closed. When high power demands place a greater load on the engine and manifold vacuum drops below a predetermined point, the power valve spring overcomes the reduced vacuum and expands, opening the power valve. Fuel from the float chamber flows through the valve and out the small holes in the sides of the valve into the passages leading to both main wells. A restriction in the passage leading to each main well controls the amount of fuel the power valve meters, thus insuring an even distribution of fuel. In each main well, the fuel joins the fuel flow in the main metering system, enriching the mixture.
As engine power demands are reduced, manifold vacuum increases. The increased vacuum acts on the diaphragm, and the diaphragm overcoming the tension of the power valve spring, moves downward. This closes the power valve and shuts off the added supply of fuel which is no longer required.
