During heavy load conditions or high speed operation, the air-fuel ratio must be increased for higher engine output. The power enrichment system which is controlled by intake manifold vacuum, supplies extra fuel during this period. This vacuum is applied to the power valve piston from an opening in the base of the carburetor body, where it is connected to passages in the main body and air horn to the power valve piston. During idle and normal driving conditions, manifold vacuum is high enough to overcome the power valve spring tension, thus holding the valve closed. When high engine output is needed, the increased load on the engine results in decreased manifold vacuum. The power valve piston spring opens the power valve when manifold vacuum drops below a predetermined valve. Fuel flows from the fuel bowl through the power valve and into passages leading to the main wells. At the main wells, this fuel is added to the fuel from the main jet to enrich the mixture. As engine load requirements decrease, manifold vacuum increases and overcomes the tension of the power valve spring, closing the power valve. The main system is then supplied with fuel by the main jet only for normal economy operation.