Engine vacuum is communicated through opening “B” and vacuum passage to the vacuum piston cylinder. During part throttle (cruising range of operation), engine vacuum is ample to hold the vacuum piston at the top of its stroke and the power jet valve remains closed. At wide open throttle or when under heavy load, the manifold vacuum drops, permitting the spring to move the pump down and open the power jet valve. Fuel then flows through the power jet and power jet and is discharged through the venter discharge passage into the air stream to supplement the fuel from the main jet system and provide the richer mixture that is necessary for maximum power. When the throttle opening or engine load is decreased, manifold vacuum is ample to lift the vacuum pump, permitting the power jet valve to return to it’s closed position and cut off the supply of extra fuel.