76 when the valve 70 is closed signifying wot condition there is no flow in the controller 38 and the pressure in the upper chamber or reference pressure chamber 32 of low pressure regulator 30 is equal to the inlet pressure p in when the valve 70 is open there is flow through the two orifices 72 74 with the resulting pressure drops so that effective pressure of the fuel p 4 is more negative the plot of air fuel ratio against air flow is essentially smooth and parallel at the two different manifold pressure i e wot and part throttle in the actual embodiment the vacuum pressure control unit 38 is integral with the carburetor 12 and the output port 76 is connected to the pressure source input 34 on the low pressure regulator 30 inlet pressure p in is taken from the air cleaner 62 after the ambient air has passed through the air cleaner element 78 the second source of pressure which is controlled by the valve 70 is the same source as operates the air valve vacuum motor 66 and the valve 70 is interposed the air valve pressure and the orifices 72 74 in the controller 38 it has been determined that there is a pressure drop across the air metering valve 10 which is on the order of 8 10 inches of water and with the pressure p 4 of the fuel.