A vacuum passage in the throttle body to the control valve vacuum chamber transmits the manifold vacuum. As the truck decelerates, the high manifold vacuum acts on the valve and over-comes the tension of the valve spring. This opens the valve and fuel enters from the float bowl into the passage around the valve. This fuel, then, flows up the vertical passage, past the air bleed and into the horizontal discharge nozzle. The fuel is discharged in the over-run control valve vacuum chamber and then down the same passage that transmits the manifold vacuum. This fuel-air mixture is discharged into the engine making the mixture too rich to burn and prevents popping on deceleration. The outward appearance is similar to a power valve but the two units must not be interchanged. Over-run control valves are identi fied by #215 stamped on one of the wrench surfaces.									F. GOVERNOR
									(1) INTERNAL VACUUM TYPE 2300G, 4150G - A Holley Centri-Vac vacuum operated engine speed governor may be incorporated on these carburetor models as a positive means of controlling engine speed. The governor con tains a throttle actuating mechanism attached to the primary throttle shaft of the carburetor, through an overriding clutch. The throttle actuat ing mechanism consists of a diaphragm assembly, governor spring, and governor lever assembly. Below governing speeds, the operator, through a simple clutch arrangement on the throttle body of the carburetor, controls the throttle body of the carburetor, controls the throttle plates in the usual manner. When governing speed is reached, a com bination of venturi and manifold vacuum acts on the governor diaphragm to close the throttle plates. Two calibrated by-pass jets in the vacuum pass ages meters the vacuum from the venturi and the manifold to provide the correct balance for proper operation of the governor. At speeds below governing rpm, this vacuum is weakened by air bleeding through the governor valve in the gover nor spinner so that no premature governing action will occur. Filtered air from the carburetor air cleaner enters the governor system through a passage in the main body and then into the governor vent tube. The air flows through an air line to the housing enclosing the governor rotor or spinner valve. A combination of low pressure from the venturi and manifold transmitted through the governor system passages, draws the air past the governor valve and through another air line to the throttle actuating unit on the carburetor. In the controlling unit, centrifugal force acting on the rotor, which has kept pace with engine speed, overcomes the tension of the valve spring and tends to close the governor valve when governing speed is reached. This greatly restricts the amount of air bleeding through to the diaphragm on the carburetor, thus allowing the combined venturi and manifold pressure differential to operate the diaphragm. As the pressure differen tial across the diaphragm increases, the dia phragm moves the governor lever against the tension of the governor spring to close the throttle plates. The governed engine speed is held constant by centrifugal force on the valve balanced against the tension of the valve spring. Any slight change in engine speed due to load variation will cause the governor valve to react immediately, either increasing or decreasing the amount of air bleed ing through the governor valve orifice. A change in the amount of air bleed will, in turn, cause an instant response from the diaphragm to increase or decrease the throttle plate opening. When the accelerator is released, control is takenfromthe
			Anti-backfire Valve
	E. ANTI-BACKFIRE VALVE -Some models of these carburetors are equipped with an anti backfire valve which eliminates popping back on deceleration. This device is located in the middle of the throttle body. The valve by-passes a certain amount of air through the valve instead of all the air flowing around the throttle plates. On deceler ation, when the manifold vacuum reaches approxi mately 22" Hg, the valve closes and prevents the air from flowing through. This creates such a rich mixture in the engine that it will not burn, thus eliminating popping on deceleration.
				NOTE
		This valve has been factory adjusted, and must be serviced as a complete assembly.