Deceleration Valve: Operation
Deceleration valve body contains a spring loaded diaphragm which is held in place by the bottom cover. Diaphragm is subjected to manifold vacuum on the top side and to atmospheric pressure on the underside (by a bleed hole in cover).
During periods of deceleration, intake manifold vacuum is sufficient for the diaphragm to overcome the spring loading and lift the deceleration valve off its seat. With valve open, vacuum draws a metered amount of air and fuel from fuel pick-up tube through the deceleration valve and into manifold.
This additional air/fuel mixture, coupled with other engine modifications, provides improved combustion. The air/fuel supply needed for combustion is fed to the engine through a hose which connects the deceleration valve to the deceleration section of the carburetor.