Idle Vacuum Valve

Idle vacuum valve uses carburetor port vacuum to control vacuum vent operation in by-pass valve. Its purpose is to dump air pump air during extended idle (1/2 minute or more). When at idle, carburetor throttle plate is below spark port, so vacuum to idle vacuum valve is zero. This allows idle vacuum valve diaphragm to be up, letting atmospheric pressure enter vents and connect directly to by-pass valve vacuum vent. See Figure.