In operation, the idle vent valve is closed except when the throttle valves are in the idle position. When the throttles are closed, a wire tang on the pump lever pushes upward on the spring steel vent valve arm and opens the vent valve. Thus fuel vapors are allowed to vent externally, thereby preventing them from entering the carburetor bores and being drawn into the engine. This prevents rough engine idle and excessively long, hot engine starting. When the throttle valves are open to the off-idle and part throttle position, the idle vent valve closes, returning the carburetor to internally balanced venting.

A temperature controlled idle vent valve is used on some models. In place of the standard vent valve, a heat sensitive bi-metal strip is used as the valve holder. This is mounted beneath the idle vent valve arm.

The bi-metal strip holes the vent valve on its seat (closed) at temperatures below 75*. When underhood temperatures are above 75* to 85* the bi-metal strip bends upward moving the vent valve off its seat. This lets fuel vapors, caused during hot engine operation, escape from the float chamber. This results in improved hot engine idle and hot starting. At temperatures below 75*, the vent valve remains closed and retains fuel vapors internally to supply extra fuel for good cold engine starting.

During hot engine operation, when the thermostat vent valve is open, it is necessary to close the valve except at idle to maintain an internally balanced carburetor. This is accomplished through the spring steel vent valve arm which operates off the wire lever on the end of the pump lever. As the throttle valves are opened from the idle position, the vent arm exerts pressure on the bi-metal strip and forces the valve closed.

The thermostatic vent valve is adjustable to make sure it closes at the proper time during throttle valve opening from the idle position.