Engine Overrun Conditions

During periods of high manifold vacuum, while throttle valves are closed (engine overrun), throttle by-pass valves open allowing enough mixture to be supplied to give proper combustion. This additional air/fuel mixture produces power to reduce effect of engine overrun. Throttle by-pass valves are connected to inlet manifold by internal drilling in carburetor bodies and open when inlet manifold vacuum exceeds 21" Hg.