Engine Overrun Conditions
During engine overrun, carburetor throttle by-pass valve supplies sufficient mixture for proper combustion. This additional air/fuel mixture provides enough power to reduce effect of engine braking. To help overcome this unwanted engine braking feature, ignition is retarded 12° (crankshaft) through action of second distributor vacuum unit. When engine overrun creases and intake manifold vacuum falls to 18" Hg (1970-71) or 17" Hg (1972-73), air entering calibrated air bleed allows throttle by-pass valve to close and distributor vacuum unit to return to normal closed positions.