Throttle Valve Operation
Connected to carburetor is a throttle valve which keeps throttle slightly open when decelerating and engine speed is above 2000 RPM (engine overrun). Throttle valve is operated by a two-way valve which in turn is operated by an RPM switch. When engine speed rises above 2000 RPM, two-way valve is actuated by RPM switch and vacuum can no longer pass from intake manifold to throttle valve. Plunger on throttle valve would then remain extended, allowing throttle to remain slightly open during periods of deceleration (engine overrun). When engine speed drops below 1800 RPM, two-way valve is again actuated by RPM switch allowing vacuum to pass from intake manifold to throttle valve, plunger is drawn in and throttle returns to its normal idling position.