Exhaust Emission Systems - BMW: Operation

RPM switch activates control valve when engine speed is above 2000 RPM. Control valve allows intake manifold vacuum to reach vacuum unit on carburetor. Vacuum unit then provides a carburetor throttle opening of 3-4° until engine speed drops to 2000 RPM. Throttle position is then gradually reduced to its regular idle position. This throttle opening of 3-4° must be maintained by adjusting throttle vacuum (dashpot) unit. When carburetor throttle valves are closed and engine is running (idle and deceleration), the dual acting distributor vacuum unit retards ignition timing thus reducing the normal centrifugal and vacuum advance by approximately 12°.