Abstract: Passage means is provided to feed atmospheric air into the vacuum chamber of a vacuum actuator of a breaker plate and a control valve is provided to control the amount of atmospheric air fed into the vacuum chamber to make the vacuum in the vacuum chamber equal to the intake manifold vacuum during engine operation below a predetermined speed and above a predetermined load and to reduce the vacuum in the vacuum chamber to a predetermined level lower than the intake manifold vacuum during engine operation above the predetermined speed and below the predetermined load so that the ignition timing of the engine is adequately retarded during running of the engine at low load and at low speed and medium load and the engine ignition timing is not retarded or is slightly retarded during running of the engine at high load and at high speed and medium load.