in normal load operating conditions of the engine , an ignition timing control unit produces an ignition timing signal in accordance with the rotation speed and intake negative pressure of the engine . in transitions , for example , from idling operation to normal load operation , and vice versa , an ignition timing delaying circuit in the ignition timing control unit produces a correction signal by detecting whether or not the throttle valve is fully closed . when the correction signal is produced , the ignition timing control unit produces corrected ignition timing signal so that , in the transient periods of operation , the ignition timing is gradually changed to a predetermined ignition timing .