Abstract: The spark initiating mechanism of the ignition distributor is operated by the operating rod of a vacuum actuator unit in response to engine intake manifold vacuum to provide ignition spark vacuum advance. The enclosed space defined by the outer casing of the vacuum actuator unit is divided into a first chamber exposed to atmosphere and a second vacuum chamber by a flexible diaphragm in operating engagement with the operating rod. An exhaust port opening into the vacuum chamber is provided with a dual valve arrangement including a normally closed valve operable to the open position by the force of the diaphragm member flexing in the direction to produce increased ignition spark vacuum advance and a temperature sensitive valve located between the normally closed valve and atmosphere which automatically operates to the closed position with ambient temperatures less than a predetermined value and to the open position with ambient temperatures greater than the predetermined value.