Abstract: The operation of the interrupting switch in the primary circuit of an ignition coil triggers a first timing circuit having a fixed interval or one sometimes modified by engine loading, at the end of which a timing interval signal is generated which lasts for a second interval. The latter is applied to switch a resistor across the timing wave generator to load down its output voltage. In this manner, an engine equipped with a simple ignition shift providing spark advance in the low speed region and then steady or slightly retarding timing thereafter, can be caused to provide a retarding priming shift that sets in at a medium speed, increases and then tapers off with higher speeds so as to cover a speed range in which the engine is particularly sensitive to engine knock and avoiding the inefficiencies which would be necessary if the simple ignition shift system had to be backed off to avoid intersecting the knock boundary.