In an electronic system for an explosion engine, a device for automatically adjusting the spark timing, wherein a follower is kept into contact with a three-dimensional cam having a surface shaped; with lifts representing the desired timing angle of the spark delivery with respect to an angular position of the drive shaft, said follower and said cam being mutually displaced in two directions orthogonal to each other and to the cam lifts as a function of variables representative of the engine operation, more particularly the engine r.p.m., the throttle angle and the suction pressure in the intake duct, and an electronic system comprising a transducer coupled to said follower to generate a first electric signal representative of the follower position, means generating a second electric signal representative of the time necessary for the rotation of the engine through a predetermined angle, means for processing said first and second signals to originate a third electric signal representative of the lag time corresponding to the desired timing angle measured by the follower, a timer operated upon the engine passing through the aforesaid predetermined angular position, so as to generate a fourth electric signal, and means for comparing said third and fourth signal as well as means controlling the spark delivery, thereby the latter takes place upon said third and fourth signals being equal.