In an ignition system in which the spark coil is wound right on the magneto armature and the spark is produced by electronic interruption of a short circuit across the primary winding, the first of a succession of rectified half waves applied to the electronic circuit unit has its amplitude reduced either by insertion of a circuit component in one half wave path of the rectifier to provide damping, or by the configuration of the armature core, or both. The magneto rotor has a U-shaped permanent magnet, the pole faces on the end of the legs of which are rotated past opposed pole faces of the armature core, which has the desired effect if the latter is of asymmetric U-shape or of symmetrical E-shape.