A capacitor discharge ignition circuit, adapted for retrofitting conventional breaker point ignition systems, provides gating of the electronic switch and spark advance responsive to an electrical signal generated by the magneto coil of the conventional system and provides force commutation of the electronic switch and a selective engine shut-off. Gating of the electronic switch is induced by a voltage drop of a fixed magnitude following a wave peak of the magneto coil signal. An inductor in series with the capacitor and the electronic switch induces a reverse bias across the electronic switch following the gating thereof to force commutate the switch. Engine shut-off may be achieved by grounding the relatively low voltage induced in the magneto coil, which, during engine operation, is stepped up for charging the capacitor.