Mags were mostly used on drag racers, and it was VERY difficult to get one spinning fast enough to generate a reliable spark. Once it's running, the spark gets hotter as the speed increases, but it was not unusual to see a mag-equipped car pushed the full length of the local drag strip before it wouild fire. On farm tractors, airplanes, etc, an "impulse coupling" was used to snap the mag around with a spring for starting. That also retards the ignition timing, so the engine doesn't kick backwards when using a hand crank or hand-turning a propeller. Most aircraft mags have no provision for advancing the timing according to speed or load.
Magnetos are very useful, on a Briggs & Stratton lawn mower engine, a chain saw, or something else that starts without a battery. On a vehicle, they're just a PITA!
Jerry
