Ignition and Starting Systems
Magnetos
A magneto is a self-contained generator of high-voltage pulses. The rotating magnet induces current in the primary winding; when the breaker points open, the primary field collapses across the capacitor and induces high voltage in the secondary, which travels to the distributor and through the harness to the spark plugs.
Two timing concepts matter. Internal timing (E-gap) is set on the magneto itself - points must open at the E-gap position, just past neutral. External timing aligns the magneto to the engine so the spark occurs at the specified degrees before TDC on the compression stroke.
Starting aids: an impulse coupling delivers a retarded, hot spark at low RPM and snaps the magneto past the compression load; a starting vibrator (shower of sparks) system uses a retard breaker for a retarded spark. Both operate during start only.
Rule of thumb
On a magneto check, expect no more than about 125 RPM drop on either magneto and no more than about 50 RPM difference between them - always confirm against the AFM. A drop that is too small can indicate ignition timing that is too far advanced.
Traps
The engine is shut down by grounding the magnetos, so a broken P-lead makes the engine hot. Massive-electrode spark plugs are cleaned and gapped; fine-wire plugs are gapped with a wire gauge and resist fouling better. Starting systems include direct-cranking starters with an overrunning clutch, starter-generators, and air-turbine starters - know the starter duty limits and cool-down periods.