Ignition systems (magnetos, ignition harness, spark plugs, timing)
Ignition systems
This section covers magnetos, the ignition harness, spark plugs and timing. The exam tests how a self-contained magneto produces and distributes the spark, and how timing and plug condition affect operation.
Core rules and concepts
- A magneto generates its own current from a rotating magnet and coil; breaker points interrupt the primary current, and the capacitor suppresses arcing and speeds collapse of the magnetic field.
- The distributor directs high voltage to the correct cylinder through the harness. Harness leads must be clean, correctly routed and fully seated; shielding reduces radio interference.
- An impulse coupling provides a retarded, strong spark for starting and disengages at running speed.
- Timing is set so the spark occurs before top dead centre, typically in the 20–25° range depending on engine. Too early causes kickback and high peak pressure; too late raises exhaust temperature and loses power.
- Spark-plug heat range must match the engine; fouling, wrong gap or a cracked insulator can cause misfire. Magneto checks should show a small, smooth RPM drop on each magneto.
Worked example
An engine timed to 25° BTDC is found set at 20° BTDC. The spark is later, so combustion pressure peaks after TDC; the engine may run hotter and produce less power. Re-time to the manufacturer’s specification using the timing marks and a timing light or buzz box, then recheck both magnetos.
Traps
Do not confuse magneto timing with valve timing; do not assume both magnetos are correctly timed because one checks good; and do not gap spark plugs by eye without the specified gauge.