Between 1985 and 1992 the National Transportation Safety Board has cited magnetos as a cause or factor in 92 accidents involving 22 fatalities and 21 serious injuries. This is surprising in that engines have two magnetos, but only one is required to operate the engine. Yet possibly this is the problem. Because the magneto is considered redundant, there is a tendency to operate it until it fails.
Magnetos are reliable and provide a redundant safety factor, but they have the potential of causing engine failure if they are not inspected and serviced. When the magneto bearings fail on one obsolete magneto model, the rotor seizes causing gear teeth inside the engine to break. The broken gear uncouples both magnetos, causing instant engine shutdown. Worn impulse coupling rivets can cause engine failure when the impulse coupling engages in flight. The adage "if it's not broke don't fix it" translates to "operate it until it fails, and hope it doesn't take the engine (and us) when it does."
These are tough words to start a book. When my shop opens up magnetos for overhaul, we see that many are not receiving proper preventative maintenance. With an understanding of the magneto's mechanical and electrical design, you have the knowledge to make the correct maintenance decisions so that your magnetos operate safely and reliably.
The magneto uses mechanical energy from the engine to generate alternating current waves. These current waves are only generated during certain intervals of the magneto's rotational cycle. The magneto, therefore, must be internally timed so that the wave energy is sent to the spark plug at the peak of the wave. This timing function is provided by the breaker points and capacitor and is called the internal magneto timing or "E" gap.
The wave energy must be sent to the correct spark plug at the correct piston stroke position. This timing function is provided by setting the engine timing.
Magnetos consist of a transformer coil, a rotating magnet, a capacitor, breaker points, and a distribution switch enclosed inside a housing...
Single ignition operation during engine run-up is intended primarily to see that all plugs are firing evenly and regularly on each magneto. If the RPM drop is outside of limits, therefore, the first question is whether or not the engine runs smoothly. If the engine runs smoothly, then the engine timing on BOTH magnetos should be checked first.
If the engine is rough on one magneto but smooth on both magnetos then check for a cold cylinder. Allow the engine to cool down, and then start and idle the engine for a minute or two on the magneto that gives you the roughness. Shut down the engine and feel each cylinder for the cold one. Most likely you will find the cold cylinder has a fouled spark plug or a bad ignition lead.
If the engine is rough regardless of the magneto setting then the problem is not ignition. Suspect an induction leak or valve train problem.
Internal magneto problems are characterized by irregular roughness that cannot be isolated to a particular cylinder. Breaker point and capacitor problems fall within this category.
A magneto that doesn't fire indicates that the the magneto's primary circuit is grounded. Suspect the "P" lead or ignition switch is grounding out the magneto, the breaker points aren't opening, or the capacitor is internally shorted.
Excessive RPM drop on one magneto and not the other indicates improper engine-to-magneto timing or improper "E" gap setting.