A magneto is an electrical generator that uses a permanent magnet to produce a current to start an aircraft's engine.
Magneto is not starting
When the impulse coupling fails, pieces will fall into the gears. When parts fall, the pieces jam the gear train. The crankshaft will then break off. This will cause the magneto to fail and so the engine will not start. If the impulse coupling is the problem, replace the impulse coupling.
Can't rotate the magneto to get timing
If the magneto is not timed correctly with the engine it will run rough or will not start.
Remove the retaining nut from the end of the magneto and remove the gear. Next, turn the gear 180 degrees. You then want to replace the gear, nut, and cotter key. Note: if the gear has an odd number of teeth, the timing will change by 1/2 a tooth.
Check the key on the magneto. A worn out key on the drive shaft will cause the drive to rotate without the magneto moving.
Check to make sure the magneto drive shaft is not broken. You can tell if the drive shaft is broken if there is scratched paint under the hold down clamp. The only way to tell if this is a problem is by the scratched paint or big timing change. Note: the shaft can have problems due to misalignment or mis-engangment which will put a side load on the shaft.
Broken distributor gear teeth on magneto
The aircraft will not start or will not run correctly.
Make sure the magneto was not turned while the timing lock was engaged in the distributor gear.
Check the excessive clearance between distributor block bushing and distributor gear shaft. This can cause a gear shimmy.
Make sure the temperature of the magneto is not too high. This can cause broken gear teeth.
Check the engine counterweights to make sure they are working properly.
This problem will be noticed if the engine runs poorly and the aircraft shakes.
Check the rough coil in the magneto. Rough coils usually break when they get too hot. If your aircraft switches from running smooth to running rough, a broken rough coil can cause this. Once the coil is removed perform a heating test by heating it to 140 degrees F for an hour then run the magneto again. If the engine runs poorly replace the rough coil.
Excessive RPM drop on left/right aircraft magneto
This problem will be noticed if the RPM instrument drops excessively.
Check for a shorted shower of sparks vibrator. This will cause it to stay in a retorted position on the left magneto. If this is the problem, replace the sparks vibrator.
Be sure you are using the correct fuel
Check for an induction leak. A leak can cause a mixture that can ignite when both spark plugs fire.
Check for a failed magneto capacitor. If this is the problem, replace the capacitor.