Pilot Description of Broken Impulse Coupling Spring

"Aircraft has a 600 rpm drop on left magneto during run-up. Otherwise engine operates smoothly."

10-52949 Impulse Coupling Spring

Diagnosis: Retarded engine timing caused by broken impulse coupling spring.
Underlying Condition leading to Failure: Corrosion pitting
Prevention: Replace impulse coupling spring on engines that show signs of exterior corrosion and at a maximum of 500 hour intervals or in accordance with manufacturer's recommendations.

Red rust caused by condensation collecting in magneto.

Additional Warnings: D-2000 and D-3000 magnetos, broken impulse coupling can lead to severe loss of engine power and forced landing. Replace spring at each annual inspection might be something to consider.

Additional Instructions:
Here are two videos I did that shows you how to wind impulse coupling springs

Bendix S20/S200 Magnetos

Slick Magnetos