As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) ENGINE BEGAN TO RUN ROUGH IN FLIGHT. ENGINE WAS SECURED. POST FLIGHT INSPECTION FOUND THE CIRCLIP RETAINER ON THE MAGNETO DRIVE SHAFT WAS NOT IN THE LOCKED POSITION, THIS ALLOWED THE SHAFT TO MOVE (AXIALLY) IN AND OUT. THIS CAUSED INTERNAL COMPONENTS TO CHAFE CAUSING DEBRIS TO CONTAMINATE INSIDE OF THE MAGNETO.