As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED ENGINE RUNNING VERY ROUGH DURING ROUTINE TRAINING FLIGHT. THE AIRCRAFT RETURNED TO THE AIRPORT AND MADE AN UNEVENTFUL LANDING. AN INSPECTION OF THE LEFT MAGNETO WAS PERFORMED AND THE DISTRIBUTOR GEAR WAS FOUND TO HAVE SEVERAL MISSING TEETH AND WAS BROKEN INTO THREE SEPARATE PIECES.