As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ENGINE STARTED TO RUN ROUGH AT CRUISE SETTINGS AND ABOVE. ENGINE BACKFIRE VIOLENTLY WHEN RIGHT MAG SELECTED . RUBBER DISK COUPLER BETWEEN MAG AND MAG DRIVE FOUND TO BE SEVERLY CRACKED AT EACH COG ON THE DISK, CAUSING THE MAG TO BECOME OUT OF TIME AT RPM GREATER THAN 1750. BELOW 1750 RPM ENGINE OPERATED NORMAL. THE MAD AND MAG COUPLER WERE REPLACED AND AIRCRAFT RELEASED FOR RETURN TO SERVICE.