As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING AN ENGINE RUN UP THE LEFT MAGNETO WAS OBSERVED TO BE NOT FIRING. UPON REMOVAL OF THE MAGNETO THE IMPULSE COUPLING WAS FOUND FAILED. ONE OF THE PAWLS WAS WEDGED BETWEEN THE PLATE OF THE COUPLING AND THE HOUSING OF THE MAGNETO. IT APPEARS THE RIVET ATTACHING THE PAWL TO THE HUB ASSEMBLY HAD FAILED. THE MAGNETO GEAR WAS WORN ON ONE TOOTH AND METAL SHAVINGS WERE APPARENT IN THE ACCESSORY HOUSING OF THE ENGINE. THE MAGNETO DRIVE SHAFT WAS BENT. THE ENGINE AND MAGNETO WERE SENT TO PENN YAN AERO FOR INSPECTION AND REPAIR.