As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PERFORMED A MAGNETO CHECK BEFORE TAXI OUT, DETECTED NO RPM DROPPED WHEN LT MAGNETO TURNED OFF FOR LT ENGINE, TURNED OFF RT MAGNETO FOR LT ENGINE, ENGINE WAS GOING TO STALL. WHEN MX REMOVED THE MAGNETO, FOUND LT MAGNETO PLASTIC GEAR SHAFT LOSE AND MADE PLASTIC GEAR VIBRATE IN FORWARD AND AFT DIRECTION. DEFECT RECTIFIED AFTER MAGNETO REPLACED.