As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON A ROUTINE FLIGHT, PILOT NOTICED A DIFFERENT SOUND TO ENGINE. UPON LANDING, PERFORMED A MAGNETO CHECK AND WHEN SWITCHED TO LEFT MAGNETO, THE ENGINE STOPPED RUNNING. INSPECTION REVEALED THE LEFT MAGNETO IMPULSE COUPLING SPRING HAD BROKEN ALLOWING MAGNETO TIMING TO SLIP 30 DEGREES TO 40 DEGREES. THE IMPULSE COUPLING SPRING WAS REPLACED AND ENGINE RUN PERFORMED. MAGNETO CHECKS WERE GOOD.