As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CUSTOMER WAS COMPLAINING OF A ROUGH RUNNING ENGINE DUE TO A BAD LEFT MAGNETO, REMOVED MAGNETO AND FOUND THE IMPULSE COUPLING SPRING WAS BROKEN. COUPLING SPRING REPLACED WITH A NEW PART, REINSTALLED MAGNETO AND TIMED TO ENGINE. GROUND RAN ENGINES AND NO FURTHER PROBLEM FOUND.