As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT EXPERIENCED ROUGH ENGINE. UPON LANDING AND GROUND RUN-UP, RIGHT MAGNETO WAS DISCOVERED DEAD. DISASSEMBLY SHOWED THAT ELECTRODE POSTS IN DISTRIBUTOR BLOCK ASSEMBLY HAD COME IN CONTACT WITH ELECTRODE FINGER ON DISTRIBUTOR GEAR. ELECTRODE FINGER WAS TORN AWAY FROM GEAR AND GEAR WAS ABOUT ONE-HALF MELTED AWAY. NEW MAGNETO WAS INSTALLED AND RUN-UP WAS GOOD.