As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED HIGH RPM DROP DURING MAGNETO DROP OFF CHECK. ISOLATED PROBLEM TO NR 5 CYLINDER BY USE OF GRAPHIC ENGINE MONITOR. REMOVED ALL SPARK PLUGS AND FOUND 1 SPARK PLUG ON NR 5 CYLINDER HAD CRACKED INSULATOR TIP. INSTALLED NEW SPARK PLUG. GROUND RUN UP AND MAGNETO DROP OFF CHECK WAS NORMAL.