As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT FROM KPDK TO KGEV, THE ENGINE DEVELOPED A VIBRATION AND AIRCRAFT WAS LANDED AT KMRN. TWO PASSENGERS ONBOARD AND LANDED WITHOUT INCIDENT. IT WAS FOUND TO BE A NR 1 CYLINDER SPARK PLUG WITH A BROKEN INSULATOR CAUSING THE ROUGH RUNNING AND VIBRATION.