As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE FLYING, PILOT EXPERIENCED PARTIAL LOSS OF ENGINE RPM AND MADE A PRECAUTIONARY LANDING IN A FIELD. UPON INVESTIGATION, FOUND THE BOTTOM SPARK PLUG IN NR 4 CYLINDER WAS NOT GAPPED PROPERLY. A PIECE OF CARBON GOT STUCK BETWEEN THE ELECTRODES OF THE SPARK PLUG ALLOWING IT TO BECOME OIL FOULED. THERE WAS NO DAMAGE TO AIRCRAFT OR PERSONS.