As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED INTERMITTENT ROUGH ENGINE OPERATION EN ROUTE. PILOT HAD FAILED TO LEAN THE ENGINE, CONSEQUENTLY, FOULING ALL 4 LOWER PLUGS. AN OFF-AIRPORT LANDING WAS MADE. NO DAMAGE TO AIRCRAFT. REPLACED LOWER PLUGS AND FLEW AIRCAFT BACK TO AIRPORT WITHOUT INCIDENT. NO OTHER FAULTS FOUND.