What was found
DURING FLIGHT ENG THROTTLE RPM PULLED TO IDLE TO DESCEND WHILE RUNNING ROUGH. ENG SHUTOFF AND COULD NOT BE STARTED. EMERGENCY FIELD LANDING COMPLETED. ACFT SENT THROUGH ANNUAL INSPECTION AND CAUSE OF ROUGH RUNNING ENG FOUND. BOTH SPARK PLUGS IN NR 4 CYLINDER ASSY INOP DUE TO OIL FOULING. CYLINDER AND PISTON REMOVED EXPOSING STUCK OIL SCRAPER RING AFTER A LOW COMPRESSION READING. OIL SCRAPER RING STUCK IN PISTON GROOVE BY BLACK CARBON MATERIAL. DARK STAINING BELOW OIL RING. COMBUSTION LEAK INDICATED. NEW NR 4 CYLINDER ASSY INSTALLED AND ALL NEW PLUGS INSTALLED. ACFT OPERATIONAL CHECK COMPLETED. NO OIL CONTAMINATION IN NEW CYLINDER.