As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) IN CRUISE, LEFT ENGINE STARTED TO RUN ROUGH. AIRCRAFT LANDED. MAG CHECK CARRIED OUT, 300 RPM DROP WITH LEFT MAG OFF. NR 4 CYLINDER LEAD DISCONNECTED, RETAINING NUT MISSING ONLY SMALL RING PART FROM NUT FOUND. INTERGRANULAR CORROSION. REPLACED.