As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NR 4 ENGINE SHUT DOWN DUE TO CONTINUOUS BACKFIRE IN CLIMB. INSPECTION REVEALED NO CRACKED CYLINDERS, FRONT CAM NO DEFECT. COMPRESSION TEST OF NR 12 CYLINDER CHECKED GOOD, NR 12 EXHAUST VALVE GUIDE CHECKED GOOD IAW PURDUE DC-6 MM MB31-3-1 (7). MULTIPLE SPARK PLUGS FOUND WITH EXCESSIVE GAP. NR 8R AND NR 12L SPARK PLUGS WERE FOUND INOPERATIVE. REPLACED ALL PLUGS. GROUND RUN CHECKED GOOD.