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Had a call this morning from an aircraft owner who had replaced the ignition harness on his engine (C85-8). The old harness wasn’t causing any problems but was looking bad, frayed, insulation cracking, etc. After installing the new harness the engine was backfiring, bogging down with advancing the throttle. He assured me that all the ignition leads were hooked to the proper spark plugs and the harness caps were correctly installed on the mags. I had him check the ignition harness part number to verify it was for a -8 engine and as I suspected the harness was for a -12. The mags on a -12 turn the opposite way from the -8 engines due to the mags on a -12 drive off the cam gear while on the -8 they drive off the crank gear. That turned out to be the problem. A simple fix is to install the leads marked for #4 cylinder on #3 cylinder and those marked #3 on cylinder #4. This also applies to A65 engines. The A65-8 uses the same harness as a C85-8 so double check the part numbers before ordering a new harness. The owner was very pleased that we were able to troubleshoot the problem over the phone since it’s a 70 mile drive from my hanger to his.