Just a thought here; and like Fleetwin says, don’t go arbitrarily replacing ignition parts on a hunch, especially MY hunch! I have never seen this on smaller OMC motors but I have on their bigger ones: How about the possibility that both cylinders are firing at the same time? The real head scratcher in your description was that the engine vibrated badly at idle BUT RAN SMOOTH WHEN YOU DISCONNECTED THE LOWER CYLINDER, AND IT WAS STILL SPARKING. Do you have a nice, open gap spark tester? Can you get it to spark at a slow enough cranking speed that you can visually verify that the two sparks are happening in an alternate manner and not at the same time? I remember watching some spark checks on V6 OMCs, where all six cylinders were firing at the same time. It sounded like some kind of space weapon….
Long live American manufacturing!
