Well, back in the day, the UL81J plug was the final recommendation for the low tension ignition models, the current Champion plug is the UL81C. I don’t ever recall seeing arcing issues using these plugs, so I am thinking there was some unique issues/problems with this poster’s coils/plug boots.

I love coming to this site because I learn new things all the time and also remember things I forgot. Seriously! I forgot the ’76 35 was still low tension ignition. Anyway, IMHO, it is still at least somewhat of a consideration about trying another plug without the U gap, at least if all else fails.

On Mercruisers, the spark plug wires have a built in resistance, by design. You used to always be able to tell when it was time for new plug wires, when you’d see arcing from the high voltage coil tower terminal to the negative coil terminal. No amount of dielectric grease would seem to help. You’d know there was just getting to be too much resistance in the wires, even though the engine still might be running okay. I think that in the case of the 35 plug boots, the same thing is going on. Maybe the NGKs have more resistance that the proper Champion plugs Fleetwin mentioned?