Re: 1960 Evinrude Fastwin 18hp Troubles

[Don]

OK, well kinda hard to say about the high speed skipping, but am guessing it is ignition related. Go ahead and try the new plugs, but you should use the J6C, not the J8C champions. When the engine "skips" at high speed, does it seem like it is bogging down, or is it a sudden/quick blast from full to partial power? Sudden/violent bursts of full power "usually" indicate an ignition fault. I know you have replaced most of the parts under the flywheel, but that doesn’t mean everything is OK under there.
I want to warn you about not properly torqueing the flywheel, do it right. If the flywheel nut is too loose, the key will shear and the tapers will be damaged. Over torqueing the flywheel nut will damage the flywheel.
In any event, I want you to try the simplest things first without ripping the engine apart again. Most parts stores/Harbor Freight/Sears sell those inline neon ignition testers. Put two of those inline testers between your plugs and the plug leads, then go for a ride (when the sun goes down) with an assistant. Ignition faults will be indicated by either light going on and off intermittently instead of a steady flicker.
While on the test ride, listen for arcing, you may be able to see arcing under the mag plate also, maybe one/both of your plug leads is bad.