1974 25HP Johnson - timing/Points. Lose 1 cylinder after long run

I changed the plugs, points & condenser on my 1974 25 HP Johnson. Starts right up. Trolls great, idles perfectly. But if I run the engine wide open for longer than ~5 minutes then slow down I totally lose 1 cylinder. I set the points (per spec) at .022. Plugs (J4C's) gaped to spec .030.

Both ignition coils were new last year. Engine is cold (running in lake superior) but I checked the thermostat and it's working as expected. Leads (both cylinders) from armature plate to ignition coils are also new (this year).

While engine was sputtering I touched the bottom spark plug cap and got a little tickle but when I touched the top cylinder nothing.

Any ideas?

Start from the beginning what are your compression numbers engine cold? Does the plug wires jump a open air gap of at least 3/8 inch brite blue snap?

Appreciate the help!!! I don' have my compression tester with me. but the rings/pistons/crankcase were all new last year. There are no marks on the cylinder walls. I can hear the suction from the rings.

Both cylinders have a yellow ish spark. Top cylinder does jump 3/8 ". Bottom does not - spark jumps to the side walls.

This may sounds too easy a fix, but since it happened to me before I'll suggest it anyway:
Did you make sure the wires that go from coil to plug are well inserted in the coils?
I had this very same problem on a 9.9hp once , and after getting back to the ramp on one cylinder, I found that one of the wires wasn't making good contact in the coil receptacle. Pushed it firmly, and voila! I was running on two legs again!
I guess the full throttle vibrations and boat banging on water at high speed was enough to loosen the wire.....

Hmmm... No I didn't. But I will! Thanks.

Exactly how are you checking for spark. You cannot use the spark plugs you need to make one or buy a open air gap tester. Is there any slop up and down on the points plate? You may have a driver coil issue under the flywheel if both coils get a yellow spark. You can make a spark tester with a old plug wire and a board, cut the wire in half and staple a gap on the board fasten one end to ground and stick the other end in a plug wire.

To test, I placed the spark plug against the outside of the crank. Yes - there is slop on the point plates!! I'll look on youtube for creating an open air gap tester. Thank you!

The engine is back and running again! thanks for the help. I cut 1/4 inch off each end of both spark plug wires. Reset the spark plug wires into the spark plug boot and coil. I also reset the points. Seems to be running again. So thanks again!

Glad to hear. Love it when it's that simple!