1994 mercury 40 hp 4 cylinder High speed miss on #3 cylinder

[bateyl]

I have a 40 hp mercury that misses on #3 cylinder only when ran over 4000 rpms. Below that it runs fine. Pulled the plugs after running and #3 appears fouled. The compressions are all between 1 or 2 psi of each other at about 110 psi.
I have replaced the plugs, wires, coils, and rebuilt the carburetors. Any suggestions would be welcome. Ive been trying to trouble shoot the switch box, trigger, and stator. Having trouble with my DVA meter.

 

[sutor623]

Did you try a high speed shutdown and check for water in the #3 cylinder? How do you know #3 isn't firing properly? Is this assuming this because of the fouled plug?

If you truly suspect its electrical you could connect a tiny tach (LCD rpm gauge) to cylinder 3, let your buddy run the boat, and see if the rpms hop around at the suspected misfire.