I'm trying to figure out what I am doing BEFORE I spend a lot of money
on parts I don't need, namely the ignition amplifier (about $200++.)

I have a 85HP Evinrude [circa 1972] that has always run great until my
dad stored it in his garage for several years untouched. The boat it
originally went with was also stored and used as my dad's junk
collector bin. I talked my dad into selling me the boat and I had no
idea it was completely rotted out. I cleaned out every last bit of wood
out of the hull and rebuilt the entire boat (at 16' Glasstron.) It's
like a brand new boat! (probably better since it's lighter than
original, I used kevlar and epoxy. I have photos of how I did it, if
you're interested) Now for the engine...

So, I have this Evinrude and it starts fine, etc. A couple weeks ago, I
took it out on a local lake for a spring test run. Within 10 minutes,
it overheated and the buzzer-alarm in the control unit went off. Uh
oh... I limped in, loaded up and garaged it. I found a water pump kit
(not easy to do) and installed a new water pump per the Secor manual.
That went fine but when I started the motor (with muffs on) the engine
overheated again. What now? Well bits of the old water pump impeller
got into the thermostat (or whatever it is Evinrude calls the thing)
and busted it up. Fortunately, I found the thermostat parts locally and
fixed it up. Now the cooling system works nicely except I noticed that
when the motor overheated a lot of the electrical tape wrapped around
the wire harness had melted where it touched the block, so I took off
all the old tape, inspected all the wires and placed them all in
plastic wire housing and tied them up nice. None of the wires
themselves or spark plug wires melted from what I could see.

Anyway, today when I took the boat out, at first it seemed to run fine,
but it missed a little more than I recall it doing before. I chalked it
up to cold water in the lake and the motor needed to warm up. I ran the
boat for about 3 hours today and as the day wore on, the missing got
worse, _especially under load_ for example, once the boat acheived
plane, the missing would smooth out and run almost good. If I slowed
down and then gave it full throttle again, it was like it was running
on 2 cyl and then the other 2 would kick on, then off, then on, then as
the boat got on plane back to smooth running. The motor idles OK... not
smoothly, but it will troll without stalling.

I am thinking I should change the spark plugs (have not checked them
yet) and spark plug wires. I understand the points are never really a
concern in this engine since they only act as a low voltage switch.

I once had a truck that ran like this and it was a spark plug wire that
was arcing to the engine block -but only under load (which made it hard
to find.)

Could it be the sparkplug wires? If so, where do you get wires for a
1972 Evinrude 85? Do you make them up yourself? If so, what's good to
use?

What about spark plugs? Any good recommendations?

Finally, could it be the coil or amp? (the coil can be obtained cheap
enough, but that Amp is a real killer...)

Thanks in advance for reading my drivel and for your helpful comments.

Jeff