Nick,
Good to hear you got it fired up. I’m still finishing up the Sea King 5. When you say it runs rough at high speed, is it popping or just running slower than it should, smoking a lot, and and sounding like it’s only firing about half the time or running on one cylinder? By the description it sounds like it’s running rich and you are leaning it out until it takes off, then it’s starving for fuel, but… I realize you say you had to richen it to make it run right, then it screams and stalls. Also, do you know your tank is working properly. It could be that it’s running out of fuel and, speeding up due to lean condition, then running out of fuel and stalling? Just a couple thoughts.
-Ben

What kind of a boat are you running it on to adjust the high speed mix. Do not run it over 4,500RPM. Do not attempt to adjust the high speed in nutral. Correct adjustment can only be achieved after the motor is warmed up on a boat. Early breaker points were designed to "Float" at 5000 RPM to protect the motor. Serra Marine advertised a stronger spring to eliminate high speed miss. Evinrude issued a bulletin that motors failing with non OMC breaker points would not be covered by there warrantee. Later OMC points had slightly stronger springs. . . 😉

That looks like a pretty nice one you have there. I’m hoping mine will be a runner with a magneto rebuild, a good carb cleaning, and a lower unit service. At least it’s complete and it was pretty cheap. It looks like I’m getting some leakage out of the top crankshaft seal. Quite a bit of oil under the flywheel.
-Ben