ORIGINAL: proptop
One thing that just came to mind...
You mentioned that you think it has an early carb on it?
Could the disc be moving on it's own just a little bit?
Sometimes, when o rings get some age to 'em, they get hard and the carb adj. can drift a bit.
I know it has an early carb on it. Main needle is on the carb housing (behind idle disk) When I got the engine it was new never used. I pulled the carb apart and put some green o-ring lube on the o-rings and popped it back in. It seems like the o-rings are making a good seal and the idle adjustment stays where I put it. (no idle mix needle - eccentric screw) Something was wrong with the carbs on Military Drone sportsters which mine are. It is acting like its running way lean on the top end and from the setting from yesterday I'm at least 1/2-3/4 turn richer than that and its still falling off. The Sudden stalling is different from the load-up and not clean out stall in it just quits.
After it stalls, it will restart with 2 or 3 flips of the prop. Idle is nice and smooth and up to about 4000rpm is nice and smooth. In fact it runs pretty nice up to just before WOT. The last 1/5th throttle is where it will sag. The weird part is I'll hold the boat on shore, run the engine to WOT and get it just singing.. Sounds great, no sag. Its only after its in the water and moving forward under near-full load that it gets saggy in the WOT area. It blows lots of smoke and oil from the exhaust while holding it stationary and revving it. This is where I'm confused.