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Well, it does seem like some sort of fuel starvation issue from what you describe. But, that is just a guess, I am not there. Try the primer test and we will go from there. Does your inline filter show any signs of more debris? Just because the clear filter shows fuel flowing, doesn’t mean it is flowing properly, to all three carbs. Those little black plastic fuel line tees that distribute fuel to the three carbs could be an issue, those little fuel lines might be kinked or coming apart internally. I even had one relatively new engine years ago that had a piece of the steel float arm hiding under the float inlet needle seat cutting off fuel to one carb intermittently.
I’m hoping this is just an issue of that lower high speed jet having some debris in it, perhaps the broken brass ear is floating around in there.
Usually fuel pump issues occur at lower speeds, the engine won’t even get to high speed. There could be an air leak somewhere also. Or, the boat’s fuel system might be messed up creating a restriction/air leak. I am trying to remember, did you try a separate 6 gallon tank/hose?