You using ethanol-free fuel?
^This. There are different jets in a carb. Think of it like this (simplified)...you have idle and WOT jets, idle does what it says, then as you give it gas the idle jet maxes out and the WOT jet starts syphoning gas from deeper in the float bowl (there is a transition period when both are supplying gas), as you continue to roll the throttle that WOT jet supplies more and more of the fuel.Gotta be your high speed jet in your carb. Soak them suckers or order a new car system. Don’t take much to clog it, and if they weren’t patient and soaked it could still be clogged and hog speed jet ain’t allowing fuel. Gotta be if runs and idles fine.
I had a 115 Mercury act the same way, it ended up being a bad power pack.Idk much about outboards myself. But I put new lines on it and it didn’t fix it. Seems to open up with muffs on it but it won’t under a load. I took it across the AR river with not issues even against the current just goes about 5 mph with 3 guys and all our gear.