That sounds like a fuel delivery, fuel starvation problem.
Vent open and not blocked on the fuel tank?
Arrow on the Primer Bulb facing towards the engine?
Inline Fuel filter? Clogged or has water in it.
Ethanol has stripped the lining off the fuel lines and is clogging it.
Fuel Pump issue?
A make is useful in such question, and then Google
year, make model 50hp engine dies are 3/4 throttle
someone may have already solved it.
this year make model 50hp engine dies are 3 4 throttle Google Search
finds this http://www.lake-link.com/forums/boats-and-motors/50-hp-mercury-2-stroke-losing-power-at-wide-open-throttle/117851
and this answer
Had an almost exact same problem with 1990's 15 hp 2 cycle johnson. Lost power at 3/4 to full power and would stall. This was a first time new problem. The motor had always run great previous. Noticed small gas leak at hose to engine connector. Replaced all gas line and bulb and connectors from portable gas tank to engine. Problem solved, runs like brand new. Hoses, connectors, bulbs, check valves and gaskets just wear out over time. Simple and not to expensive of a fix.
Well, I opened the hood on my motor the other day, just to see if I could find any fuel line leaks of any sorts. I did smell gas immediately. Looked around and didn't notice any sort of leaks in the fuel line, so if there is one there it's pretty small. I did open the carb cover and there was quite a bit of gas on one of the three holes. Think I found the problem there, one of the gaskets had fallen off and was inside of the cover. Another one of the rubber gaskets is cracked a little, so I don't have a good seal to that carburetor cover. Not sure if that's one of my problems or not, as I believe all that cover does is help with emissions, but I'm not motor expert. Thoughts, anyone?
Your fuel line might be coming apart from the inside because of the ethanol in the gas. I had that problem before. The gas line looks great but on the inside is where the problem is. See of you could borrow a take and a hose from somebody and give that a try.. I use Mercury quick clean in my motor a little goes a long way.. Don't over do it.Hard to tell a man Folllow the directions.. Is that a Oil injector motor if so might want to check your oil level and make sure the lid is tight..