I had very much the same problem with my 1986 15 hp Evinrude: It would bog down when in gear as I advanced the throttle. Could only get near wot if I riched up the mixture by a full half turn of the low speed needle valve. But then, to run again at low speed or idle, I would have to turn the needle valve back in by a half turn.
Long-story-short, it turned out there was air leaking past the low-speed needle value "retainer." That's the red neoprene bushing with a little tab that's just outside the carb body near the outer, threaded end of the needle. In the first of three carb jobs I had done in trying to fix my motor problem I tried to force this retainer into the keyed recess on the carb, and got the retainer in there but cockeyed. What was happening is that at higher throttle air was being sucked in, making the mixture much too lean. I put a new retainer on the needle shaft and let it just butt up tightly against the carb body. The needle valve threads appear to hold it in place well enough. It has always seemed to me the retainer's outside diameter is too big for it to fit into the retainer recess with the tab fitting into the key, at least on my 15 hp carb. After this fix my motor runs great at all speeds.
If this is your problem it's a very easy fix. I wasted two whole seasons trying just about everything else. If air is not leaking past your needle valve retainer, I would look for an air leak somewhere else, like the fuel pump or fuel hose from the pump to the carb.
Good luck!
