Sounds like it’s running on one cylinder, to me. Of course, diagnosing over the Internet is almost as bad as reading tea leaves. What have you done, or not done, to the ignition system?

Have you cleaned/rebuilt the carb? Any gunk in the slow speed circuit could affect the idle. Start with slow speed no more than 1.5 turns open.

Have you pulled the spark plugs to see if one is wet and not firing?
Dave

sounds to me like you need to get the carb adjustments dialed in… with the high speed open that far, the motor will be way to rich, and could exhibit the symptoms your describing. Try running the engine full throttle… very slowly turn the high speed adjustment knob to the right until the motors rpms speed up and smooth out. Continue this very very slowly (to the right) until your motor begins to slow and big down, due to being to lean. While continuing to run at full throttle, turn the adjustment knob to the left (to open) until your motors rpms come back up. The idea is to get that sweet spot between the edge of being to lean, and to rich.Now you can proceed to the low speed side. This is done at idle, in gear., and must be fully warmed up. When turning the low speed dial clockwise (very very slowly) the motor will eventual start to lean cough. Now you can tweak the adjustment open till it smooths out. I hope this all makes sense. I think you power loss is due to (at least in part) your high speed adjustment being way to ruch.start with 1 Turn open on the low side and 3/4 turn open on the high side. FYI those motors cough at idle till they warm up, that’s normal. Of course all of this is assuming the ignition system is in good order.

That is a good outboard. If the 304940 boss gasket is damaged or missing it will allow fuel to bypass the high speed adjustment needle valve and could make it difficult to get a proper adjustment. The taper on the needle valve must be smooth without any notches. . . 😎