I have a johnson 55e76e on my pontoon boat and it works great at idle and low throttle levels, but if you open it up it runs great for 30-45 seconds then loses power dramatically. If you drop it back to idle for a minute or so you can get the 20-45 seconds again. Sounds like a fuel issue...
I have a 1976 55Hp johnson on my pontoon boat and it idles beautifully as long as the water level hits the motor at the same place. If I take someone along and they walk to the front of the boat, raising the back of the boat the idle speed increases. If they walk to the back it reduces, often...
