runs just a little rough

1993 Nautique Excel. Awesome boat with pic coming soon. Learned the hard way that fuel goes bad real quick and prevented starting. Some fuel additive and new gas seems to have helped. Now Im trying to work out some small kinks. When I start the boat in the morning it runs great at 1000 rpm. But when I try to put the throttle to neutral, the rpms drop to 500 and the boat runs real rough and soon stalls. I tried backing out the idle set screws 3/4 of a turn. That seemed to have helped in that it allowed the boat to run at 500 rpm so that I could at least engage the prop and move forward. Still, the boat seems starved for power. Im not a mechanic but Im guessing its one or more of these possibilities. (1) old fuel has gummed up my carb and fuel system and is still inhibiting performance. (2) the idle on the boat is set too low or needs to be adjusted (3) Maybe, just maybe, the protec system is starting to fail and decreasing performance in a cylinder or two. Anyway, just trying to gather some general thoughts or experiences from folks. I should mention that when I give the boat some gas from idle it accelerates great. If I go real easy on the throttle, it kind of clanks a little like its underpowered. Anyway, impress me with what you know. thanks

Just how old is old gas?

The first thing to do if you suspect bad gas is run on a fresh can directly to the fuel pump.