If it is only an idle adjustment (which I do not think it is), you can adjust the idle speed by the idle stop screw on the bottom carburetor. It is a phillips head screw. Do not adjust the screws on any of the other carburetors. They are for carburetor balance.
The idle in neutral for that motor should be 950 plus/minus 50 rpms.
Most likely, you are only running on two cylinders and have a spark or carburetor issue.
I generally recommend to run the engine and remove the spark plug wires from the spark plugs (one at a time) and see if there is a drop in motor rpm. You see what happens with each spark plug wire. If you remove one and there is no change, that is the cylinder that has a problem....could be spark or could be fuel (carburetor issue).
You should use an insulated pair of pliers to pull the wires off of the spark plugs. However......standing in 3ft of water and pulling the spark plugs might be a little "jolting"...if you know what I mean.
So maybe find someone with a dingy or back the boat into a slip, so you can do the test from the dock.
Mike