2006 Mercury 90 HP 4-stroke idle problem

I have two 90 HP motors. One runs like a top, and the other fails to idle. I have run a ton of carb cleaner through, as well as sea foam. I recently drained the fuel tank for the problem motor. I have a water-fuel separator. I have been trying to figure out why it won't idle for some time. I cleaned the fuel filter on the back, there was a little crud in it, and the inline filter just above it seemed fine (I can blow through it, and for good measure, I sprayed some carb cleaner through it the wrong way). Today, I ripped into it, pulled off the manifold, and dis-assembled the carb (or whatever they call them these days). It had a bunch of crud in it, which I cleaned up real good, put together, and tried again, but no luck. I didn't notice any jets or anything, unless they are on the float side of the carb thing.

This engine has always had trouble maintaining idle, even when I first got it. How do I adjust the idle? Also, I read in another post that I should check the float position. Does anyone know what this value is? I also noticed the valve on the top of the 'carb', which is like a tire valve - is this for a can of carb cleaner, or for air to blow it out? Should I suspect the injectors? Can these be cleaned?

Oh, and I should mention, that although it doesn't idle, and runs a bit rough at low RPMs, it feels like it is right on the edge of working, so I am thinking if I can tweak the idle up just a little, running it a couple of weekends with some Techron or some such cleaner might make it all better. Does this seem reasonable?

Thanks in advance,

-Kevin