one of the two engines 2007 350 mag 5.7 mercruiser idles at 480-520rpm whilr the other idles at 700-720. I think it maybe the IAC valve. There was a serpentine belt issue where there belt was breaking down and a lot of "dust" was in the engine compartment. I examined the air muffler in the IAC and it was full of this dust, also under and in the fire arrester was dust.
Is there any way the IAC can be tested to be bad. I disconnected the wire interface while the engine was running and the engine stalled.
Could it be partially clogged and will taking it off and cleaning it do any good or should I just replace it.

It could be partially clogged. Air-flow is essential for the IAC, cleaning could help depending on the amount of damage. I would also suspect the IAC valve as being your problem, as long as the problem did not exist prior to belt insident.

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If you have two engines the easiest way to trouble shoot the IAC is to swap them. They pull out easy enough. The problem is more than likely the "Muffler".
I would run the boat without them and see if the problem still exist. Hope this helps