I have a 95 merc with a 502 efi that wont run in idle position, so I'm thinking the iac is bad. I removed the flame arrestor but the throttle body wont budge. What am I missing here? Do the 4 studs have to come out?

yes

Thanks, they seem like someone has locked tighted them in . Ill double nut it and see if I can get them to turn.

the stud has a machined face up against the TB correct ?? like a washer

Yes it does.

Do you have to remove the throttle body on the big block versions to swap out the IAC?

I dunno but it looks like it could use a good cleaning as well.
a little background, took it out the other day stated up ran great , took a 10 mile ride upriver and shut down to float . No start. After a while I gave it some throttle and it fired right up but dies as soon as you go to idle to shift.

You don't have to take the TB off to remove the IAC but either that or the plenum needs to be removed to gain access. That said, what you said here makes me think you have vapor lock going on and not the IAC

I know all about vapor lock , but this is different. Came home on a rope and tied to dock overnight, next morning same deal, no start at idle but quarter throttle and starts right up . Let it run for a few minutes and dies as soon as You return to center .

Yep that would be the IAC