I have three 502 MPI Magnums (circa 1998) in my boat. Any way of raising the idle on these? i'd like to raise them 200 RPM's for better control while docking but don't want other complications as they run good otherwise

If I remember correctly Theseus a screw that is locked in place from the factory that can not be changed. I am sure you can change it through the comp or put a plastic cap on screw and see what rpm rise that gives you. That would be a presidential solution and not the best. What are your Rpm now. You really don't want to go above say 900 or you will kill your drives.

No you cannot do it mechanically because if you open the throttle plate to raise it the computer will shut down the idle air control to bring it back down, and if you reach the end of IAC travel then it will not idle cold. You could have the ECU's tuned with a new idle speed though.

That's true it would only work while motor was warm. Was tryng to figure way around burning three comps just to change the idle. If the idle is really low on all three but equally low then the are prob set right from factory and truly should be where they are. You don't want to bump those drives to hard and damage them

The ecm has a desired idle speed that is set from Merc. when the programming is done. You can not change it.
Eddie

There shouldn't be any real reason to raise the idle rpm. Just give it a little throttle as needed when docking.

You need to check the minimum air rate. do this by plugging the IAC passage and then check RPM, if it is off do to wear in the shaft you can adjust it up to spec again.