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

What kind of boat? What are they idling at now?

It would need to be reprorammed, if you mechanically raised it the iac would go crazy trying to lower it and you would run into all kinds of issues,Smitty