If your idle AFR is rich it can cause IAC surging. Just something to keep in mind if you want to calm down the IAC at idle.

I went through all that with the Holley, that can all be tuned out. Watch the timing map when its surging, see whats going on. Then when you shift in gear, watch it there also and you can adjust all those cells. I had 900 HP, shifted at 700 like a honda. (after hours of effing with it lol)

What Launchpad475 said, I think the most amount of time I had on mien tuning was idle tuning, in and out of gear, heck even just with the cut outs open and closed changed the IAC about 10%. At the end of the day, I pulled timing in the idle area(about 8 degrees) and she would idle nice and smooth at 750rpms. Anything less then that and she would hunt. I think some of this is due to my rather large intake plenum and the IAC being far away from the valves.

usually the hunting is too large of a change between cells, merc usually has idle timing at zero then ramps it up to teens-20's in gear. So for example if you have 0 degrees timing in the 700 column, and 20 degrees in the 800 column, when your shooting for 750 its hunting between the 2.

Are we speaking of Carb'd or EFI engines. Assuming efi since I see IAC in the discussion. How are you adjusting idle? Via tuning software?

Yes

I have been tuning Holley carbs since I was 16 back in 1973.
It really really feels weird to sit in the back seat and make changes on a laptop to the engine as it is running and listen to the engine change.
I won't go back to a carb ever again.

I totally agree.. I've dyno'd twice now with EFI and the only time I touched the engine was to set initial timing.. :)