[bobcowan]

I don't use an IAC at all. I set the idle the old fashioned way, with a set screw. Yes, the idle may vary a bit depending on conditions - temp, altitude, etc. But not enough to matter.

I don't see why you'de want to retard the spark just for idle. But my computer is fuel only, so I don't really know. My spark is controlled by the distributer with a simple curve. Again, the old fashioned way.