Big drop in idle when fans kick on???

[Pman1961]

With the previous 351w 385hp crate motor I did not have the issue of the idle dropping when the fans kicked on. Maybe a few rpm but not noticeable. My current motor was built and run on a dyno at KCR with total timing set 34degrees advance. I don't know if anything to curve the dizzy was done and will call to discuss Monday. Dana, I've got limited experience messing with distributors. Can the curve you mention be achieved with different spring rates or do I need to take to a Engine Builder that can curve the dizzy? My purpose to adjust the timing was I would think retarding a few degrees may help preventing the run-on when shut off. I did have some old gas and that may be a culprit as I put fresh fuel in today and seemed okay but never got the car to full temperature before shutting off. Elmariachi thanks for the good advice. I'll test and make follow up tomorrow. By the way my car is a 2006 Superformance with a factory ford alternator. I'm sure nothing special but I've always noted that when the lights are turned on the ammeter takes a big hit. Never a problem with the battery though. I'll check the voltage drop on the alternator also. Any idea what I should expect and what are the pro's and con's of running a higher amp alternator? Thanks guys!