RPM vs Vacuum for Idle Screw Setting

[MercuryMarc]

I just rebuilt my Holley 1940 carb and it is running OK on my 69 200. I have set the idle mixture screw to get the max RPM on my tach (with the slow idle screw backed out to completely close the throttle plate). The manual says to set it to max RPM but I have also read to set to max vacuum. Is there a difference? And what vacuum port (the carb spacer port)?
Thanks-
Marc in SF
63 Comet, 69 200/C4
http://mercurycomet.net

 

[mugsy]

I have used both methods to set all that stuff. I had a 1947 replacement carb at the time on my '65 Mustang. I found generally that the "highest vac" settings were a little leaner than the "highest RPM" setting. My previous experience with carbs were with GM products and their manual states to use highest vac. Anyway, I went with the highest vac, to run leaner and save gas.

Also remember that stting the idle speed and the idle mixture is an interative process i.e. set the idle speed, then set the idle mixture, then re-set the idle speed and then re-set the idel mixture. Go back and forth until you are happy. But set the idle speed first and end with the mixture.