Strange tuning response on my engine?

[DanEC]

See what everyone makes of this. I noticed the last time I adjusted my idle mixtures and idle speed that when I pulled the cap off the vacuum port on the carb to hook up the vacuum gage - the rpm increased??

I replaced my S&H filters with K&E filters and re-adjusted today and had the same result - although I think to a lesser degree. After adjusting the idle mixtures (primarily by rpm but also checking engine vacuum) I experimented with pulling the vacuum cap off on each carb individually and the rpm increased 100 rpm with open vacuum port. I adjusted the carbs using the same carbs vacuum port as a precaution. And I made sure the choke was wide open.

This is the first engine I've run in a long time without vacuum advance - I'm used to the engine speed falling when I pull a vacuum hose loose but that is due to loss of vacuum advance. If the engine speed increases with open vacuum port that would seem to indicate it's set too rich?? But why wouldn't the idle mixture screws correct for that. The engine seems to respond normally to idle screw adjustments. Vacuum at 850 rpm idle is 13 - 13.5.

Any thougths? Much ado about nothing? Seems to be running fine but I don't have many miles on the engine yet.

Forgot to add - 459 FE with 427 LR dual quad intake and 600 Holleys, medium hydraulic cam, worked steel heads and 9.1 compression.