I was looking thru some old excel files for different images and ran across this link that shows a "cut-away" of a Holley Power Valve along with some carb tips in general.
It goes into a slight bit of tuning Holley Power Valves, but does acknowledge that the Power Valve might be open with the "wrong spring setting" Power Valve but that would not cause the engine to run rich if the carb were idling.
The only way the Power Valve can supply extra fuel to the engine is if the Carb Boosters are in operation. So the Hot Rod Guys finally got that story right in print.
Jeff Smith also touches on a couple of other carbs so it those carbs and info interest you, read those tips too.
Here is the link:
http://www.hotrod.com/articles/carbs-tips-tricks-cheap/
Tom V.