The power valve opens when the manifold vacuum goes below the number on the valve. The number has NO effect on the amount of gas used ... it only regulates WHEN the power valve opens. You might want to measure the manifold vacuum at WOT under a load. Helps to have a passenger look at the gauge, especially if you do not have one in the dash. Check to make sure the vacuum is below the number of the power valve.
Standard Holley power valves are rated between 2.5 and 10.5 "Hg vac. There can be two problems with using the wrong valve.
If you put a 2.5" Hg valve in your car and the minimum vac at WOT was say 3.0" HG, it would never open. Tremendous loss of power.
On the other hand, if you used a 10.5"Hg valve and the engine had a very rude cam with a lumpy idle and was at say 8" HG at an idle, the power valve would be open all the time. Your exhaust would be black and it would run like a pig.
The most common power valve is a 6.5" Hg, so you are all right with the valve you got now.
You might want to get a book on Holley Carbs. The best one IMHO is by Dave Emanuel
http://www.buy.com/retail/product.as...51&dcaid=17902