The valve uses vacuum to open and close. When you stomp down on the gas pedal the vacuum in the manifold drops to a very low number, this makes the power valve open. So when you start with a low vacuum number of 6 inches.. the valve may already be opening and now you've got to much gas in the manifold - once you turn up the idle screw a little, the idle jumps way up because you've already got plenty of fuel in the manifold that you don't/didn't plan on being there. I'm not certain or familiar with that carb, if it is a Holley copy you can get a power valve eliminator and run without it. Or, see if you can get a different power valve with a lower setting?? But I think your vacuum signal is to low already.

Again, talk with the OEM, the folks that built that carb can help you tune it best. HTH.