Joe,
I am using the originally installed 6.5 power valve.
I have no flat spots, idles @ 850 all day and provides a grey plug and pipe soot after cruising @ 3000 rpm for 30 min. WOT is right now, no stumble, no hesitation, all the way to chipped redline (6500 rpm).
Zoomer hit on some important points.....all very good. I will explain my system, join the two and hopefully correct the problem. We're pullin' for ya
Go back to the beginning: (and I am sure you have covered these points, but I missed two small pieces of -AN rubber core stuck in my float!!)
1. Very Important!! Fuel pressure - REGULATED @ 6#s, unrestricted. Be careful if using a return line to the tank, you must have sufficient back pressure. I am using a dead-head system. Too much pressure will lift the seats - when the engine heats up, at idle, you can actually see the fuel dripping in the back bowls. Your fuel floats are preset and should be fine, but very small particulate can hamper their operation.
2. Presuming your carb is clean the idle/transfer circuit is the next place to check. If any idle screw needs to be turned out more than 2 full turns, STOP. You have other problems. If <2.....great, then make sure all four corners are equally adjusted. The transition slot should already be adjusted, but can be checked when the carb is off. Simple way: slot hole should look like a "square" not rectangle.
Set idle @ ~ 850 rpm and set timing.
3. Altitude. I am @ 399' ASL. Higher altitude requires less jet, ~1 jet per 4000' adjusted altitude.
4. I use the originally installed jets.
These are fundamentals, give these a shot. We will move to more complex adjustments if necessary.
GOOD LUCK, Joe.
Edit: You may be trying to compensate for a shooter/accelerator pump adjustment with a fatter power valve. If your A/F is OK from transition to mid range, you may need only to adjust accel pump/fuel delivery cam.
Just thinking out loud here............