I've got a 383 SBC with a holley 750cfm (0-3310S) and a single plane high rise manifold.
It idles ok has loads of go when on the gas but at very light throttle is coughs/misses/judders (1400 rpm/ 30 mph just when you need it!).
It has about 9" of vacuum at idle and a 5" power valve. Vacuum when the problem occurs is about 15", a little throttle and the vacuum drops to below 10" and the problem goes.
I've tried upping the spring rate in the vacuum secondary and this didn't help but just gives a slight hesitation when I go on the gas.
Not sure if it makes much difference but it's in a light car (fibreglass T) and has a lightweight flywheel.
I'm getting to the pulling my hair out stage so any suggestions welcome.