What intake used there?
Don't mess with the restrictors, if it occurs at low amounts of throttle then you are nowhere near the power valve open. If you can get it to hiccup while sitting in drive then have someone do it while watching the exhaust pipe for any possible puff of black smoke. If concerned about PV opening then plug the hole with commonly available plug and then test for the flaw again, but DON"T PUSH THE ENGINE HARD, you can damage it. It will drive at light load and low throttle though so you can test more. If plugged at least you know it's not PV. You can disable the accel pump as well and SLOWLY ramp rpm up to see if it does it. Which accel pump check valve in the fuel bowl under pump cover, the ball check or a silicone disk type? Has to do with how fast the accel pump activates.
You need to verify the timing, timing slow will make it flat spot. The plugs often cannot be read for a while until they color up, it doesn't happen instantly. Look up very high inside, they begin to color up high where the porcelain meets the steel first. You really need to know idle timing and total full all-out as well. The vacuum advance is closely hooked to that depending on which unit you have, it may or may not need to be manifold vacuum. If less vac adv, you use more initial timing lead if more vac adv you need less lead, the two are hooked together and have to add up to make sense on the total, why you need numbers of all of it. If you don't have enough timing you could easily be working on a carb with nothing wrong with it.
Leaving carb where it is idle setscrew wise, pull the carb and look underneath to see how far up on the slots the throttle plates are, you are only supposed to be either just barely exposing the bottom of the slot or maybe .020" or so, if higher you are too rich then it goes super lean when you run out of transfer slot, think of it as mini-accelerator pump until both the shooter gets working. You don't have the mixture screws very far open which leads to believing possibly too far up in the slot, a common mistake. The mixture screws are not set by turn amount, rather by vacuum, one may end up different from the other but if too far one circuit is clogged.
If truly right off idle most likely the transfer slots are too exposed. Drop the idle speed back down too low to get the slots right (while carb is off) and just test to see if the flat disappears, you can bring up idle speed in other ways later.
You can adjust pump cam to have a big space and then none at all and take careful note of what changes. Or bigger shooter vs. smaller and the same. The vacuum gauge WILL drop when the motor dies, normal, an effect not the cause.
If the exhaust manifold has an O2 port you can put an O2 in there to tell if rich/lean by using an A/F ratio gauge.