Help With Carter Afb

Is this a new carb? does it stumble immediately when you stomp on it or is it happening just when it reaches 2000 rpm?
Is it only during quick throttle movements? If its immediate then I would revisit your accelerator pump adjustment. I'd say about .010 or less gap between your pump arm and the adjuster. Its possible too that if it were misadjusted and you went wot that it could have damaged the pump diaphragm (generally would have to be way off adjustment but is possible).

I think you are on the right track with timing advance curve and the accelerator pump first. (I usually do Holley or GM carbs, so some of this is general advice.) If the pump circuits and nozzles are clean, the nozzles are big enough, and the rubber parts (plunger on AFB?) are good, adjustment is all that is left. The power valve in a Holley, or the power enrichment circuits in any carb might be problem.

Brainstorming, how tight is the spring for the secondary flap? If it is flopping open too soon, it can bog. Try adjusting that tighter. The worst thing that will happen is your mileage will increase. After that, low float levels are a more remote possibility. There is not much else left.