I'd think your problem is an induction air leak. could be at carb flanges, or betwixt manifold and head. Could also be a leaky vacuum hose or fitting on intake manifold. Could also be loose throttle butterfly shaft bushings, although this one would more likely be the cause for uneven idling - if the butterflies are completely closed, I wouldn't think that bushings would admit enough air unless they're REALLU loose.
If you get all the air shut off, the engine will stop. If closing the throttles doesn't do it, then the air is coming from somewhere else. Spraying WD-40 on suspected leak sources will likely prove revealing when engine is "idling."
Good luck with it!
ps