Have you thought about where the air comes from for idling? A number of places:
- leakage around t/shafts
- PCV
- idle air bleed [ s ].
- gap around t/blades.
You now have 4 extra t/blades & two extra shafts for idle air, compared to a single 4 bbl carb.
I suspect that you have some leakage past the 'closed' secondary blades. Remove carbs & hold up to the light. If you see gaps, you may be able to re-set the sec blades to close more. If you any additional bypass air via drilled blades, then these need to closed/reduced.