The end carbs have idle circuits for a reason, it was to keep the fuel from going stale in the bowls when they were not in use, as if the end carbs would go for very long without getting opened up!!
The end carbs operate off a vacumn source that originates in the venturi of the center carb, when it is wide open or has high air flow, it creates a vacumn signal that goes to the pods on the outboard carbs and opens them up. The end carbs have a tiny kill bleed that bleeds off some of the vacuum to keep the ends from opening until the engine can handle the additional flow.
In short, the end carbs do not open under normal driving/idleing conditions. Under idle conditions, they do contribute to the rich/lean condition of the idle system. This is kind of a simple explanation, but basically covers it. Lee