While adjusting my carbs that hose is open to the atmosphere.  Should I plug it or just leave it open?

Thanks in advance. 

if you mean the large one that attaches to the back of the air filter housing, it's essentially always open to the atmosphere - just inside the line of the filter, which is irrelevant from a vacuum standpoint.

unlike the efi system, the carbs can't see any pressure/vacuum other than what runs through them or the intake manifolds/balance tube.

Thank you sir!

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