Need help with a fitting on a Holley 2 Bbl carb, please.

I think capping that tube would cause an "Inbalance", and could cause a problem, if I remeber my 50 yr old lessons on carbs .
The only other vent may be inside the air cleaner, (low pressure) iirc.
If you cap it, - poke a hole in it,
jmo
Yup. I'm pretty sure this is what RRR was trying to tell me last night with his response, when my brain wasn't engaged properly.

(Hmmm: Brain synchros not functioning for me? Possibly!)