I have been sold out of the 2100/2101 since the day after Noah landed the Ark, so don't have one to look at.

GUESSING - it appears to be a home-made bowl vent instead of venting the bowl into the air intake. Most of the Carter YF carbs have a 1/4 inch tube coming up into the air intake, and this appears to have been removed and plugged in your picture.

I think I would open the hole into the air intake, and plug the hole to the outside.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
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The Carburetor Shop