Jeremy - that port is the vacuum source for the automatic choke used on the 2101s.
I just checked a couple of 2100s carbs (manual choke), and this passage is not on my carbs.
The gasket to the left is for a 2100s.
The gasket to the right is for a 2101s with automatic choke.
Have no idea why your 2100s has the passage. Since its only function is to provide a vacuum source for the automatic choke, which you should not have, it may be plugged. The easy way to plug the passage is to take a split shot sinker just slightly larger than the passage, and gently press it into the passage.
Jon.
