The Stovebolt Site Forums The Shop Area Engine Shop Yep, the YF is better

I don't have a 2100S to look at, but I do have a 3211S (the successor to the 2100 but re-designed with a lower profile and auto choke) which came to me virtually brand new except that it had been sitting in the original Carter cardboard box in Baton Rouge for over 50 years and that cardboard plus abject humidity had caused the outside of the poor thing to look horrible. Since I can't afford to replate it, I painted it with paint that is guaranteed to not have a problem with gasoline. Let's take a look at how it is vented:
1st image below shows the exterior vent hole (red pointer). This allows atmospheric pressure to be present constantly in the float bowl and this model plus your 2100S was designed not to function without that vent..
2nd image below shows the throat vent hole (yellow pointer). This is also constantly open and its job is slightly different.
3rd image below shows another style of the throat vent (a brass tube). I've heard some folks call these vents "Balance Vents." The idea here is this vent allows a compensation for the ambient atmospheric pressure (in the float bowl) and the difference caused by air flowing through the throat. Here's how I understand this: if you happened to live on the coast or anywhere else where you were at sea level, you'd naturally have almost 15 pounds of pressure inside your float bowl. But...if you had a restrictive air cleaner, due to the air flowing through the throat you will have less pressure as a consequence. If that is the case and this vent is either absent or plugged up, you will lose the balance needed (balance between internal and external, that is) and this will cause your carburetor to run richer.
4th image below shows a different style of the float bowl vent. This is a variable vent...open a bit at idle and then closed as speed increases. There is a spring-loaded appliance with a rubber seal sitting atop the diaphragm link inside which at idle sits a bit away from that vent hole and closes when you start driving. Why? Well, if you're idling you need that vent open so as to let any vapors to vent to the atmosphere. Remember, air passing through the throat at idle should be low and won't have as much of an effect. Then when you increase speed, that vent hole closes and the carburetor is designed to revert back to the internal vent scheme. And the way these were designed (at least as I understand it) is if that vent hole is open when you're driving at speeds faster than idle then you'll run too rich. You'll run too rich presumably because the pressure in your float bowl will be higher and that wasn't how this particular model was designed to operate. It was designed to enjoy the same pressure in the throat as in the float bowl. At least that's what I think. But you don't have to worry about that with the 2100S. Like the 3211S it was designed more simply vent-wise. Hope that helps.