Some AVS carburetors have a hole on the top of the float bowl that closes when the throttle is closed. Why? Why do some applications not have it?

it's a feature that appeared on the later emissions carbs.

Are you talking about the rubber flap that overs up the hole in the top when the throttle is applied?? My original AFB on my 67 440 has this feature also....4327S Carter.....

Yes. It's connected to the accelerator pump linkage.

It's called a bowl vent and it opens at idle so pressure does not build up in bowl and push fuel out into carb which would cause a rich/rough idle.

The AVS carbs that the bowl vent near the acc pump are for non-ecs cars.The carbs that don't have the bowl vent on the acc side have a nipple on the r/h side of the carb in the top half near the choke linkage,a hose runs from that nipple to the crankcase breather on the r/s valve cover.This style carb is for ecs cars that have a "sealed" evap system. John.

when gov't emission regs really started to get tight in '68 they no longer allowed gas fumes to be vented directly to the atmosphere. Not sure why it has to be open at off idle (either to the atmosphere or to a charcoal canister)

If it wasn't open to the atmosphere, once the fuel level in the bowls dropped under acceleration, it would create a lower pressure in the float bowl, preventing fuel from flowing out the discharge nozzles.

Quote:

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If it wasn't open to the atmosphere, once the fuel level in the bowls dropped under acceleration, it would create a lower pressure in the float bowl, preventing fuel from flowing out the discharge nozzles.

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The valve closes at any throttle position other than idle. The bowls are vented primarily through the air horn to insure pressures above the fuel in the bowls equals the pressure above the venturies, regardless of any potential air flow restrictions (i.e. dirty air filters). The idle bowl vent intended purpose is to release evaporating fuel vapors outside the air cleaner under hot idle & heat soak engine off conditions. Without the idle vent valve, fuel vapor collects in the air filter housing making for hard hot starts and unstable over rich hot idle.
As stated, for E.C.S. the idle bowl vent valve was enclosed and fitted with a hose nipple to collect and later reuse these vapors.