All three carbs have a bowl vent in the standard location, cast into the rear of the air horns. This provides atmospheric pressure in the bowl and that's why the engine runs fine regardless of whether the center carb has a working second vent.
The vent on the center carb, if present, should only be open when the throttle is at the curb idle position. Curb idle for an emmisions carb is fully shut; that is key off, so throttle-stop solenoid is retracted. In other words, the external vent is closed when the engine is running.
See here: pages 32.
http://www.hamtramck-historical.com/ServiceHighlights/1970PassengerCarHighlights09.shtmlWhether there is enough vapor that could exit the vent and
start a fire does seem rather dubious. Every vehicle until the vapor savers came along have open vents. The only difference being that most were protected by the air cleaner. Doing so keeps dirt out as there is air flow into the bowl as fuel flows out and fuel level drops. This is not to say that once there is fire, fuel vapor present would not contribute to its growth - of course it will!
