Originally Posted by dkamp
No, it needs to be vented to atmosphere external to the engine... totally ambient...
And just in case someone's wondering... no fuel gas vents through the vent- the vent's purpose is to allow the demand regulator's diaphragm to modulate properly. If it was sealed in a cavity, the pressure inside the cavity would prevent the diaphragm from moving in response to demand vacuum... on a cold day, pressure in the cavity would 'lock off' the fuel supply, and on a warm day, pressure would be so low, that it'd flow fuel with no engine operation.
If you look at the Garretson KN, you'll see that one of the features is a 'primer' button. This is generally used to purge air from a propane line when an exchangeable bottle is used... everytime you remove the fuel hose, propane in the hose wanders out, and air wanders in... when you reattach the tank, air in that hose has to get pulled out... and it takes lots of cranking to get it started up. Press the PRIME button for a little bit, and all the air will be purged out. Well, if you look closer, you'll see that the vent is not far from that purge button... and the purge button simply presses on the diaphragm.
The fuel controller works on the principle that air pressure inside the VENTURI of the carb (the part where it's narrow... where a liquid fuel's main jet would normally be found) is LOWER than ambient pressure.
Realize that a carbeurator works because the pressure drop inside the venturi LIFTS fuel up from the carbeurator bowl, because ATMOSPHERIC pressure is in the bowl, pushing DOWN on the fuel level. If you were to plug off the carbeurator's vent... or connect it to some other partial-vacuum location, then the fuel draw from the bowl would be incorrect, and worse yet, unstable... so your fuel mix would be likewise errant.
The fuel controller (zero governor, negative pressure regulator) takes place of the liquid fuel's float bowl level... and the needle and seat... in one simple component.
And as an aside note... the 'power valve'... the large adjustable valve between the fuel controller and the venturi feedpoint... has NO control of the mixture at ANY point other than full throttle. It simply limits fuel at full throttle. At any other speed, it's the fuel controller's gain adjustment, which is essentially linear, based on the difference between ambient atmospheric pressure, and the demand pressure signal it senses in the venturi.
I entirely understand the purpose of the vent and how the regulator works.
My understanding is carburetor bowls on better setups are typically vented into the air filter, so the bowl sees the same pressure the carburetor sees. Besides keeping dirt out of the bowl, this also tends to help keep the balance correct as the filter gets dirty etc.
My thoughts were if you vent the demand regulator in the same manner, it's seeing the same thing the inlet of the carburetor is vs the venturi vacuum, consistently including as filters get dirty.