[Formulajones]

Originally Posted by Ben M.

The carb port for the vent is not a true vacuum port, it's much weaker than that and doesn't pull much vacuum at all (stick a vacuum gauge on it and see). You can unplug it and it won't cause any trouble with the engine idling. On the 2 port canister cars, it doesn't pull a vacuum on it and just captures the fumes passively. On '72, the only time they didn't want it functioning was before the engine had fully warmed up as it increased emissions ever so slightly - obviously not enough to keep the system in later years.

The standpipe vent at the tank in '69 vented the fumes to the atmosphere and in the name of reducing emissions and reducing smells of gas, they closed the venting system more and more as the years went on.

On my 70 Formula, with the Quadrajet, the canister is actually a 3 hose system. One is like you said, a vent to the carb, not a vacuum assist. But it does have a smaller 3/16" hose hooked to vacuum on the carb. A 3rd hose going to the tank. However no visible switch on top of the canister.

Odd how these systems vary from year and model throughout their use. 2 hoses, 3 hoses, vacuum switches, no vacuum switch.

You make an important point on the 72 comment and not wanting to work until the engine warmed up. That's why the vacuum hose that ran from the switch to the carb runs through a thermal switch on the intake first, that triggers vacuum at a certain temperature. This switch was also used for things like vacuum advance.

I bypassed that on my 72 Chevelle and no longer have the thermal switch installed so the charcoal canister works full time. I've had it this way for the last 35 years with no ill affects.