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.
|