> Is this correct, or is there some piece missing that is supposed to make
this a
> one-way flow?
Yup, that is exactly correct. The main function of the carbon canister is
to absorb and hold the gas fumes while the engine is not running. In order
for that to work, there has to be flow from the carb bowl vents (and fuel
tank vent) into the canister. Then once the engine is running, fresh air is
constantly pulled through the canister to purge fuel from the carbon and
leave it ready for the next cycle.
The vacuum connection is only to the "constant depression" part of the carb
(in front of the throttle plates), so normally much less than manifold
vacuum.
> I had always assumed that the suction would pull on the crankcase and
> the gas tank,
It's only a very small suction at best, since the whole system is open to
the air through the carbon canister. But there is some, because the
canister itself (and the lines to it) presents some restriction to flow.
-- Randall
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