Hi,

I have the classic "clunk" that I believe is the gas tank collapsing
from the vacuum.  I know something isn't correct, because when I take
the gas cap off after a drive, I get a great hissing as air goes into
the tank, and on occasion, I get the clunk.

I had my 91 Westy to the mechanic for some other stuff & he had a quick
look at the charcoal filter valve while he had it on the hoist.  He
indicated that it was sticking, but got it working again.  Alas, the
"clunk" didn't go away.

Now that I have a bit of time, I am looking into this myself.  I don't
understand the theory of the system though.

I BELIEVE the idea is that when the gas tank expands with heat, the
vapors are supposed to flow out through the charcoal cannister, exiting
at the bottom (when the engine is not running).  Likely, when the engine
is running, the vapors should go to the air cleaner?

What confuses me a bit is how the system is supposed to work to allow
air INTO the tank with fuel consumption.

Holding the cannister in my hand, It seems like I can suck & blow any of
the connectors & air flows.  It doesn't seem to have any valving inside
the cannister, though the resistance is quite high (but I am using my
mouth, remember!).   It seems to me the way that the system is plumbed,
air should just come in the bottom of the cannister to equalize the
vacuum.  But it can't be that simple..

As a proof of concept to ensure the "clunk" is truly the gas tank, I
have disconnected the line that runs from the gas tank to the charcoal
cannister & put a fuel filter on the end of it to keep dust/dirt out of
the fuel tank.  That should allow very easy air flow to the tank and
prevent a vacuum from forming.  I know I'll have to be careful to not
fill the tank too full.

Can someone enlighten me on the theoretical workings of the charcoal
valve?  The Bently describes all the bits & testing procedures, but
doesn't actually describe the workings of the system.

Thanks,
Steve Williams