All advice I've read seems reasonable, but solution could be very simple.  Had this problem on my 84 Westy and found that the breather tube that goes from the filler pipe to the top of the tank had fallen off.  To check, turn wheels all the way left and slide your hand down the side of the filler pipe.  You will find a nipple and hopefully a hose a couple of inches down.  Check that hose first as it can be replaced, with a little effort and moderate flexibility, by reaching down through the wheel well.  If its not that hose, you're probably looking at one of the other problems mentioned and a more diddicult repair.
Good Luck!
Cory Sammons
84 Westy
N. Bridgton, ME

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