I has this, and ended up replacing the venting stuff per Van-Cafe:
http://www.van-cafe.com/home/van/page_42_114/gas-overflows-when-you-fill-your-tank.html

Problem solved :o)

On Fri 01/13 2017 07:40, Gordon wrote:
> I've been having a harder time filling my 89 Westy's gas tank more and more. Something is blocking or causing back pressure such that the gas station nozzle is made to stop. I am forced to slowly feed the tank an ounce at a time and it takes forever with much spill over.  I've tried several stations and it isn't them, I'm afraid.
>
> I've heard of some filter? Or charcoal? In the way there?  Anyone know what part may be the culprit?
>
> Thanks,
> - g
>

toodle pip,
--
       Dug Smith          | Terry Pratchett: It's almost
(mailto:dug@dugbert.com) |  impossible to ride a rock and roll
(http://www.dugbert.com) |  motorbike and stay on for three verses.