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