This is a subject we tried to beat to death in a previous forum thread that may have been a year or two ago, but I'm pretty sure we didn't reach a conclusion and a solution at that time.
Many of us seemed to be unable to fill our tanks with gas (or that which passes for gas these days) and have the nozzle automatically shut off as it should when the tank is full. I and others know this to be a problem on our Cs and Ds, and as I recall the complaints came in from owners of of several other years of Letter Cars as well.
An interesting (and ironically telling) facet to the conversation was that some folks said that they didn't remember having the problem many years back when their car was near new, but it now has become an issue they can't figure out. Why should it now be any different than it was?
I think I have finally got the answer. I wish I could take credit, but some member who saw it on the forum - and whose name I apologize for not remembering - pointed out the cause which is simultaneously both obscure and obvious. If you look inside the filler tube, you will see a long metal stamping attached to the inside of the filler tube. That stamping forms a vent and that vent must be oriented on the TOP of the filler tube to work right.
The reason the problem didn't occur on new cars back in the day was that the installers knew about this quirk and put the tube in correctly. But when we restore our cars and pull the tank down, we don't notice the feature inside the tube, and, as in my own case, we may even install the tube back in the car exactly upside down.
The solution is easy. On cars with a straight filler tube running through a gasket at the quarter panel and an O-ring at the tank end, you just have to spin the tube until the vent stamping is at the top. I know this works on the C, and maybe it does on all years of Letters.
I had a special spanner wedge tool welded up that neatly catches the cap notches at the top of the filler tube, and in 30 seconds I re-positioned the orientation of the tube. For those of you who have the same problem, you can try your own method of rotating the tube if you like, but my offer is to let anyone use the tool I made if they wish.
I have sent the 2 pound tool off to Bob Porecca in New Jersey to use it first, and he can cover the shipping to send it on to the next person needing it and so forth. We'll just keep it travelling around the country to where its needed. Just let me know if you want it and what car you're going to use it on, and I'll track it and let you know when it might get to you.
If you want to see photos of the tool, let me know off the forum and I'll email them to you.