Update.

Captain Obvious had the right idea ... looks like the tank itself is clogged. Pulled the lines off the tank with it 1/2 full and ... NOTHING. Not a dribble of gas. Drained the tank and dropped it. The inside looked like it had shag carpet the varnishing was so bad. Have run POR-15 tank cleaner through it a couple times and its looking better, but still can't even get compressed air through the return or supply. That doesn't sound normal. I'm guessing they are blocked somewhere in the tank. The cleaner doesn't seem to be clearing it. Any ideas?

I did start it up off of a gas can after I pulled the tank ... sounded great! Of course it still doesn't have an exhaust so it sounded really nasty ... my son wants to leave it that way :-)

Bummer. Shag carpet!! That's nasty! The good news is that you've identified the problem, right!

I think there's some internal baffling and some kind of filter inside the tank. I've (thankfully) not had to mess with that myself, but I think I remember some threads about it. And some pics.

I had the same problem and I use an extra speedo cable and plunged the metal line in the tank and it freed it up.