You likely answered your own question when you stated that your tank is filthy. Picture flat flaky pieces of crud swirling around in the tank ending up clogging the hole in the bottom of the tank (if that represents your tank design) or the screen on your in-tank fuel pickup. This type of fuel delivery malfunction can be frustrating, as it can come and go when the crud moves around, on and off of the fuel pickup or the hole in the bottom of the tank. My recommendation, regardless of any other issues that you may have, would be to have the tank cleaned at a radiator shop or better yet, buy a new tank. I have suffered your pain, and swiftly solved it with a new tank.
Ditto on the tank. FYI, I took my original 49 3100 tank to a radiator shop and had it boiled out and pressure washed and it still filled my fuel pump and fuel filter bowls with rusty debris.
Many people have had good luck cleaning their tanks out and resealing them so that is an option, I went w/a new tank and haven't looked back.
I believe it's already been mentioned but you should disconnect the fuel line at the tank & fuel pump then blow it out to see what comes out. Very old fuel makes a hardened molassess type gunk that will clog lines etc. My fuel line had it. It wouldn't even pass any fuel. I filled it w/carb cleaner and let it sit over a weekend w/one end elevated. What eventually washed out was what I described earlier. I must have went through 2 cans of carb cleaner before I got nice clean fluid out of it. It works great now. I drive my 3100 every weekend.
Good luck
Dave
