My 69 won't start with under 16 gallons- help!

[bmxmon]

haha, thats really odd. So if you have it filled it would continue to run until its out of gas, but if you let it run until under 75% full then shut it down it wont start back up? Are you trying to start it up again slightly after? Im wondering if your fuel line is running too close to something hot and is cooking the fuel, causing vapor lock. I wouldnt think you have something inside the tank since it would have suction while running but not while off, meaning whatever crap may be inside the tank would stop fuel while running.
I would check your fuel line route and make sure it stays away from hot things, even the intake can heat the line too much.