yeah, it may sound dumb, but it sure sounds like a pinched vent line. easiest enough to check - the tank will look like it imploded. i don't think it is a carb problem. but for fun, i would try a different fuel supply (on the ground) and see what happens. sure sounds like the tank is draining about halfway, and then not drawing fuel - i would suspect some kind of crack or leak in the tank carb line - including the brass tube. another thing worth trying - with a friend - is to fire her up, and then hold the airplane vertical and advance the throttle. if it dies fairly soon, that's a pretty good indicator that you have a fuel draw problem. oh yeah, the other possibility is that fuel lines are reversed. we have all done it...

P