no fuel to carb on crank

when i try to start my 1975 Plymouth Valiant 318 after letting it sit for a few days it cranks, starter engages fine, but no firing, pour fuel down carb starts dies 10 seconds later, repeat a few times, run time gets longer until it maintains a idle, my guess is after a while fuel is flowing back down the line into the tank, as no smell of fuel or leaks on the ground, but not sure, any ideas?