I'll just bet your pump is sucking air, or the tank is not venting.
Sucking air is usually because the jumper-hose at the back is perforated; the hose that connects the sender to the hardline.
Some cars use the charcoal line for venting, which eventually rusts up solid.
So try again with the cap cracked. If the problem persists, check/ replace the jumper hose. If yo have to use gearclamps, install 2 on each side with screw-heads staggered 180*.
If you still have a problem; do a fuelpump delivery test. With the engine warmed up and idling slow as per normal, reroute the fuel line to a clear plastic container;the car will run a minute or two with the gas that is in a full bowl. I'll bet the pump is lazy cuz the arm return-spring is gone weak or just gone. If it is lazy, fill the carb again and this time run it at ~500 rpm for 1 minute. The pump should deliver 1quart when new.
For a lo-perf slanty I wouldn't throw it away until it gets to half that..... unless it is lazy; in which case just pull it off and check the spring and the arm.
