Your problem is the hot ethanol fuel bubbling will not lift the float to close the needle and seat. You need to put a return on the system as they did on all carbureted engines 74 and newer. If your using a mechanical pump use the style in the fist picture. With a electric pump and a holley style carb use a return pressure regulator and the style in the second pictures.
I had the same problem. I use to start it with the pump off and turn it on and off until fresh fuel reached the carb. Once I installed the return it fixed the problem. I always install a return on all mechanical pump cars as seen in the first picture. Problem cured.
My race engine never did it with C16 race fuel. Only when putting pump gas to go to car shows.
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