Gas tank hook up 71demon

Son's demon has a gas tank that has a vent atthe top of the tank and the sender has 3/8"inlet , there is a second line there i beleive 5/16 "dia. Where do we hook the 5/16 line to ?or do i block it off?

We are having fuel delivery issues with this car , starts and runs then stalls and then no fuel in carb , changed filter , checked pump it seems ok , when you take the fuel line off at the pump should the fuel just pour out of the line ? This doesn't any help would be great thanks

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So does it starve for fuel after driving awhile?
Or does it starve out after initial start up?
What are the conditions when it starves, Hot? Under a load? Idling?