[Jack21]

You should have a vent tube on the tank that allows air in as fuel is consumed. Ideally, this should be a check valve so air only vents in. Since the early '70's, this inlet vent has connected to a carbon cannister to trap fuel vapors from venting to the atmosphere.

Without a vent, in the cap or tank, to let air in, a vacuum would form in the fuel tank and not allow the fuel pump to pump fuel to the carb. The engine would stall as if running out of fuel.

Heater control vacuum valves can be used to do this.