[Jason in TX]

Yeah. Your carb is sucking gas out of the tank. If there is no vent, eventually it would in theory create such a vacuum as to start imploding the tank.

The same theory as if you had an air tight bottle full of water and a straw. Start sucking the water out of the bottle using the straw. if you kept drinking without letting air it, it would start caving in the bottle, and would be really hard to suck the water out.

Your carb is having a hard time sucking the gas out of an air tight tank. When you sit, air gets back into the tank and it works again.

You don't want a vented cap on a radiator because the water would bubble out the cap.