Quote:
Originally Posted by Elrod
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.
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The carburetor does not suck gasoline from the tank. The gasoline which is sucked into the manifold is from the bowl. The bowl is vented to the atmosphere and always at atmospheric pressure. The gasoline level in the bowl is controlled by the float and float ("needle") valve. When the fuel level drops in the bowl the float valve opens and more gasoline flows into the bowl. No vent in the gas cap keeps stops gasoline from flowing into the tank.
Cars with fuel pumps are a different story. If there isn't a vent into the fuel tank then the fuel pump may cause the tank to collapse as it pumps fuel out of the tank since there won't be air to replace the fuel.