Non-vented Fuel Caps
The USA spec fuel cap fitted to many MGB’s & Midgets was unvented and as a result sometimes there is a big suck of air when you undo the fuel cap. – to such an extent that the bottom of the tank on the Midget of ours with such a cap gets sucked up a couple of inches and there’s a big metallic clang when the cap is taken off and the tank fills with air! The obvious question that then arises is whether this affects the fuel delivery from the pump.
Our experience is that it does occasionally play silly beggars with the fuel flow but not often. It seems that the only time it interrupts fuel supply is when fuel is low and the bottom of the tank is sucked up the pick-up is left high & dry on an island of tank bottom with the fuel around the edges. And the gauge is fooled – reads nearly 1/2 too! We have learned that when the fuel gauge starts to increase it’s a warning of this about to happen!
So in summary the non-vented cap doesn’t seem to inhibit fuel delivery when the tank is at negative pressure. And presumably the very large number of cars sold in the USA would attest to that too!