Car in bits again...
AO Silver Member
Join Date: Feb 2007
Location: Hampshire, UK
Sealed fuel systems need a 2-way valve. When you use fuel the valve needs to allow air into the tank to replace the fuel, otherwise a partial vacuum can form and restrict the flow of fuel to the engine.
On the other hand, the fuel expands as it is heated, and the valve needs to bleed off this pressure otherwise it can overwhelm the needle valves in the carburettor and before you know it, you have a sump full of petrol. That's a pretty extreme occurrence, but it has happened to me on another car.
A small vent hole drilled in the filler cap can work, but there will be some evaporative loss of fuel.
I used to have my fuel tank vent to air via a simple roll-over valve, but the garage used to reek of petrol after a day or two. I now have one of the Tweeks bi-directional valves on the end of the breather pipe, and there is no noticeable fuel loss now.
The Americans are very hot on evaporative fuel loss, so the valve you have may be fine for the job. Try blowing through the valve in either direction - there should be some minimal resistance each way.
Alfanauts are GO!!!