Probably a silly question, but what actually is this for? I read somewhere that you can put a bicycle tyre valve on it to stop fuel spillage, but which way round?

The basic question is: is it to let out excess fuel if pressure builds up (e.g. on a hot day, or to let some air in to relieve the vacuum effect when some of the fuel has been used (or both)? If both, then the tyre valve idea would seem to prevent air/fuel going in one direction

Anyone got any ideas?

Both.

Just run a length of tube up from the tank and then back down to just below the chassis

Doesn't fuel then evaporate?

IIRC from reading somewhere on here, the tyre valves method used 2 valves. One facing one way, one the other on different pipes. One lets air in, to replace the fule as its used. The other acts as a breather to let any pressure build up due to heat etc out...

Why cant someone just make a fuel cap with the breathers in? Or are they available?

Whatever system you use fuel will evaporate unfortunately....

David

It will but in tiny amounts. Until the advent of emmissions tests all cars were vented this way.

Now most modern cars vent into a closed carbon filled cylinder, to store the vapour. At engine start up a solenoid vents this vapour into the intake manifold - this keeps the vehicles overall emmissions lower

My fuel cap does have the vent built in - it's off a Caterham.

[Edited on 14/2/05 by JAG]

I ended up using a breather valve from a jet bike, only £5. Allows air in easy, but very little air flow out (just enough to allow for heat expansion). As most jet bikes spend a lot of time upside down in the water they must work just fine!