Fuel System Breather Valve info

In an attempt to answer the question "Why does my tank woosh when I take the filler cap off" I have opened up a breather valve.  The pic below shows from right to left:

1. Black cap glued to gray body - outlet goes down to the back of the tank to the trumpety thing
2. Cream cap with one way valve (lets air in only) and small hole in centre
3. Cream Valve seat
4. Valve carrier - with valve pin that seals the hole in the valve seat when it lifts
5. Ball weight
6. Valve body with outlet that goes to the tank



So, as far as I can see, there are three possible scenarios:

A - Tank heats up and air/vapour wants to get out
B - Tank cools/fuel used and air wants to get in
C - Tank causes fuel surge into breather

The top of the unit is the black end.

C is straight forward I think - The fuel surge will lift the valve body and close the valve ensuring that no actual fuel escapes

A - A slow release of air/vapour is possible past the gravity valve and through the single small hole in the cream cap

B - Fuel use and tank cooling will create a slight drop in pressure and would suck air in - this would come in via the one way valve in the cap which has 4 holes and through the unvalved hole

So either way the tank should pressure balance over a period of time.   I always argue that the tank should wooosh (suck in air when opened) as this is what I've experienced with loads of cars.

I suspect that this is because the amount of fuel used is greater than the air drawn in due to the small holes.

This would mean that eventially you'd get fuel starvation due to the low pressure in the tank or the tank would collapse!!!  Clearly this is wrong and the stripped valve would indicate that no wooshing should be heard.

Over to you lot…..

Come on........somebody

took mine apart, it got blocked, made is rly hard to start…not delivering enough fuel, crap running, and fuel leaking out through trumpet breather, massive woosh, petrol coming out of petrol cap etc….

it caused a hell of alot of problems, i replaced with a second hand one, and its started doing it again….thing is, it can only get blocked up when dirty fuel goes up the breather pipe, and the valve doesnt work….dunno y really…inline filter did have a tiny bit of crap in it…which is enough to block something that small i suppose….

u mentioned a while back u had loads of breather valves, how much wud u want for one inc PandP

Thanks Josh

replacing mine today , just want to check the pipes are right before i replace them just in case that is the problem and not the valve.. black pipe onto the top and clear pipe onto the bottom of the valve ?

just answered my own question, i used the 'suck it and see ' method. first pipe didn't notice much, breather pipe. next one lung full of petrol fumes. must be connected to the tank, sorted.   ps don't try this at home kids  :wink: