Adv tank install - fuel cap question

[Hambo]

I have a '95 1100 with the original plastic tank, which I am in the process of replacing with a 30L Adventure tank. Upon removal of the fuel cap I note there is a black plastic 'bladder' mounted below (see picture) which is connected to one of the two internal overflow / breather pipes. I assume this is in place to prevent overfilling? However, I understand from on-line micro-fiches that the Adventure tank does not have such a system, instead using some sort of saftey valve?

Anyway, I have no problem to re-install my original 1100 fuel cap, the only thing was, I was hoping to follow a mod posted on this site to maximise tank capacity by drilling a hole in the filler neck to release trapped air. If the black plastic 'bladder' is in fact an overflow system, then the filler neck hole mod could be waste of time? Alternatively, I could blank it off?

Basically, what the f*** is it and how can I best maximise my fuel capacity?

 

[John Armstrong]

I'd guess it's something to do with emissions control, ie you don't want nasty unburnt fuel spilling onto the ground. Best try and find some non US parts fiche if you want to get rid of that.

 

[Tarka]

That`s on mine too.....and it`s not a US model.

As far as I can tell,it`s a combined overflow and breather,as the filler cap isn`t vented.

On U.S. models I believe the other (outside,bottom ) end of its hose has a carbon filter on it....hence the warning in the Clymer manual not to mix up the tube with the one from the filler neck water drain....presumably as you don`t want to have a carbon filter full of water...nor to have unfiltered fumes or neat petrol escaping from the breather/overflow.

UK ones are unfiltered and I`m damned if it makes a difference if you ever do mix them up during a fuel fiter change (unlike mixing up the fuel lines..).

I`ve pondered removing the device and sealing off the rigid pipe at the filler end,and fitting a vented cap....you`d get quite a lot more fuel in the tank then......

Thoughts ??