Mk 2 Carb - Filler Cap

[RussGLAuto]

Afternoon gents,

Quick question, Mk2 Carb car, vented or non vented filler cap?

My original vented (I think ) cap didn't seem to provide a seal with the filler neck as I could remove it without unlocking it, filling my garage with fuel vapour. Not ideal!

Bought a replacement from Halfords. It fits, but when I unscrew it there is a huge 'gulp' from the tank as if negative pressure has been created in the tank.

No, they don't look the same...

Trouble is, my original obviously has a key barrel in it. How do I change the barrel if I buy a new vented cap??

Ideas on a reply please...

Thank you!

Russ

[GT_II]

Are you sure your old cap doesn't just need a new seal?

The original fuel cap incorporates a one-way valve, allowing air in to prevent a vacuum forming as the tank empties, but holding in pressure to prevent vapour loss. If you blow and suck into the hole at the inside end of the cap, you will see what I mean. I usually find the tank is pressurised when I remove the cap to fill up, presumably as the fuel releases vapour as it returns from the hot engine.

I don't know if this would count as vented or unvented.

[Kdc169]

to be honest on my red mk1 gti sportline, the tank creates a vaccum when running and i end to open cap every 2 trips out to let it breathe, ive tried various things like cleaning antigravity valve, pumps, new caps.

[RussGLAuto]

Are you sure your old cap doesn't just need a new seal?

The original fuel cap incorporates a one-way valve, allowing air in to prevent a vacuum forming as the tank empties, but holding in pressure to prevent vapour loss. If you blow and suck into the hole at the inside end of the cap, you will see what I mean. I usually find the tank is pressurised when I remove the cap to fill up, presumably as the fuel releases vapour as it returns from the hot engine.

I don't know if this would count as vented or unvented.

I did think that, and ended up making my own out of 2mm rubber. Didn't last 5 mins and split. I can't find a part number or any for sale anywhere without buying a whole new cap.

Ay ideas?

Cheers

Russ

[GT_II]

I think the fuel cap seal is 811201557A, available as NOS.

[RussGLAuto]

After some searching, it seems the cheep Halfords cap is the non vented version. Hence the vacuum building in my tank even after a short run.
The Carb models require a vented cap to avoid damaging the tank with the negative pressure.

The below link takes you to a catalogue of specialist fuel necks, caps and guess what, seals! The closest to the OEM cap is on page 17 - T28 cap. Will try and match that with the seals on page 32...

https://www.blau.co.at/data/Kataloge%20 ... 9_BLAU.pdf