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fuel cap vented or unvented?

Posted by chrisv

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fuel cap vented or unvented? #1
chrisv chris vine Gawler , Adelaide, Australia 1979 MG MGB	Posted: August 19, 2012 02:10AM Registered: 1 year ago Posts: 383

Need to replace my rusty fuel filler cap on my 79 B.
Do I need vented or unvented?
Cheeers

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Re: fuel cap vented or unvented? #2
MGB567 Barrie Braxton Sunshine Coast, Australia 1966 MG MGB MkI "Money Guzzler"	Posted: August 19, 2012 02:32AM Registered: 2 years ago Posts: 5,299

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=230

M-E only has vented so I assume the answer is yes?

CKD 11/66 first registered 8/5/67 owned since 3/77. Undergoing nut and bolt restoration. Tartan Red (now). 18GB +40 balanced with almost all new internals. Peter Burgess big valve fast road head. Piper 285. TT exhaust. ARP everywhere. 123 ign. Needham 4synchro c/r box. Stock rebuilt/replaced suspension. Superpro bushes. New brakes all round including all pipes in SS flex. Interior redone. CAMS approved roll bar and side bars. Lots more. Hybrid of o/e and show/fast road car. Not for sale - it's my toy!

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Re: fuel cap vented or unvented? #3
Peter-Sherman Peter Sherman Melbourne, Australia 1975 MG MGB V8 Conversion	Posted: August 19, 2012 09:22AM Registered: 5 years ago Posts: 4,279

if you don't have a charcol canister, vented.
If your's is ex American you will need to get an American cap, or replace the neck, which is often cheaper and you can get lockable versions of cap for get real $. Adrian Ackhust of AAAutomotive will oblige, he's local to you
http://www.britishv8.org/MG/AdrianAkhurst.htm

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Re: fuel cap vented or unvented? #4
chrisv chris vine Gawler , Adelaide, Australia 1979 MG MGB	Posted: August 19, 2012 05:06PM Registered: 1 year ago Posts: 383

Thanks
Is there a way to physically identify whether the cap is vented or not?
Mine is ex USA with charcoal cannister but has twin SU conversion.

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Re: fuel cap vented or unvented? #5
Jim Lema Sunny Seattle, USA 1967 MG MGB	Posted: August 19, 2012 05:14PM Registered: 9 years ago Posts: 1,431

The charcoal canister had a line that runs back to the fuel tank. There is a gas/vapor separator on the right hand side of the boot fender. If that system is still connected, then the tank is vented and you can use a non-vented cap. If you have removed the lines from the system you may need a vented cap.

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Re: fuel cap vented or unvented? #6
Peter-Sherman Peter Sherman Melbourne, Australia 1975 MG MGB V8 Conversion	Posted: August 19, 2012 08:13PM Registered: 5 years ago Posts: 4,279

If your's is like mine the American filler neck has the 'attaching flange' on the outside of the neck. The British/Aus style has them on the inside. If you want to turn a non venting cap into a venting one, drill a tiny hole about where the blue X is, The center rubber seal is on a spring so it is compressible. The hole bypasses the seal.

You may already know but the idea of a "sealed" tank is that any air that gets in or out has to pass though the charcol filter away in the engine compartment. This charcol canister is under a slight vacuum and a line runs back to the drop out vessel up in the corner of the boot, and a line runs down to near the filler neck.
An issue we currently have is that the new fuel tanks don't have the 1/8" (appox') vent outlet under the filler neck. I did try running without a sealed tank but found that the EFI heats up the fuel a little and , there was a constant, quite strong, smell of petrol via backdraft. I put a vent line into the filler neck itself, which then acts as a vapor collector.

~~http://www.mgexperience.net/archive/Line_off_the_Gas_Tank_and_what_its_for/1287508~~

Actually , that last photo from crossed out link is wrong, I'll deleted and replace it with a correct configuration. hose shoul go from tank to bottom of drop out canister. Then from top of canister to engine compartment.

Edited 1 time(s). Last edit at 08/19/2012 08:27PM by Peter-Sherman.

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Re: fuel cap vented or unvented? #7
chrisv chris vine Gawler , Adelaide, Australia 1979 MG MGB	Posted: August 19, 2012 09:43PM Registered: 1 year ago Posts: 383

Thanks
All clear now, a picture tells a thousand words
Chris

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Re: fuel cap vented or unvented? #8
MGB567 Barrie Braxton Sunshine Coast, Australia 1966 MG MGB MkI "Money Guzzler"	Posted: August 20, 2012 08:18AM Registered: 2 years ago Posts: 5,299

Re #4 Chris if you'd mentioned your car was ex-US then I wouldn't have answered at #2 (as I know next to nothing about US cars but I would have guessed that they were vented via their smog set-up). But fortunately Peter also has an ex-US car so now you're cooking with gas!

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