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MG Midget and Sprite Technical - Fuel leaking out at cap

Hi folks. I have this age old problem, which affects many BMC products in that if well filled or used enthusiastically petrol comes out at the cap and runs down and stains the paint work etc. Has any one found a solution in a well sealed but vented cap, or an in filler baffle? Any wisdom appreciated.
Slowly

Hi Slowly(?),
I'm the first to call out on p*ss-poorly made parts but these are usually the modern replacements, the original parts were on the whole long lasting and reasonably made.

I often fill my tank to the very edge of the back of a vented cap and drive enthusiastically but only have problems if I do so and the weather get unexpectedly hot.

I put a cheap vented cap on 9 years ago with no problems so if you have the problem through different vented caps it could be the problem lies elsewhere and not the vented caps.

Just quick (and not thought through) thoughts -

. do you have an oversized or under insulated back exhaust box perhaps?

. a petrol pump that needs venting(?)

. you should have an un-vented cap?

More details of your car might help - model, year, fuel system fitted, standard/non-standard, etc..

More and better answers will follow from other more knowledgeable posters, good luck.
Nigel Atkins

Apart from export-market cars with complicated emissions control equipment, all Spridgets have single fill and feed pipes on the tank, and all breathing / venting is done throught the cap therefore this has to be a vented type.
Original caps do suffer from shrinkage and deterioration of the sealing rubber, and will eventually leak. New replacement caps are very likely to use crap quality 'rubber' seals and may well leak after a few months.
One solution that worked for me was to carefully cut a new seal from an old inner tube and fit it to the original cap. You can also make a new seal out of leather or plastic if you can find suitable quality and thickness material.
David Smith

Thanks for the responses. Car is a frogeye, with original cap which feels very "tight", but the seal may not be effective. The car gets used for autotest and fuel leaks even when down to a couple of gallon.I will try a new seal. Thanks
Slowly

If all else is well then lots of modern things have a flappy flap part in the filler pipe that seems to stop this happening
Rob Armstrong

If you are using the original cap it is vented to atmosphere without any type of non return valve so fuel, can be pushed out of the breather hole. You either need to fit a later cap that has an integral non return breather or fit a non return breather valve to the fuel filler neck and then seal the original vent hole in the cap.
Daniel Stapleton

Just dont fill it up all the way isat i do... my past caps did this and leaked

also my seal around the sending unit leaked also

prop
Prop and the Blackhole Midget

This thread was discussed on 04/11/2016

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