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MG MGB Technical - Vented Gas Cap ?

Hi Everyone,

Quick question. I bought a gas tank from England for my 1979 B and it does not have the threaded hole for the fuel vent line. Rather than taking it to have a connection solder/welded in I have heard I can just use a vented gas cap as it does exactly what the fuel vent line does. Also in the previous post it says the old vented gas cap does not fit on the newer model (like my 79 B's). Is this correct? Is there anywhere in the states I could buy the correct vented gas cap or do I have to buy a filler pipe that will work as well??
JRB Mr

JRB - You will need to purchase the early type filler tube in order to install a vented cap. Both should available from Moss. Cheers - Dave
David DuBois

You should also seal off the old vent line between the tank and the canister as left open it is a source of unfiltered air into the engine, bypassing the charcoal granules and so not purging them. It will also disable the anti-runon valve.
Paul Hunt

Thanks Paul. I am pretty sure I have all the lines disconnected as I have fitted SU carbs and desmogged. Ill double check.
James

On the inside of your cap is a circular rubber seal attached to a springy metal base which compresses a little to give you a good seal when the cap is put on. Just drill a very small hole inside the circle of the rubber seal, in the metal base. This will not be visible when the cap is on.
Or attach a hose fitting to the rubber joiner that joins the tank to the neck. The neck will then act as a vapour trap.
Peter Sherman

Thanks Peter, I was thinking about doing that. Just worried when filing her up with Gasoline. If the vent pipe becomes filled with gas.

Thanks

James
James

I bought a new tank a while ago when I was fitting the V8, same deal, no vapour vent. I had to get an engineers certificate for the engine swap and wanted to leave the emissions set up as close to original as possible. Also with the fuel injection system the fuel recirculates from the engine and the tank gets quite warm. With a venting cap you will get a smell of petrol backdrafting as you drive along. I tried it. It rather spoils the whole "open top motoring, fresh country side air" deal. I doubt that this would really be a problem for you so local regulations permitting, the pinhole in the inside cap seal would be easiest. Not very ecofriendly though.
Don't drill all the way through the cap, just the inside springy bit so that the rubber seal is compromised. You might try just pulling off the rubber seal. Mine slips off and on quite easily.

I eventually attached a hose fitting to the metal neck.
In fact I later read that just prior to the the introduction of the vapour vent tanks, in order to meet regulations, the factory did pretty much as I have done. That was about 3 years and >50000 km ago. No problems at all except when removing and fitting the neck (collision repairs etc) because of the fitting I need to pull off the boot petrol cap rubber surround (the bit you see from the outside) off backwards to get the neck clear of the boot. Minor nuisance. I missed the obvious, and much too late I thought of attaching the fitting to the rubber neck tank joiner. 2 big washers either side and a tight fit would squeeze the rubber and make a good seal. Of course the rubber necks are generic, buy it by the meter stuff (most auto places) so it doesn't matter if you mess it up.

The vent tube should go to the bottom of that drop out canister/tincan up behind the tail light. The top outlet of the canister should go (via under the car)to top of the charcoal canister in the engine compartment,there should also be a vacuum line going between the top of the cannister and the manifold. The big bottom outlet of the charcoal canister should be unobstructed. The idea is that when the engine is running there is a slight vacuum in the top of the charcoal canister which draws any vapours from the petrol tank. When the engine is not running any vapours will be filtered out by the charcoal.


If petrol gets in the neck vent tube (from either filling or from tremendous surges of acceleration :) ) it gets drawn up to the drop out canister behind the tail light in the boot, and just stays there until next time the engine is off. Then it just runs back down to the tank. Thats what it's for. If your drop out canister/can has dissappeared, a very large efi type fuel filter will do. One of those soup can size ones (About $20 Kmart). Any sealed container with an outlet at the top and at the bottom.


Peter

"With a venting cap you will get a smell of petrol backdrafting as you drive along"

Depends if you buy a vented cap or just drill a hole in a non-vented! Pukka vented caps have a spring-loaded valve that only opens when a certain amount of vacuum (much less than would be required to cause fuel starvation) has developed in the tank. This prevents fumes escaping, as well as fuel in a roll-over.
Paul Hunt

This thread was discussed between 15/04/2009 and 24/04/2009

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