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MG MGB GT V8 Factory Originals Technical - Fuel tank venting

Hi All,

I know that many of you have done away with the charcoal canisters. I would like to know what's been done with the fuel tank venting on the later models.
Can the canisters be used with just vacuum applied?
My car had all of the canisters disconected when I bought it and I didn't put them back in when I installed the V8.
I'm getting tired of a stinky boot.

Kelly
Kelly Combes

If you have a later car, you can put in an early filler neck and vented fuel cap and plug off the emission lines. Or if you dont want the hassle of changing the filler neck out, you can drill a tiny hole in the non vented cap.
Jake

I plugged that vent line that comes up through the trunk, then I drilled a small hole in the center of the hinged flap inside of the filler neck, and then I drilled another small hole on the side of the filler neck just outside of the car, but almost under the gas cap. I can take a picture if that would help.

Justin
Justin

The gas smell in the trunk is most likely due to the deterioration of the 5/16" rubber lines in the trunk that connect the fuel tank to the separation tank mounted on the right hand side of the trunk.

In my 4 cylinder days my original rubber lines broke off and my entire luggage smelled like gas.
Jim Miller

The venting hose goes from the tank to the separation tank in the trunk and then through the floor of the trunk to a steel line which runs under the car to the charcoal canister in the engine bay.
When I removed my charcoal canister I found that the steel vent line was also plugged so I removed that too. I couldn’t see any use for the separation tank in the trunk, so I removed that as well and ran the SS Braided line from the tank to the fitting which runs through the trunk floor that the separation tank use to use.
Tank is vented.

FWIW
Bruce
Bruce Mills

Okay, venting the tank sounds simple enough. I guess I'll just plug off the steel line that runs to the engine bay.
I'll be sure to replace the lines on the seperation tank or remove it.
Thanks for the replies.

Kelly
Kelly Combes

A vented fuel filler cap doesn't just have a hole in it, it has a spring-loaded valve so that when the vacuum rises to a certain point, and not enough to affect running, it opens and lets air into the tank. The main reasons for the valve are that a) fuel doesn't dribble out in a roll-over accident, and b) fumes don't escape when the sun shine son it. Get a proper vented filler cap if you are going to block off the vent pipe.
Paul Hunt

The small drop out tank is faily important. When you accellerate hard, which we V8 types can and often do, the fuel slops to the back of the fuel tank and can get sucked up into the vent line, thence into the manifold. Also vapour can condense in this little tank (which is of less than 500ml capacity and located high up behind the tail light) and dribble back into the tank, rather than blocking the line.
The idea of the charcol canister is this,
A vacuum line runs from the fuel tank, to the bottom of the drop out tank, thence (via under the car)to the top of the charcol canister. Also from the top side of the canister a line runs to the manifold. The bottom of the canister is open to atmosphere. The idea is that when the motor ISN'T running any fuel vapour which gets past the drop out tank, will vent cleanly through the charcol. When the motor IS running a very small vacuum is created in the tank and the charcol does little.
My new fuel tank didn't have a vent line, so I put one in the filler neck, as this could be removed to a safe distance for this purpose. We are required by law to have a fuctional charcol canister/fuel tank venting system here.
In the 1800 there were some other vacuume lines to do with crankcase air, which V8'ers don't care about.
Peter

When the engine is stopped expansion of fuel and air in the tank and float chamber causes vapour to escape through the charcoal filter, which traps the volatiles, to the atmosphere. When the engine is running a small amount of air is drawn through the charcoal in the other direction to purge it of the vapours it has previously absorbed. The restricted suction from the engine, together with the relatively large port that is open to the air, means that there is never really any vacuum or pressure in the canister or its vent lines to tank and float chamber. At least, there wasn't until the anti-runon valve was added to close of the atmospheric port, and the vacuum that *was* then developed was applied to the carb float chamber(s) to suck the fuel out of the jets and instantly stop the engine. All engines should have a crancase ventilation system of some type, from quite early on in the MGBs life this was a positive (despite it using negative pressure :o)) system using inlet manifold or carb suction. When the charcoal cannister was added to the MGB they simply plumbed it to the rocker cover so that the venting of the canister and engine were done in series which is preferable to two parallel.
Paul Hunt

Hmmm, So,using the canisters in some sort sounds like a good idea. Not using the space for anything anyway and I have two sets of them. There's a spare vac port on my carter 400 that can be used.
Kelly Combes

This thread was discussed between 08/04/2004 and 11/04/2004

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