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MG MGB Technical - Charcoal Absorption Canister...

One of the few things left undone on my '73 B/GT restoration of 2004-5 was the charcoal canister. I've had a faint intermittent gasoline smell in the car over the last year or so and I'm on a multi-pronged approach to the problem. The fuel hoses are all relatively new and in good shape. There is no evidence of any leakage on the hoses or at the carbs. Even so, my carbs are going to see Joe Curto shortly. The gas tank is new and does not leak. But I have questions about the fuel-vapor handling system.

The charcoal canister is plumbed correctly and the anti-run-on valve is functioning as it should. This car has been desmogged thoughtfully and neatly (pump and its plumbing removed and holes plugged). I had thought that the canister would come apart to clean it out and replace the charcoal but I'm studying one close-up and I don't see how it would come apart. How does one service these?

I'm also suspicious of the vapor separator and its plumbing. The hoses, as well as the separator itself, are NA according to Moss US, but I have little doubt that under the steel braiding, there is little left of the original hoses. Since this car is a GT, any fuel smells from these hoses would end up in the cabin.

Any suggestions on how to deal with these things are gratefully welcomed!

Allen Bachelder

It's possible that the soldered-on vent pipe off the top of the tank cracked a little around the joint. Happened to me on a brand new tank when I was tightening on the vent hose.

Allen. The one "spare" charcoal cannister in my collection screws apart at the base for cleaning and renewal of its contents.

The hoses from the fuel tank to the vapor seperater, and from the unit to the hard line going forwards, are standard fuel line hoses.

I have seen some of the rubber with stainless steel mesh covering, used as replacement hoses. But, the only example I have which is, most probably, original, used standard rubber hose. Have had no problems replacing the existing, cracked, lines with standard, rubber lines.

Much information, if it can be found, in the archives on rebuilding the charcoal cannisters. Most revolve around using the fibre-glass and activated charcoal for fish tanks.

Les Bengtson

As others have mentioned, the bottom of the vapor canister unscrews. The contents can then be removed. If you don't have a pet store nearby, the charcoal can be reactivated. IIRC this proceedure came from John Twist: Empty the contents into a shallow baking pan, spread it out evenly. Light the charcoal with a propane torch. You may have to relight it several times, and wave the flame around over the whole surface. Let it cool, then reassemble. I've done it and it does work.

The vapor seperator hoses came in two flavors - early cars used braided stainless covered with screw on fittings on the ends. Later cars used plain rubber hoses that push onto tubes on the seperator. If you can't find replacements for the early braided hoses, remove the collar below the nut (I use a grinder). The end fitting (with barb) can then be removed from the hose and then reinserted into a new hose with a new hose clamp.

Tom Sotomayor

I rebuilt the cannister on my '74 a few years back...and added new carbon...It was easy and a noticable improvement a'la resudual fuel vapors....

For a long time I could smell gas when I drove my 74B. I finally took the charcoal cannister off then apart. The charcoal I dumped on a steel cookie sheet and finally put the sheet in a warm (not on) oven to try to evaporate any old fuel that got into it. This seemed to work very well but I could still smell gas! On the right side of my trunk by the fender is a small round tank into which vapors are pumped.there they collect, become liquid again and drop down into the gas tank. I discovered that the hoses were real loosey goosey at the ends! They were well connected but where the hose attached to its end fittings they were bad. This was cause of the smell! I replaced the hoses and the smell was gone! If the smell on your car seems stronger in the truck, maybe you should try this. Good Luck Bob
Bob Ekstrand

Thanks to all of you. Lots of great info here. BBS participants (BBSers?) do it again! I've already figured out how to unscrew the bottom from the canister.

My vapor separator hoses on this 73 are the steel braided kind with the screw-on fittings. ' Glad to get some good advice on how to replace these too.

Again, thanks all!
Allen Bachelder

I am in the process of putting a 74 back on the road and was minded to remove the charcol system altogether.

I had thought that this was a power sapper - is it not?

The hoses (push on) are all shot to pieces and Moss lists them as NLA. If I am going to keep this on, then:
1) Where can I get replacement hoses, and
2) How can I test it all to see if it is working properly?


Paul Barrow

Paul, that is one emission device that does not have any effect on power. You can fabricate hoses from generic hoses from an automotive parts place. There is some testing information in the emissions section of the Haynes mgb manual. The canister just needs to be clear so that vapors can be filtered through the charcoal. A diagram of the system is shown on page 91 of the Haynes manual.

Clifton Gordon

Paul. As Clifton notes, there is only an "up side" to the charcoal cannister system.

As to hoses, go to a good parts store, something like NAPA rather than the discount parts stores, and tell them you need an emissions quality hose for the hose from the center nipple of the charcoal cannister to the rocker arm cover and the hose from the front tappet cover to the mushroom valve/carbs. This hose is also known as "oil resistant" hose and, like fuel line hose, is designed to hold up in the presence of oil/gas and oil/gas vapors. Heater hose does not work well in these applications. The smaller hoses, both on the charcoal cannister and the vapor separator can me made from standard fuel line with no problems.

Les Bengtson

Well, the only downside so I'm told, is the space it and its plumbing takes up in the engine compartment, so freeing-up this space is the only benefit of removing it. However in a 73 model year and later with anti-runon valve interfering with any of the plumbing will almost certainly disable the valve, which can be a negative factor.

I'll have to question the class of the hoses though, the tank and float chamber hoses are going to contain fuel vapours virtually all of the time, parked and running, whereas when running the hose from the canister to the rocker cover is continually drawing air from outside, the only vapours in it will be those purged from the charcoal granules from prior expansion of vapours from the tank and float chamber. There shouldn't be oil or its vapours in *any* of these hoses, only in those from the front tappet chest cover to the PCV valve or carbs.
Paul Hunt2

Thanks Guys - that is all very helpful info. I will proceed with all of the above!


Paul Barrow

This thread was discussed between 01/05/2008 and 03/05/2008

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