MGB & GT Forum

I'm about to pull the engine on my weber downdraft equipped 77 MGB to renew the clutch. I thought I'd take the opportunity to address some of the oil leaks - side covers, valve cover bushings, etc while I had the engine out. Was thinking through things last night and had a thought that crankcase pressure might be at least contributing to the leakage. Compression is good, leakdown is less than 20% across all 4 cylinders. I looked over the charcoal canister and breather lines and, sure enough, discovered that when the weber was installed they plugged the line from the charcoal canister to the carb that draws air thru the canister. My understanding (though this emissions stuff confuses the hell out of me) is that the canister gets "cleaned" by low vac from the carb drawing fresh air thru the ARO valve. The side cover line is plumbed into the air filter, so I threw a T into that line and plumbed the canister into it as well. Any thoughts as to weather that sounds reasonable? Also, should I consider renewing the charcoal seeing that it has been running for years without having air drawn through it? Any other crazy ideas? Thanks!

Renewing the charcoal cannister is a good thing. Its easy. I just used fish tank charcoal or you can wash then bake off the contaminants from the original stuff. Any reduction in crankcase pressure is good.

No, that's not how it works. The charcoal canister is flushed by air being drawn through the vent on the bottom. On your car this would have been plumbed through the ARO valve to the atmosphere. The other line from the ARO to the carb was there to apply a vacuum to the carb float bowl in order to stop fuel flow and thus shut off the engine. It has nothing to do with flushing the canister or venting the crankcase. Since you have a weber, that line and the ARO are no longer functional. You could probably remove the ARO and vent the bottom of the charcoal canister directly to the atmosphere to ensure that a failed ARO won't block the vent. You should definitely not put a tee in the line from side cover to carb. The carb needs to pull a slight vacuum on the side cover so that crankcase gases are pulled out of the crankcase. A tee in the line would short circuit that route. The top of the charcoal canister should be plumbed to the valve cover and to the gas tank so that fumes are pulled from the canister and gas tank through the crankcase and into the carb.

The charcoal canister may or not be plugged. Usually not. Lines to and from it however are very susceptible to blockage. Even then, the problem is usually the routing of the lines, including fresh air into the crankcase and a slight vacuum on the side (tappet) cover to remove the vapors and reduce the pressure within the crankcase, (which may be excessive due to 'blow-by' the rings.

Simply adding a T to a line may or may not have any effect. A thorough analysis is needed.

I believe - but am not sure - that the charcoal canister system works with SU carbs, not with Webers.

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Jim K is a grease-stained wretch

Sorry, hard to explain. Not referring to the aro to float bowl line, that's gone. The line I teed into the breather line is the one coming out the top of the canister next to the line running to the valve cover. It should be running to the carb and getting low vac upstream of the throttle plate. There's no provision for this in the weber so inside the air filter box is the best I can do. I am starting to think that a seperate fitting rather than the T might be a better idea as if there is enough pressure coming out the tappett cover it may pressurize the canister.

Sorry, but that is still incorrect. However, note that I made a mistake in my earlier description. There are three lines on top of the canister. One goes to the valve cover as a source of vacuum. The second goes to the fuel tank to vent vapor. The third goes to the carb float chamber (on an SU or ZS carb) to vent it. A line from the ARO goes to the intake manifold to tell the ARO if the engine is turning or not. The canister base vent goes through the ARO to vent to atmosphere.

The way the canister works is that a vacuum is pulled from the carb (or air filter in your case) that sucks on the crankcase side cover vent. That mild vacuum sucks air from the charcoal canister via the connection on the valve cover. The charcoal canister is vented on the bottom so that the vacuum from the valve cover pulls fresh air through the canister (on ARO-equipped cars, that vent is closed by the ARO so that a vacuum builds up in the float chamber vent and stops fuel flow, but this is irrelevant for your setup). If you tee the vacuum line between the carb and side cover to the charcoal canister (which is open to the atmosphere), you will eliminate the vacuum that is making the entire system work. There absolutely should be no tee in the line from the side cover to the carb/air filter.

Here is a nice picture of how the system is plumbed and a quickie description of how it works. org.uk/arv3e.htm



Edited 1 time(s). Last edit at 2014-10-01 08:43 PM by davester.

Ugh, I don't know what it is about this stuff but it just gives me a headache. I see what you are saying, maybe having the float-bowl line plugged makes sense with a weber. It is running fine, crankcase pressure can't be too high (side cover breather is not blocked and isn't blowing much at all). I will see what happens regarding oil seepage with fresh gaskets, etc. they seem to be leaking pretty badly which made me think of crankcase pressure, but maybe they're just old and british.