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MG MGB Technical - Evap loss control system

I smell fuel in the garage. All the "smog" stuff has been removed from a recently purchased 71 MGB. All that remains is the vent tube from the tank. Everything else is gone...I mean everything! Smell seems strongest around the carbs. Any help would be appreciated. BTW, the car runs great. No "run-on", etc.
John

John. I do not know what year the charcoal cannister was required. Moss' catalog seems to indicate it was 1970. If this is correct, you should have a line going from the vent near the filler neck to a vapor seperation can, then, forwards to the charcoal cannister. This system vents the fumes into the charcoal cannister. You should also have the overflow lines from the carbs attached to the charcoal cannister. If not, that would be your problem. The charcoal cannister takes nothing away from engine performance and is a necessary part of the system which should not be removed when "de-toxing" the engine. Les
Les Bengtson

Les,
I have a 74.5 with most of the pollution stuff removed, it does still have the carbon cannister however. The PO also converted to a Weber carb. Neither the air filter housing nor carb have a place to connect to the carbon system. I noticed that the intake manifold does have a fitting for a vacuum take off (I imagine it would be for vacumm assist brakes). Would the system work if I connected to that? I don't have an extreme gas smell problem at this point and I really don't want to drill into anything to install a fitting.
John
John

I see on page 404 of Bentley's MGB book that there appears to be 2 hoses coming from the carbs, joining to a "Y" going to what is called an oil separator. On my car I see two small holes, perhaps a 1/4 inch dia. each...left open. No hose, nothing. Does this help?


Les,
I had a 71 30 years ago. I know there was a canister there. A mechanic removed the smog pump when it froze, but this treatment seems extreme.

Thanks,
John
John

John. You should have a large rubber hose running from the large nipple of the charcoal cannister to the valve cover (if the cover is original). This is part of the PCV system. The Weber DGV series in vented inside the air cleaner (at least my two seem to be) and do not have an external vent to hook to the charcoal cannister. Cap off that nipple. If you have a DGV, get an air filter having a 90 deg elbow and run a line to the front tappet cover to the elbow. This provides a very slight negative air pressure within the crankcase, similar to the original system. Some simply run a tube down from the tappet cover to below the engine, called a "road draft tube". This system did not work all that well in the MGA and early MGBs which is why the factory came up with a better system about 1965 (from memory).

You are correct that the fitting on the intake manifold is the take off for vacuum for the brake servo unit. I do not believe it is used on your car. If not, make sure it is sealed up properly to prevent an air leak in that leg of the intake manifold.

Les
Les Bengtson

John in Oregon. As I remember it, the proper carbs for your car have the Y connection going from the carbs to the front tappet cover. This is the same principle I mentioned to John in RI. You should also have two over flow brass nipples, located below the fuel line nipples, which carry the fuel fumes to the charcoal cannister. If your set up is similar to my 79, and I believe it is, the fuel tank vent line should come forwards from the trunk, run along side the battery cable and come up on the right (passanger) side of the engine compartment. It is a steel line. From the steel line there is a rubber hose to connect it to one nipple on the charcoal cannister. The other small nipple goes to the carb over flow vents. The large nipple goes to the 90 degree elbow on the rear of the factory valve cover. At the bottom of the cannister is a large vent going to a rubber hose which is your vent to the atmosphere.

Hope this helps. If not, keep asking and we will get your problem solved. Les
Les Bengtson

Les,
Yes, I have the Weber DGV downdraft. It is vented through the valve cover to the cannister and I do have the hose going from the tappet chest to the air cleaner housing. I am glad that you said that was the way it should be set up with the Weber. I wasn't sure if I needed anything else. Thanks.
John

I recently went through reinstalling a charcoal canister, and it brought up another question - where does the gas go?

3 lines from the top of the canister - one from the carb balance/overflow (on my HIF setup), one connects back to the tank, and one to the valve cover. At the bottom there's a line to the anti-runon valve.

Inthe event the carbs do overflow, seems like the canister would just fill up with gas. eventually I suppose you could push it (and maybe some charcoal?) back to the tank down that line, but under normal running circumstances there would be a slight vacuum in the canister (from the valve cover line). So do you suck the gas from the canister to the valve cover? Seems like even if it's just vapor you wouldn;t want the gas vapor drawn through the engine/PCV.

Thanks
Steve
Steve Aichele

Steve, check out Paul Hunt's site. It gives a good explanation of the emissions plumbing along with a good diagram:
http://www.mgb-stuff.org.uk/
click on spanners, then fuel, then North American emmissions plumbing.
If I understand it correctly, the carbon cannister is not designed to stop raw gas overflow. It is designed to capture gas fumes coming from the carbs. Under normal running conditions the anti-run on valve will be open allowing fresh air to be drawn into the carbon cannister purging the carbon of the fumes through the engine. Upon shutting the engine the anti-run on valve closes and creates a vacuum drawing unspent gas fumes into the cannister and preventing the engine from running on. The cannister itself has filters and screens to prevent any carbon from escaping and the carbon is supposed to absorb the fumes. I rebuilt my cannister, quite easy, replacing the carbon with aquarium carbon and the filters with cannister filters bought at Auto-Zone.
John

The vapors or fuel that goes into the canister is filtered through the filter and drain tube into the air. Fuel souldn't be getting to the canister. If it does there is a fuel leak or some other defect. John, since you say the gas smell is around the carburetors I would thinks you have a problem in the carburetors. Look for incorrect levels, too much fuel pump pressure if you have an aftermarket pump or leaking float needle valves. Older MG's didn't have evaporative emission systems and when functioning properly they don't have a gas smell. If you want to install the evaporative emission system, (It does help keep the air a little cleaner) look at the diagrams on page 20 of the Moss catalog to see what is needed.
Clifton Gordon

John. No problem. The way you have it hooked up is the way I have it hooked up and both seem to be working fine. Les
Les Bengtson

Clifton. You will get the smell of fuel vapors, both around the opening to the vent line from the gas tank and the carb overflows, if the charcoal cannister is not present.

You are correct that the older versions of the vehicle did not evidence these smells. However, the fuel tank was vented through the fuel filler neck cap and, from time to time, you would smell fuel fumes when near the rear of the car (especially just after filling the tank in how weather). With the carbs, the overflow vents were connected together through a hose and copper tubing system which vented down near the bottom of the engine, providing a form of "road draft tube" for the carb vent system.

Anytime you have open lines which serve as vents to a part of the fuel system, you will have fuel vapors present. It really sounds like the PO of John's car "de-toxed" it without understanding why certain systems were there in the first place. Hence, he simply removed parts, but did not do so in a manner that would either leave the good things in place (charcoal cannister) or return the system to the earlier type of configuration. Les
Les Bengtson

Les, Thanks for correcting my errors.

Clifton
Clifton Gordon

Clifton. I do not know if I have "corrected" any "errors". I own a 68 BGT and a 79 roadster. My daughter owns a 77 roadster which we rebuilt together. Thus, most of us are working on the cars which we are most familiar with and are not fully aware of the full range of MGBs which evolved, quite considerably, over the years. Thus, if we are to be chastised because we speak from our own experience, with a somewhat different model of MGB, would be contrary to to the ethos of this BBS. We each contribute what we know and what we have experienced. If I "am right" and you "are wrong" it would, merely, indicate that I was a better guesser than you, not that you have made an "error". Neither of us owns the car in question and you, like I, are offering up our "best guess" analysis.

You have, over the years, offered up many "best guesses" which have turned out to be exactly the solution to someone's problem. Problems that I, and many others, have failed to diagnose exactly.

Therefore, there is no need to state something to the effect, "I may be mistaken here". Without seeing the vehicle and meeting the owner, we are all "mistaken" in what we suggest. We suggest what makes sense to us, and, quite often, others suggest what makes sense to them. In the end, most of the problems get solved. That is a good thing. And, what we are here for.

For myself, I value your imputs and read all of your postings because it allows me to learn more. I greatly appreciate that.

Les
Les Bengtson

what do you do if you do not have the factory valve cover with the elbow vent on the back, but have an Alloy Cover with no vent? what do you run to the large nipple on the cannister?
Thanks Paul
R Slice

Can I tap into this thread? I just put a '73 B/GT back in service after a nine-year hiatus. The anti-runon valve gets too hot to touch after the car has been shut off and I note that it's still energized even when the ignition is switched off. Since it's wired in series with the oil pressure switch, am I right in presuming it's the switch? Or is it the valve? For the moment I've just disconnected the valve and I shut the engine down with the clutch if necessary.

Here's a potentially embarrassing confession: in 21 years of MG passion, I've never serviced a charcoal cannister, nor have I noticed a need to. What kinds of problems can a "challenged" cannister create (i.e., how can I tell it needs service)?

In answer to Steve's question; yes, fuel vapors are drawn into the valve cover where they are mixed with other crankcase vapors and recycled back through the carbs via the line from the front tappet cover through the "Y" and the hoses.

If memory serves, crankcase ventilation worked in reverse on the early B-series engines. The carb air cleaner on my Magnette was connected to the vent on the valve cover such that crankcase vapors were "sucked" from the top and the vent line from the front tappet cover to the atmosphere under the car actually drew in fresh air - and a bit of road grime...

Allen Bachelder

' Forgot my original question. I just installed new carpeting in the aforementioned '73 GT and was wishing the pieces for the right rear "pocket" could fit better - which they would were it not for the vapor seperator can. Is there a way to plumb it out of the circuit without losing anything?

Thanks,
Allen
Allen

Allen; The oil pressure switch should be closed when there is oil pressure and open with no oil pressure. The switch supplies ground to the anti-runon valve through the slate/yellow wire. Sounds like your oil pressure switch may be in the closed position when the engine is off. After turning the ignition switch off there is 12 volts on the slate/pink wire at the anti-runon valve until the switch is turned on again.

I have never serviced a cannister either. I think we are polluting a little more than those who service their cannisters but there no affect on how the car runs.

Clifton
Clifton Gordon

Clifton,

Thanks a lot for your answers. This oil pressure switch was fairly new when I parked the car in 1996, so I thought it unlikely. And I had never heard of one failing before.

But you just inspired another thought. Perhaps the slate/yellow is shorting to ground somewhere, or the contact pin on the valve is shorting to the case. I'll have to run a couple of continuity tests tomorrow and see what I find.

Thanks again. I responded to your post on the "how old are we" thread. You are an inspiration to this 65 year-old punk!

Allen
Allen

I installed a Mikuni side draft (Weber DCOE look alike but a much better carb for daily use) about 15 years ago on my '73. I have it plumbed as follows:

1. Canister to vent pipe on rocker cover.

2. Hosed from oil separator on front tappet cover up to OEM Smith's PCV (this is the only PCV that works correctly - trust me - I went through dozens of other types). Just belly up to the table and pay the outrageous price for the Smith's PCV. Later on diaphragms are cheap.

3. The down pipe from the Smith's PCV is connected to the inlet manifold.

No. 2 and 3 are a very important part - simply connecting the front tappet cover pipe to the air cleaner results in an oily mess in the air cleaner and does not properly ventilate the crankcase anyway.

You need the PCV to regulate the flow of crankcase fumes into the inlet manifold. When manifold vacuum is high (throttle closed or idling), no crankcase fumes will flow and your mixture won't be upset. When manifold vacuum is lower (throttle open and cruising) the PCV will open and allow the engine to burn the crankcase fumes. Under these conditions, the mixture is not affected very much because the crankcase fume flow is low compared to the air-fuel flow from the carb.

Trust me on all of this - this is an engineered solution - the design I came up with was after much research and testing. The result is proper crankcase ventilation with no oily mess in the air filter and better yet, no oily mess throughout the engine compartment.

It's worked great for 15 years.
Richard Smith

This thread was discussed between 23/06/2005 and 26/06/2005

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