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MG MGB Technical - Gasoline in the Emission Canister

Recently I overhauled my Zenith Stromberg on a 1979 MGB and was so proud that it was running good. A couple of days later I noticed a change in the idle. It was easy to start from cold and idled low. Once warmed up the idle was slightly high. After about an hour driving slow in the country the idle was too low to remain running at a stop sign. About this time I could strongly smell gas fumes. Upon inspection under the hood I noticed that the small hose on top of the first emissions canister (the one that connects the line back to the gas tank) was cracked and actually leaking wet gas. What do you think is happening to cause gasoline to enter that area? Backup from the tank or some kind of overflow from the carb? The emissions pump was disconnected a long time ago (no emissions standards here). Would that little electric switch on the outgoing of the second canister, if not working, cause this? (I don't know whether it is working or not). I also noticed the elec fuel pump seems to be constantly running when the motor is off and switch is on. Any ideas, any one? Thanks.
Lee Gooch

Your carb is flooding from a worn, dirty, or mis-adjusted needle and seat.
John H

I put a new Grose-jet needle/seat during the overhaul. It could have got grit or needs further float adjustment i suppose. But would a flooding carb not cause the engine to speed or flood out? Thanks for the info John.
Lee Gooch

Lee
I had the same problem on my '79 midget, 150CD but basically the same, after rebuild. My thoughts are that the ZS carbs do not like that Gross Jet, I don't know if it has to do with the fuel pressure or what but I changed back to the original needle/jet and the problem went away. It did'nt hurt to clean the carbon canister charcoal and screen also. Clean your old one or order a new original.
Hope this helps.
Mike
MK Mike

Lee-

The fuel bowl on the carb is overflowing through the vent. You need to check the float for proper adjustment and to make sure it is not full of fuel. You need to make sure the needle and seat in the fuel bowl is clean and will seal when the bowl is full.
Are you using a stock fuel pump?
Kimberly

As others have said it is the float valve not seating. The carb isn't flooding, the float chamber is overflowing which is different. The difference in fuel level in the jet between correct float level and when the fuel level rises to the overflow is marginal. Hence the change in mixture and effect on engine *running* is marginal.

Grosse Jets(?) used to be recommended, but more recently they have changed manufacture and are now said to be worse than the originals. Conversely the originals now have viton tips and are said to be much better. Either way I've never had trouble with the originals in 40 years.
Paul Hunt 2

Thanks everyone. Very helpful information. I have a new "regular" jet that came with my overhaul kit but didn't use it in favor of the Grose Jet (after reading that it was preferable). Guess I'll reinstall the old kind. My fuel pump is an aftermarket... maybe it's not as compatible with the Grose Jet as well.
Lee Gooch

I've never had to deal with this directly, but some aftermarket pumps require a regulator to reduce their pressure for the MGBs. Somebody else on this thread will be able to address this better than I. Lee, can you post the name and/or pressure specs for the aftermarket pump you're using?

FWIW,
Allen
Allen Bachelder

Lee. The most common replacement pump is a Faucet (Fawcet?) pump made in Australia and available with a number of names printed on it. This is the square "pump in a box" looking thing. There are two versions of this pump, the high pressure and the low pressure. The high pressure pump definitely needs a regulator installed. I am using the low pressure pump, without problems, on a car with a Weber DGV installed.

David DuBois has a tech article on the fuel pumps, including specifications for the SU pumps and how to wire in a back up fuel pump, on my website. Look at the MG section of www.custompistols.com/ and go to the articles. Dave's are listed under his name.

You can use the basic SU specifications to determine how well your after market pump might be working. A fuel pressure gauge is relatively inexpensive and most also serve as a vacuum gauge, making it a useful tool to have available.

The archives have a lot of information on rebuilding the charcoal cannisters using aquarium charcoal and the glas-fibre that sits on top of the charcoal. Your 79B should have two charcoal cannisters with the second cannister serving as an over flow from the first. Both will need to be examined.

Les
Les Bengtson

Thanks Allen and Les. That's interesting. I've had the car a little over a year and the fuel pump had already been replaced. I just looked under the car and could'nt see a name on it. It's pretty small, stainless and cylinder shaped (almost looks like an in-line filter except it does have wiring hook-up). Under the hood, however, is an in-line regulator (just before the carb) with a dial displaying 1-5. I wondered what that thing was for... It currently is set on zero. Maybe a higher number setting would lower the pressure you think? I'll check that website you suggested, Les. Also, thanks for the tip on the aquarium charcoal. The canister was pretty clogged up.
Lee Gooch

Lee. I would disconnect the wire from the points to the distributor (if you have a points type dizzy) or, I would disconnect the wires going to the input side of the coil and insulate them (electronic distributor).

Hook up a fuel pressure gauge to the output of the fuel pressure regulator. Switch on the ignition switch to the run position. Read the fuel pressure gauge and play with the regulator to see what happens as you turn the dial. You want about 3 psi fuel pressure going to the carb.

When finished, turn off the ignition switch, reconnect the fuel line, reconnect the wiring and take the car for a test drive. It should run better.

But, you still need to rebuild the charcoal cannisters if the system is to perform in an optimal manner.

Les
Les Bengtson

Lee, the viton tipped needles seem to work beter than anything else, you can order them from Joe Curto.You can find him on the web.
John H

'Facet' pump, I believe http://www.fuelsystem.co.uk/web-facet.pdf

The SU pump only delivers about 2 or 3 psi, when aftermarket pumps are used they often deliver more than this which causes overflowing, and need a regulator as has been mentioned. All-in-all the original SUs are preferable, but that still leaves two types - points and electronic. I've had a Moprod (plastic bodied) electronic 'plug compatible' on my V8 and it was nothing but trouble, eventualy I had to replace it and used a refurbished SU points-type and have had no more problems.
Paul Hunt 2

The following is a link to rebuilding canisters
http://www.theautoist.com/carboncannister.htm
Kimberly

Les, if the emissions system is no longer running (pump belt is off) and I've dumped out the old charcoal from the first cannister, do i still need to rebuild with the aquarium charcoal, etc. or can I leave it empty?
John, my rebuild kit from Moss Motors has an original-type needle in it. Do you think this would be viton tipped?
Paul, thanks for the advice.
Kimberly, thanks for the website.
Lee Gooch

Lee. Yes, you need to rebuild the charcoal cannister system. It does something entirely different than the air pump system.

Many of us feel that the air pump performs no useful purpose from an environmental stand point. We feel that, rather than actually doing something about pollution, it merely dilutes, through the introduction of fresh air into the exhaust port, the emissions which will be measured at the tail pipe. Thus, our impression is that the purpose of the air pump is to defeat a specific form of emissions testing and that it does not have any benefit, whatsoever, in reducing the actual emissions from the engine--only the test readings are reduced. And, it takes one or two horsepower off the available engine capability. Others disagree and there have been no definitive tests on this feature. But, I would remove the air pumps on my cars were it not a required portion of our emissions inspection.

The charcoal cannisters do not have any controversy. They are an excellent idea. The fuel vapors from the gas tank and the carb are allowed to be drawn into the charcoal cannister. They will, then, be drawn into the engine and, eventually, into the intake manifold and the vapors burned in an efficient manner. Very few problems associated with the system when the various vent lines are open and the carb is not overflowing as yours was.

Given a choice in the matter, I would hook up a charcoal cannister system to my 68 GT so that it does not vent to the atmosphere if I could pull the air pumps off my 79s.

Please. Do me a favor. Do yourself a favor. Do all of us a favor. Keep the charcoal cannisters, rebuilding them as necessary. You will help the environment. Even better, your engine is set up to run with that system and, while it is possible to add an air filter to the rocker arm cover and set up the fuel tank to vent to the atmosphere rather than into the charcoal cannisters, the factory system is a rather splendid piece of design engineering.

Thus, some concerns about the environment, a nod towards originality (as long as it does not interfere with performance) and a longer lived engine cause most of us to conclude that the charcoal cannisters are a good thing rather than a bad thing.

I hope you reach such a conclusion yourself.

Les
Les Bengtson

Doesn't the air pump put more oxygen into the exhaust thereby encouraging the burning of any unburnt hydrocarbons ejected from the cylinder? Partial combustion being more polluting than full combustion? In a similar vein I have seen spark plugs fitted to catalytic converters, although in that case it was to burn the hydrocarbons inside the convertor, so warming it up much more quickly, which it has to be before it does its job anyway. It was said at the time that catalytic converters are fine for long journeys or short journeys in warm ambient conditions, but for short journeys in cold temperatures they have little effect.
Paul Hunt 2

Paul. The air pump has been is use for about 40 years now. Up until 1975, US cars used only the air pump with various other systems being added on when the emissions requirements were made more stringent. In 75, the cat was added and unleaded fuel became a requirement for use in such engines. So, when we discuss this topic, we have to remember that we are dealing with old engine designs which had various pieces added onto them to meet emissions requirements rather than the modern engines which have been designed around the emissions systems. The new systems are quite effective and the engines run very well. The fact they are quite complex is something owners have to live with.

As to the function of the air pump on our older cars, my belief is as I have stated--their function is to dilute the burned gases.

If the addition of fresh air were truly to cause the exhaust gases to continue to burn, we would expect to see a different design of exhaust system--manifolds and down pipes, to take into account the extra heat generated by this change to the existing system. That, as we all know, did not happen.

I do not have an engine/emissions testing lab available to me. My reasoning is based on more simple testing measuring the temperatures of the exhaust system using a high quality infa-red thermometer.

If the air pump, injecting fresh air into the exhaust gases, is to cause them to begin burning again, I would expect to things to be present. The first would be some form of triggering device to cause the exhausted gases to re-ignite. There is none on these older systems. The second thing is I would expect some form of significant increase in the exhaust system temperatures when the air pump is hooked up and running. This temperature rise would be present in both the exhaust manifold and the header pipe. I have not been able to document such a temperature rise when testing with the air pump disconnected, then reconnected and taking readings of both straight exhaust and exhaust with air injection.

One may question if the added air makes the cat function in a more efficient manner due to the higher presence of fresh air. Again, while there is a temperature rise of over 100 deg F between the pipe going into the cat and the pipe exiting the cat, the presence of air injection does not seem to cause the cat to work more effectively.

Considering the results of my testing and the clutter the air pump and associated systems add to the engine compartment, I do not feel that it is a system I would leave in place had I a choice about it. Add in the fact that it requires a small amount of engine power to turn the air pump and we see a slightly reduced fuel economy over a car with a similar specification engine and no air pump. A third factor, which does not bear directly on the emissions is the fact that the air pumps do not seem to be particularly long lived. I have had three of them fail over a ten year period.

For these reasons, I would not object to anyone who wanted to remove the air pump doing so.

Les
Les Bengtson

Most interesting Les. I plan on rebuilding the canister. Thanks.
Lee Gooch

"some form of triggering device to cause the exhausted gases to re-ignite"

Notwithstanding the spark plug added to cats to promote ignition of unburnt fuel in a cold and hence enriched engine, a triggering device certainly isn't required for a hot engine, as many of us have experienced explosions in the exhaust when suffering intermittent ignition, when the first successful ignition after one or more failed ignitions causes unburnt fuel in the exhaust to ignite with a loud report. A quick Google search on 'air pump emissions' produced these two links at the top of the results which both say that the air pump reduces pollution by igniting unburnt fuel, as well as their being sufficient heat in the manifold to support combustion. Tubular manifolds, and cats, can glow red-hot after all.

http://www.familycar.com/Classroom/emission.htm#AIR%20INJECTION
http://www.carcare.org/Emission_Control/air_pump.shtml
Paul Hunt 2

Paul-
I have to agree with Les. Consider the fact that the hot exhaust gases pass through the exhaust system in pulses, not as a steady flow. When a high-pressure pulse of exhaust gases is passing the air injector port in an exhaust port, an anti-back-flow valve prevents the hot gases from entering the air injector system to any meaningful degree. The air injected into the exhaust port can enter only when pressure has dropped, which is after the inertia of the pulse of exhaust gases has carried it past the air injector port, leaving a low-pressure area in its wake that permits the anti-back-flow valve to open. This being the case, the air that is injected is always sandwiched between the pulses of exhaust gases, mixing with it only after entering the turbulence inside the muffler, at which point temperatures have dropped to the point that combustion has ceased, thus accomplishing little other than the dilution of the exhaust gases exiting the exhaust pipe. Way back when the system was introduced, the EPA measured for pollution in only terms of a standard of “parts per million” (PPM, as they called it back then), thus the diluting system helped to satisfy the EPA standards, along with such tricks as leaning out the fuel-air ratio and changing the ignition timing curve to initiate combustion earlier during the compression cycle. However, several ecologically-minded scientists and liberal politicians, emboldened by having succeeded in forcing a ban on leaded fuels, loudly protested that the technology being used by the shameful capitalist auto industry was a sham perpetrated at the expense of the poor suffering masses in order to prevent them from being forced into spending millions of dollars of their precious plutocratic profits on the development and use of “meaningful” technology. When the EPA responded to the political pressure by changing its test standards to reflect the actual total amount of pollution emitted, manufacturers quietly dropped the air pumps and switched to catalytic converters.
Steve S.

Paul. Steve has said most of what I would have said and added a few facts that I was not aware of. Modern cars seemed to have dropped the air pump back in the mid 1990's according to the mechanic I talked to today.

When, back in 1969, I purchased a 1968 Mustang, the first thing my next door neighbor, a long time Ford mechanic, recommended was removing the air pump and plugging the holes in the exhaust manifold that it pumped air into. He told me that the air pump served no useful purpose, took a couple of horse power from the engine, and got in the way when working on the engine. At that time, it was against the law for a mechanic to remove the air pump, but there were no laws against the owner doing so. Hence, he provided "instruction" while I did the work.

This is an issue that I have discussed with various mechanics over the years, including the mechanics who operate our "Emissions Waiver" station, the people who "actually have a clue" (clew in British) in our emissions inspection system. The universal perception by people who do not have a vested interest is supporting the theory that an actual improvement is taking place in the exhausted gases is--there is no improvement taking place. There is only a dilution of what actually exists.

Modern, computer controlled, systems are different that what we find on our old cars. With old cars, performance and reliability were the main criteria used in production. In modern cars, low emissions is another criterium that has been laid upon us. But, even if one removes the air pump (as was done on my 68GT long before purchase) and uses the car as a daily driver, I can drive the GT for the rest of my life and not produce as much pollution as would be necessary to build a new car and drive it during the same period. The majority of pollution is in the manufacture of the vehicle. The actual operation of the vehicle is only a minor part of the total pollution series.

When one owns a sports car, one tends to be somewhat keen on maintenance and performance. We, as a group, tend to keep our cars in better tune and the average individual. My daughter's 77B does not have a cat installed on it. When we purchased the car, a Weber DGV had been installed along with a header. The car had been repainted and the decal which might have indicated that it had come with a cat installed was removed as part of the repainting process. The local authorities have a book which lists, "1975-1980 MGB, some have catalytic converters installed". The car passes the required emissions testing standards on a regular basis, more so since the original engine was rebuilt. An engine in good condition, properly tuned, is a low emissions engine. Especially when one considers the pollution cost of building a new car. From a systems perspective, running an MGB and keeping it in good tune, is one of the most "green" things that we can do. Even better when we remove the power hungry, often failing, air pumps from the equation. It makes it easier to service.

Les
Les Bengtson

All theory aside, last year at annual inspection my 74 MGB failed because it didn't have a smog pump. They don't check the operation of the emissions, just verify it's there and the air pump is turning. I had to install the air pump and associated equipment and go back for a recheck. After I installed the emissions gear I immediately had backfiring in the exhaust, even with a cold engine. It would backfire each time I shifted and any time I took my foot off the gas pedal. Never tried adjusting anything. The emissions equipment was hooked up correctly. After getting my inspection and removing the emissions equipment the backfiring stopped. I realize there is probably something wrong with one of the components.

FWIW, Clifton
Clifton Gordon

Clifton. Aint (sp?) it great? "We're doing our part for the environment." A friend told me of a V-8 conversion which passed the tail pipe emissions test--i.e. they were not above the required amount of pollution expelled from the exhaust, but did not meet the "equipment" portion of the test. Thus, the car failed and would have required several hundreds of dollars to meet the requirements. Owner sold the car "out of area".

Once, when my daughter's car had to go through emissions, I took it over to my friends, Bob and Gil Schaulin, for assistance. They had another car, also a late model MGB, for emissions testing. Gil and I drove the two cars over. The car Gil was driving passed. The car I was driving failed for "cracks on the gasket of the fuel tank cap". Pulled the cap off of the car Gil was driving, in front of the inspection people, put it on my car, and passed the inspection. They did not even require that I drive through again when I explained that I had a new cap at home and would put it on when I got home. Which I did.

I am a "green". I have two daughters, both of which hope to have children. I would like to leave them a better world than I have grown up in. I think that most of us "moderates" feel this way. But, we need to understand what is a real improvement and what is an "add-on" which does nothing of value except meet some government mandated standard. If there can be a real improvement in the operation of my MG engines, one which will provide better performance at the same emissions, or one which will provide better emissions at the same performance level as now, I will find the money to purchase it, then, I will install it.

Here in Arizona, they (being the local newspaper, the "Arizona Republic) are identifying emissions originating in China as being part of our "emissions problem". I do what I can. But, we need to look at these things as a world wide problem, not, simply, as a local problem.

Les
Les Bengtson

Les
I too always try to do my part for the enviorment. My '70 BGT has had the emissions equiptment removed by a PO. I would like to hook up the charcoal canister again. I still have the piping back to the gas tank but I have a Alum valve cover with no ports. No Air Pump, I also have installed HIF 4's and the intake manifold does have 3 unused "ports" fittings. Could you tell me how/if I can run the hoses to get this to work? Obviously I know that the overflow will go into the canister as well as the tank return line. What about the large port at the bottom of the canister? Any suggestions are apreciated.
Mike
MK Mike

Mike. I can give you an idea, but my 68 GT does not have the charcoal cannister and the other three family cars are rubber bumper models and there may be a slight difference between your original set up and theirs.

The large port on the bottom of the canister is the air vent and should go to a fresh air line going forwards towards the front of the engine compartment. On our RB cars, there is an anti-run on valve plumbed into that line. Moss has this illustrated on page 21 of their catalog. If your model year did not have an anti-run on valve, the Moss catalog would seem to indicate there was just a tube attached which might have gone downwards towards the bottom of the engine compartment. Perhaps someone with either your model, or a similar one, can provide this information.

The big nipple on top goes to the rocker arm cover and provides the source of air for the PCV system. If you have the alloy valve cover without the elbow, it should have some form of vented cap, similar to the earlier model MGBs. In that case, go to a good parts store and get a rubber nipple cover for that port.

The other two, smaller, nipples go to the fuel tank and to the carb fuel bowl over flow/vents. Connect them to the charcoal cannister and there should be no problems.

Les
Les Bengtson

Anti-runon was only fitted from 1973, but when it *is* fitted the valve has a third, small port (in addition to the medium port that connects to the bottom of the canister and the large port at the bottom open to atmosphere) which is connected to yet another port on the inlet manifold, FWIW.

If you have an alloy rocker cover with no PCV port it should be fitted with a vented and filtered oil filler cap or you will get no crankcase ventilation. If you want to reinstate breathing via the charcoal cannister you will have to fit a port to the rocker cover, with an internal restriction of about 1/16", and use a non-vented oil filler cap. The fuel filler cap should also be non-vented, if the pipe from the tank is sealed it probably has a vented filer cap at the moment as well.
Paul Hunt 2

Paul
My system seems pretty much stock except for th alloy rocker cover. I believe the oil cap is vented but it's a aftermarket flat chrome type to match the cover. I have the vent tube off the gas tank to the vapor seperator in the trunk. My idea is to keep this system with the charcoal cannister in tact. My model did not have the anti run on valve. I guess I could modify the oil cap and fit some sort of nipple to it as Les suggested.
Mike
MK Mike

This thread was discussed between 19/12/2007 and 31/12/2007

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