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MG MGB Technical - 79B still running rich

Hi folks, I cannot seem to get this car to run anything but rich. It has a stock engine with ZS carb and 25D4 dizzy. I have had the carb apart and cleaned and made sure cold enrichment system is working, rechecked timing again but nothing seems to help.
One thing is after the engine is hot and idles for a while it just doesn't feel snappy when I kick open the throttle fast, it seems like it labors slightly until the rpms get up there.
New points, plugs but same coil and wires that were on the car when I bought it. The engine runs smooth just seems like it loads upo after a while.

Please offer any suggestions as to my next step.


Did you have the carb rebuilt by some one who know how to do the job properly? I have rebuilt so many so called "rebuilt stromburgs" that have had nothing but a few new gaskets and a needle & seat installed. The Jet needs to be pressed out from the body and the auto choke needs an overhaul and adjustment. Additional parts that don't come in a kit also need to be aquired. Only after it has been done will it work like it should, and they do work well.
Jim Pelletterie

Dear Paul. My first suggestion is that you note the year of your car and whether any modifications have been made to it. The Z-S carb was used from 75-80 and the Lucas 25D distributor was used from 62-74, so some changes have been made somewhere.

In the mean time, I have some tech articles which may be of use to you. My website is and, in the MG section, has tech articles by me and several other Enthusiasts, some of which may be of use to you now, all of which will be of use to you in the years ahead.

In addition to the year of your car, and any modifications you may be aware of, it would be good to know what needle you have in the Z-S carb--some are richer running than others. It would also be of interest to know how much you know about the Z-S carb. John Twist, of University Motors Ltd, did a tech article on "Those Damned Z-S Carbs" which, if it is still available online might be of interest. John has a "Tech Manual" available, at a modest price, which includes that article and several others of interest.

The Z-S carb has some problems, including the fact that it should be rebuilt at a regular interval (about every 35K miles according to one source) and, as some have claimed, was designed to be replaced at about 60K miles with a new carb. Thus, the Z-S was a complex carb which required a significant amount of maintenance. The plastic diaphram requires replacement at regular intervals, about every two years if my original experience is anything to go by.

The "water choke" is also subject to malfunction and can cause excessively rich running.

There is information on these problems in the archives.

My solution, in an area which requires emissions testing, has been to replace the Z-S carbs with the Weber DGV conversion. The twin SU conversion might be a better alternative where there is no emissions testing. But, we are limited to a "single carb" as per the original factory specification by our emissions inspection people. The Weber conversion provides both a better carb and is acceptable to our inspections. A lot of information, in the archives, on the conversion to the Weber DGV series and to the conversion to twin SUs.

Les Bengtson

Hey thanks guys for your suggestions. The dizzy was changed obviously from the orig. When I rebuilt the car I went thru the choke system and made sure all passageways were clear and all linkage and vales worked smoothly. I followed many articles from Twist and others. I checked the bi-metallic spring and adjusted the position. I replaced the diaphragm but not the mixture needle (it looked ok). I cannot tell what size needle is installed since I have nothing to go by.
The choke mechanism seems to work good.
If I had a vac leak it would run lean correct?

I do not want to change the carb to another style, I would prefer to get this one to work.

But I am open to other suggestions as to my nect step. I will check out your website Les.
Thanks, paul

If it's still rich, there's something wrong in the carb.
Twist's article is good, and most of it is in the Moss catalog. Excellent for general ZS sorting; however, The only definitive article on the choke is:

He got almost all of it, better than anybody else. There is a "reader's comments" or similar section, where I have pointed out a couple of fine points that must also be dealt with.
The main mixture needle is removable and marked as to spec, and the specs are in most of the catalogs. Precise checking figures are available online to measure the needle, to really tell if it's worn.
I generally agree with Jim P's statements, but DO NOT press out the jet unless you really understand everything about the carb, and have the parts on hand. (Jim, where do you get the "additional parts..."?)
FR Millmore

Ok, I did read Paul Bunyan's article on the choke which was very good to say the least. I believe I went thru it as he indicated and checked all points.
Getting back to the needle, where can I find out which one I have and how can I tell if it is worn? Is there a seat that goes along with the mixture needle?

Not sure I follow you about the "jets" being pressed in? Can you explain further. Thanks for the help to date.


A rich running car could also be a symptom of a poorly functioning ignition system. Are you sure the distibutor is functioning properly? The right ignition curve can mean the difference of 2 needle sizes! Do you have points or an electronic trigger?
Jeff Schlemmer

I had a rich running 1980 B and as you say no amount of adjusting would correct it. I spoke with John Twist and he quoted a section from his article"Those Damn Zenith Strombergs". In it he says rarely does a needle disassemble itself, but it does happen. The metering needle is held in a small barrel against spring force by a very small pin. If this pin breaks, the needle pops upwards by 1/8" or so, and no amount of adjusting can correct for such gross misplacement of the needle. The shoulder of the needle can always be seen on the underside of the air piston, If it has slipped up inside, and the metering needle has a lot of up and down (against the spring) movement, then the pin should be checked and replaced. A small piece of a paper clip works wonderfully. When I checked the needle on my ZS I found that the pin was missing completely. I took a piece of a paper clip and made up a pin. I was then able to lean out the carb over several short runs up and down the street to test it. My car now runs beautifully.
Hope this helps.
DK McNeill

I rebuild a lot of these carbs and have the special tooling to press out the main jet. On most carbs the jet is coroded with varnish and other gunk and the small orfices are plugged. Don't try to press out the jet without the special tooling or you will be replacing the carb. The jet is not available but they can be cleaned and replaced It seems that the needle wears before the jet. The number on the needle can only be seen if the tiny pin is removed from the needle adjuster. any needle that has 50k on it is usually worn on one side thus making the mixture rich. The choke assembly takes more time to renew as the main body. the needle that controls the artificial enrichment has a very small o-ring that needs to be replaced. the needle jet assembly has to be pressed out and the o-rings have to be replaced. Once everything is together the internal parts must move smoothly without any bind. The best kits for these carbs are from Royze. O-rings and the proper gasket are included. Extra parts needed are a new needle, a core plug and a antirun on diaphram on the CD150 carb and Proper screws.One of the biggest problems is the use of dashpot oil. there is another o-ring in the piston stem that must be pressed out and replaced. Just incase you are wondering, I had the tooling specialy made and is not available from any one sorce.
Jim Pelletterie

Jeff, that was one thing I was wondering about, since I went thru the carb and believe it should be in good working order. I have a new set of points in the dist. and set the dwell at 58. I was thinking maybe the coil wasn't putting out the voltage thus not burning the fuel enough, but I had also read it would result in a rough running motor and mine runs fairly smooth.

Jim, I replaced the timy o-ring in the choke ass'y and believe it is working properly. How can I check if the enrichment valve is leaking by? I read something about this? Is there a way I can tell if the seat is shot without removal?

Thanks guys, paul

New development for all who are following this thread.
Just checked the plugs today after a long drive and #1 was white, #2 & #3 were black and #4 was dark brown color. So with these results where do I go from here? What kind of adjustments do you think need to be made? Does the stock carb/manifold give uneven distribution to the combustion chambers? Thus the different colors in the plugs?


Lets take this one step at a time. Although you replaced the tiny o-ring on the enrichment needle did you remove the brass seat in the choke assembly? it gets tapped out thru the brass bleeder thru the small soft plug at the base. there are two o-rings on it.
What type of plugs are you using? Are you using the factory air filter? If you are is the exhaust leaking at the manifold-converter joint. I have seen a air filter that is new on the outside completly soot black on the inside. this would cause problems
Jim Pelletterie

Paul. I am not particularly competent to speak about the Z-S carb. It sounds as if others, far more competent than I, are doing so.

I am, however, competent to speak on the mixture distribution in the cylinder head. Due to the design of the cylinder head, having a "Siamesed" exhaust port for cylinders two and three, these two cylinders tend to run slightly lean, while cylinders one and four tend to run slightly rich. Normally, cylinders one and four will show very slightly sooty plugs while cylinders two and three will show a relatively clean plug. This is due to the exhaust gasses of cylinders two and three not being removed as effecively as with cylinders one and four (which, each, have their own exhaust ports, while two and three share one).

The traditional wisdom, which is often true, is that "90% of carb problems are ignition related". It might be worth checking out your ignition system, expecially the spark plug wires, to see what is happening. Your test results do not conform to the "standard practice" and may indicate that an ignition problem is the cause.

My website, has some tech articles on how to check out the ignition system. The article on testing the plug wires might be of use to you. At the very least, it will eliminate one possible problem.

Les Bengtson

Jim, yes I did remove the brass seat and replace the two o-rings in there and replaced the plug.
I am using Bosch Platinum gapped at .035 (I think). I am also using factory air filter (new filter from Napa). Are you saying that if I have an exhaust leak at this joint it could suck the exhaust into the filter and cause a bad mixture or just a dirty filter?

Les, your head explanation is opposite from what I am seeing. 2 and 3 are rich, does that mean something of a problem? I guess I better start looking at the ignition system. But the way it starts and runs it doesn't appear that there is a problem with ignition but if there is just think how much better it will run. I will check out your site first before just replacing parts (wires, coil, etc.)

Thanks guys, paul

If the exhaust manifold gasket is leaking at the converter the new airfilter will be black inside which will cause a extremly rich mixture. I have absolutly no luck with the platinum plugs. every MG that has entered my shop with these plugs came in for poor drivability. I prefer The NGK's
Jim Pelletterie

Paul. I do not know. All of my personal experience, and all that I have ever read about this system, indicates that, due to the siamesed exhaust port between cylinders two and three, they tend to run leaner than cylinders one and four, which have their own exhaust ports.

One thing to consider is that blackening of the spark plugs can be caused by more than just a rich condition, although that is why plugs are normally sooted up. The reason that the plugs become sooty is that the fuel/air mixture is burning in an inefficient manner. Thus, it does not burn cleanly and leaves some degree of residue on the spark plugs. This is, normally, caused by excessive fuel in relation to the amount of air available--i.e. a "too rich" condition.

I wonder if it would be possible to create a situation where the left over (inefficiently scavanged) exhaust gasses in cylinders two and three could cause the mixture to burn, but not in an efficient manner, thus resulting in the plugs fouling? That is the only theory that I can offer from the description available.

If this is the case, then enriching your mixture, slightly, may cure the problem. At least, it is worth trying.

Les Bengtson

I don't have any experience with platinum plugs, but is it possible they are not quite a hot enough heat range for your engine? Is there also a possibility that they are not all within the same heat range exactly? Have you tried switching the position of the plugs in their respective holes? Like using the sootiest one in the cleanest hole, then testing again?

With normal plugs, maybe platinum too, the common wisdom is to check the plugs immediately after a hard run, shut down with no idling (for the best reading, anyway).

Maybe you need hotter plugs with your driving conditions, changing to cooler if you race or run fast a lot.

Even a bit of soot out the exhaust after driving sedately around town, then giving it the gas, seems to be the way my car likes to run. My common everyday plugs are not really loading up, though. Better than too lean, that's for sure. That can get expensive.

This thread was discussed between 07/09/2006 and 20/09/2006

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