MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - LE idles roughly


My '80 LE idles roughly when cold. It has the Zenith carb with the water choke. The car starts very easily (2nd crank even on cold days), but will hardly stay running when cold unless I keep giving it gas. It also sees to load up a bit when I can finally get it to idle when cold. It takes a couple times of revving the engine to make it clear up. The carb has been rebuilt (new needle, and the choke was rebuilt).

Any suggestions?

Thanks in advance.

Dave
David Plantz

Dave. All of my cars having carburators have this problem. It is a problem I remember from those long lost days of my youth. You always needed to let the car warm up before it would properly perform. My only suggestion is to let the engine warm up for about a minute, drive it slowly (i.e. don't wind it out) for several miles to let the engine warm up (oil squirting out of hole the size of the head of a pin), the transmission to warm up and the rear end to warm up. Thus used to be the gospel for sports car people. Mayhap it still is?

Les
Les Bengtson

How cold is your cold? I'd agree that they should only get gentle driving until they are fully warm or nearly so. Doing so allows you to run on less choke, but full bore from cold is bad for the mechanicals anyway. My son's BMW M Coupe (just a few years old) comes with the warning not to use full power until it is fully warm. But then its 'full power' is quite a bit more than even my V8. Letting them warm up instead of driving off straight away has always been said to be a bad thing, not least for economy.
Paul Hunt 2


Hi Les and Paul,

I think I did a poor job of explaining what my car is doing when cold. I always allow it to warm up before running the engine to higher rpms. The trouble with this one is that it just can't keep running when cold. Our '77 B with the same carb setup idles great when cold and warm. Same goes for our TD , '72 B and '74 BGT. It just seems like a setting is off with this carb. Temps have been in the 30's here lately.

Many thanks,
Dave
Dave Plantz

Dave. Many people, used to driving modern cars with fuel injection and computer control, do not realize that a car with a carb takes a long time to warm up properly and will run roughly until it does.

As you have pointed out, you are not one of these and have other cars to make a valid comparison with.

Check for air leaks. Check the mixture and check for proper choke operation. All of these can be a factor. Also check that the carb heater, that flat, black spacer with the one wire going to it between the carb and intake manifold, is working. It was designed to both warm the mixture and to help the carb reach operating temperature more quickly in cold weather. If you do not have a manual choke, you may wish to consider adding one. I was never really fond of the water choke system, even back when there were still BL trained mechanics to work on them for me.

Les
Les Bengtson

David,

Les and Paul have forgotten more than I know, but I do have a Zenith carburetor, and it does act differently when very cold. There are a couple of good websites you might want to look at before you give up on the autochoke:

http://www.theautoist.com/zs_water_chokes.htm

http://www.paulbunyan.net/~jasko/choke/

The first one talks specifically about the "hard to start" problem, and gives some troubleshooting steps. I've identified one possible problem, but haven't been able to test it out yet.

The second is much more comprehensive, telling you how to completely rebuild the choke.

Hope this helps.

John

John English

Could well be a problem with the auto-choke, you have to be careful enough to set these up properly when you have a manual, more so with an auto in my experience. They need a certain amount of fast-idle as well as mixture enrichment, which should vary as the mixture is varied. I had an auto on a Ford V6 Essex equipped Scimitar and it took me a lot of trial and error to get that right. By contrast an SU manual is not much more than checking that the correct bits move at the right time.
Paul Hunt 2

Dave. The information that John provides should be of value. I purchased a new 79 B, to the Personal Export Delivery (US spec with the cat and a couple of decals in the trunk) while stationed in Germany. It was maintained by the dealer while in Germany and, when back in the us, by former dealers. Back then, I had a fairly good income and very little time. (About 113 days per year on the road in such "garden spots" as Tegusigalpas.)

Thus, I never learned much about the Z-S carb as it was maintained by people being paid to "make it work right". I did not have to meet the Arizona emissions control standards because, under the Soldier's and Sailor's Civil Relief Act, I got to register my cars in Florida, with did not require that I have them emissions inspected.

Thus, your comment that, "Les and Paul have forgotten more than I know" is not correct. Specifically, in my case. I suspect in Paul's too.

My 79 LE came with a very well performing Z-S which could not be tuned to meet emissions standards. I could not do this. Professional mechanics could not do this. Hence, a conversion to the Weber DGV which seems to be allowed under the Arizona emissions standards (they only know that this year has a single carb and do not know the difference between a Weber DGV and a Z-S).

I do know that the needles kept getting leaner over the years. I also know that my 68 GT runs poorly when cold, mediocre otherwise, when I install the "emissions testing needles" for emissions testing.

If you want to keep the car original (mine was on a "Salvage/restoration" title which significantly lowered the resale value--but my daugher had to have it), a richer needle might be the cure for your problem. Burlen Fuel Systems would be the place to check with. Perhaps a slightly richer needle, if you do not have to pass emissions testing, would be the key?

Should you have to meet some form of emissions testing standards, might I suggest the "Les Bengtson approach--Fugg it!". My cars are tuned, as close as they possibly can be, to the required emissions tuning. They will be tuned to meet emissions standards at the time of the test. I do not give a ratt's tutu about the fact that my best performing needles give .005% more CO than the standards they require.

Should you decide to dig into this subject more deeply, there is a great need for someone with expertise in properly tuning the Zenith-Stromburg 175CD carb. Please pass on what you may learn.

Thanks, Les
Les Bengtson


Hi Les,

Your suggestion of using a richer needle sounds like a good idea to me. Right now, the mixture is set to full rich, but I never get any black smoke at the tail pipe. I know it's still running lean. I went back and checked the float height, and the needle is one for the later year cars. One thing that I haven't checked is the brake servo. I had one of those leak on another '77 that I had a few years ago. Most of my MGB's with the Z-S carbs would always run terribly rich. Ultimately I'd like to put a set of HIF 4's on this car. Also, the induction heater is plugged in and getting power, but I'm not sure if it's working. Thanks again for your help.

John,

Thanks for the Z-S carb info. Those websites are helpful.

Paul,

When I rebuilt the carb, I followed the manual as far as setting the clearances on the choke. I had the choke rebuilt professionally. The mating surface was warped as well. Many thanks for your suggestions.
Dave Plantz

Do you have the stock air filter on ? Cause in aviation carbureators ice up at altitude and when on a decent you always put on the carb heat or else the carb does exactly what you stated, and the whole original air filter monstrosity on the B I think trys to counter act that problem with that metal flap that moves to take hotter air from the manifold.
James

This thread was discussed between 04/03/2006 and 06/03/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.