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MG MGB Technical - Carb problems 77B

I've started having starting problems, it usually starts up ok but unless I keep the engine reved up it it loads up really bad, starts running on less than all 4 cyls. The exhaust is producing black smoke.
Decided to unhook the water choke. Wired the choke off but engine still runs awful.
The car appears to be all original and I have driven it a couple of thousand miles with this system working fine. I think the carb is a Zenith/Stromberg.
I searched the archives and wonder if a different carb may be the answer. Thank you for any response Jim.
JWG Geeslin

Whilst a different carb might cure it, so will a different car. What I mean is replacing the whole thing may be a far bigger hammer than the nut warrants. Black smoke indicate rich mixture, so your starting with the choke may be on the right lines.
Paul Hunt

Pauls right. If you need adjustment info for the ZS carb and choke look at http://briefcase.yahoo.com/bc/bubblechaser343/lst?.dir=/MGB+Maintenance&.view=1

Mike
'79B
Mike Janacek

JWG, it sounds like something is stuck open or plugged up in the carb. You probably just need to clean and rebuild it. It's not that difficult a job and will make a world of difference in how the car runs. You can get a rebuild kit from Victoria British or Moss. There are also some great resources on the web for tuning the Z/S carb.
As for putting on another carb it’s not that complicated but will require parts and time to do correctly because you need to change the exhaust and the intake both. I kept cracking the stock intake/exhaust manifold on my 79 so I changed to a Weber DGV 36/32 and 72-model exhaust system and removed all the smog equipment. Now it feels like a different car. Just keep in mind that it is illegal to modify the emission system on a 77-model car and if Alaska has an emission inspection you mat get in trouble.
Eric

I'm not sure if you've disabled the choke completely.
There is an o-ring on the choke metering needle that
can leak. In this case, you will run rich even
if the choke is "off".

In addition to the pointers above about rebuilding,
check if the diaphragm is torn -- this will cause
the carb to also run very rich.htm

Finally, check out Rick Jaskowiak's excellent
web site on choke rebuilding.

http://personalpages.tds.net/~jasko/choke/ZSchoke.htm






Ronald

In addition to the above:
Have you tried leaning the mixture with the metering needle? It takes a special tool or you have to take the air cleaner off the hold the piston so it doesn't rotate.
Is the air cleaner element ok? And is the air cleaner on? ZS run like crap without the filter.
Check out picture 16A in the website Ronald mentioned. If this o-ring is worn, it will run rich whether you close the choke with the water setup, manually or with your wire.
Rick Jaskowiak

All of the suggestions so far are things to try, especially the easy stuff like changing the air filter.
If all that doesn't produce results contact me directly about the ZS carb I have in my garage.
Randy Olson
holtian143@hotmail.com
1977 MGB with twin SU's
randy olson

Thank you all for your ideas, I will review all the information sources you have pointed out and check further into the internal condition of the carb.
Looks like I will have about 6 months of winter to figure this one out. Jim
JWG

Jim,
I had a similar problem with the ZS carb on my 79 'B after reinstalling the original carb and assoc gear after failing an inspection. Try looking in the new Moss Motors MGB catalog on pg 85 for a lot of tips from John Twist. I completely removed the choke assy and covered the holes in the carb body, and the car still ran rich and emitted black smoke. I finally discovered that the jet was worn, I could slide a 1/8" drill bit easily into it. If you can do this, it needs relacing. I ordered a new one (they advise professional installation) but if you're careful and take your time, you should be able to do it. Just make sure you line up the emulsion holes and tap the new one in to the same depth as the original. The new jet solved my problem, no more running rich and the plugs are a nice tan color now.

Good luck,
Mike
Mike Haag

I agree with Ronald- check that diaphragm and go ahead and replace it anyway if the choke turns out to be the culprit. It's cheap and good insurance.
:>)

Mike,
I too am having some trouble running rich with my ZS,
What did you use to cover the holes when you removed your choke. I might like to try this also.

Ronald,
Thanks for the advice you gave me on my thread "Small leak at ZS carb" It looks like I will need to take the choke assy completely apart and renew the o-rings and such. One question still. Should I remove the entire carb again, or can I just remove the choke assy with the carb in place? I just had the carb off a month ago to install a new heat sheild.
John
John F

John, you can easily remove the choke assembly
by itself,(just three screws) but you may want to
have a new choke-to-carb gasket on hand in case.
Ronald

John, I used some clear shipping tape I had around to cover the 2 matching holes in the carb body. I wouldn't use scotch tape, need something a little stronger, but something that can be removed fairly easy when you're done testing. Run the car until it is at normal operating temp, then shut it off and wait a few minutes so you can work on it without getting burnt. Start it up with the choke out of the way, and look for leaks around the holes you covered up. Any leaks, shut it off until you get a good seal before trying it again. As others have said above, it's a good idea to try easier things like a new diagphram, and replacing the o-rings in the choke assy. I tried all that, including a new needle, to no avail, that's why I needed to separate the 2 devices to see where the problem was.

Good luck,
Mike
Mike Haag

Thanks, Mike and Ronald,
I have the choke off the carb now, it is on the bench I will be tearing it down this week as time allows. I still have the o-rings and a spare choke-to-carb gasket, as I did not use these parts from the kit last fall when I rebuilt the carb. I will be using Rick Jaskowiak's tech article to guide me.
As I mentioned earlier I have recently developed a fuel leak dripping off the choke assembly, which appears to be coming from the area of the enrichment tube plug. Which btw I discovered is missing entirely (the plug).
Am I correct in assuming that fuel is supposed to circulate thru the choke assy?, and if so will it come spewing out of one of the carb body openings if I try running it with the choke assy off? Even if taped? or are those openings for "air" only?
The carb to choke gasket is in great shape. Should I use any permatex when reinstalling maybe the "Black" non hardening permatex.
I will also check the diaphram, and I may change the needle while I have the piston out, currently I am running with a 45H, I did pick up a 45M (high Altitude)this summer, maybe that is what I need as I do live in Denver, and often drive it up into the mountains. I did check the apperance of the jet last fall and it was nice and round not oval. However I did not measure the size of it.
I will keep you posted as to how I make out.
John.
John F

If all else fails, a 2 1/2 inch SU with a four-bolt flange will bolt straight on. You will need to change the air cleaner and run a manual choke. Your fuel consumption should improve by about 10% on the old ZS. A variety of British vehicles used these, including Morris Marina and some of the landrovers. The early Rangerovers ran the same flange size, but I am not sure if they would suit your purpose. I found my SU at a swap meet for A$10. Very easy to tune, too.
Live long and prosper, Greg
Greg Taplin

I am the new owner of a 1976 MGB. Not currently running.
It has a newly rebuilt 1972 engine. I'm told the single carb from the 76 does not run the enginge properly. I'm told that it needs a "weber". Can any one tell me what carb and model/set up a 1972 MGB was stock. Also I'm told the exhaust system does not fit beacuse the 1972 engine does not have emissions set up. Has anyone else done this?

Thanks
Tim
Tim OK

The 76 originally had the single ZS carb; the 72 had the HIF-4 twin SU's. Neither need the emissions - the 72 didn't have it originally, and the 76 runs better without most of it. John Twist has a great article on desmogging the later MGB's. You could put on the Weber, but I put on twin SU's and the car runs great.
Randy Olson
1977 MGB twin SU's, less emissions
randy olson

Seems to be a couple threads going on here.
John from Colorado...Fuel does flow thru the choke when the enrichment needle is open. Vacuum from the carb throat sucks it out of the float bowl. If you tape the openings, the vacuum and route to the float bowl are interupted and no fuel should spew out. It probably won't hurt anything to use some gasket sealer, but only very sparingly.

Tim from Wisconsin...Do you have smog checks in your part of Wisconsin? If you do, that may determine what you can do with the carb(s) and emissions.
The 76 and 72 engines are basically the same, and will run with a ZS, SUs or the Weber. You may want the SUs or Weber, but you don't "need" to get rid of the ZS. A properly tuned ZS will run your car just fine. Getting the ZS carb properly tuned can be frustrating, and leads many to do a conversion to the SUs or Weber, me included.
Like Randy, I run dual SUs with minimal emissions on my 78. Emission controls changed through the years but the engine will run setup like the early models (basically no emissions), or the later models (full emissions), and anywhere in between. If you have no smog laws to follow, it's up to you.


Rick Jaskowiak

Thanks Rick,
I will try tape over the openings now that I have the choke assy off the carb. At least that way I will know if it is running rich because of the choke, or if I will need to explore other avenues. And thanks for your great tech article on the ZS choke.
John 79B
John F

This thread was discussed between 09/10/2002 and 17/10/2002

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