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MG MGB Technical - '77 MGB Carb Adjustment?

I have a 1977 MGB and I have been replacing parts since I bought it in 2000. The original ZS. carb has been rebuilt by Britsh Connection in Bellflower, California. The car has a real high idle sometimes (3000rpm), and sometimes real low to a point where I have to keep feeding it gas or it will die out. I don't know how to get the air and fuel mixture just right and the car has been to a British Specialist about 4 times and they haven't fixed the problem. What advice do you have for a novice in MGB tuning?
Anthony Berrelleza

Anthony. Find a better British Specialist. Switch to a Weber DGV. Les
Les Bengtson

The huge fluctuation in idle speed from 3000 RPM
to stalling sounds like an autochoke problem.
Check out Rick Jaskowiak's website -- even if
you don't rebuild the choke itself, studying his
superb explanation of operation might help isolate
the problem.

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

Ronald

Les:
I know you're a proponent of the Weber DGV... you have provided some good comments to me in a past thread on fine tuning.
I have not gotten any marquable results from any suggested adjustments prior to storing the car for the winter. So, I am now considering maybe its time to have my 32/36 DGV rebuilt. I am looking at either a local mechanic who claims he can do the job (he's rebuilt other Carbs, but not a Weber ????... they're all pretty much the same, he claims ?)Or send the unit to Performance Specialist in New-York state (they have a website you referred me to look up.)that specialize in the Weber line and have them do the rebuild job. I have the Madden book on Webers. I would not attempt to rebuild myself even if it looks straight forward in the book. This is off topic but I am interested in your thoughts. thanks.
(1978 B from USA origin - Weber conversion by a PO)
Richard Standish

Richard. I am a "great fan" of the Weber because I have to be. Here in Arizona, we have annual emissions testing for all cars made from 1967 on. Some of the newer cars do not have to be tested for several years after purchase, but all 67-80 MGs must be tested annually. For the 75 on cars, the inspectors look and make sure you "only have one carb". Thus, the Weber is an acceptable replacement for the aging Z-S. While one version of the Z-S is available from Burlen, I believe it is not the exact version used on the MGB. It is also much more expensive than the standard 32/36 DGV series conversions. All this makes me a fan of the Weber DGV, for use on rubber bumper cars, until I move to somewhere I can install a set of twin SU carbs. Although, Brit-Tek is now stocking the 38/38 Weber with the simultaneous (sp?) opening throttle plates. This carb might be just the thing for performance use on an RB car, but I will not know until I have a chance to use one for a while or get to speak to someone who has this set up.

Long way around to answer your question. You can probably rebuild the carb yourself if you have a good book. I used the Pat Braden book on the Weber and the rebuild worked two times out of three attempts. Still need to find time to find out what the problem is with the one carb.

With that bad carb, and because I needed it back on the road, I purchased a new DGV (the manual choke model) and installed it. Was not particularly expensive, about $225 as I remember. Worked well and we are starting off with a new carb rather than a 15 year old carb that has been rebuilt at least once, perhaps more. So a new carb is also a possibility.

If a new carb is a possibility, and money and emissions are not limiting factors, the Weber 38/38, as sold by Brit-Tek, might well be an outstanding carb--perhaps as good as a set of twin SUs.

As to whom to have it rebuilt by, if you rebuild it and screw it up, it is a learning experience. If you pay to have it rebuilt, your only learning experience is that it costs X number of dollars to have a carb rebuilt. So, the ideal situation would be to have your local mechanic, for a fee, supervise your rebuild of the carb. If that is possible, you will probably pay as much as if you had it professionally rebuilt, but will have gained both a rebuilt carb and the knowledge to do the job yourself.

So, you are balancing the cost of a new carb, which should be a "known good starting place" against the cost of a rebuild on a carb which may not be capable of new carb performance, even after rebuilding. But, if you rebuild it under supervision, you will have gained a new skill and will have a greater appreciation of what is involved. The Weber is very similar to the Holley and other US/Canadian carbs. This means your local mechanic is correct in "they are about the same" and any knowledge you pick up will be useful for a variety of carbs, not just the Weber.

Please let me know what you decide to do and how it works out. Les
Les Bengtson

With regards to Anthony's problem with the Z-S.
California has a very strict visual inspection.
Even if the tailpipe emissions are within spec
using the Weber, a 75-on MGB will still in theory
fail the visual since the carb is not OE.

_However_ I think I saw a posting where someone
passed Calif. visual inspection using a Weber
downdraft.

Some others have passed the visual using a single SU
carb instead of the Z-S. Since the SU is
also a sidedraft, all but the most astute (to
put it politely) smog techs would pass it.
I noticed that Moss is now selling the single SU
conversion for the Z-S along with VB.

A final option is to buy a new Z-S from Propermg.
As Les noted, this is not the MGB carb. It looks
like a TR-6 carb, since it has the thermal
compensator. But it would definitely pass
visual.

One final thought, make sure the throttle shaft
is not binding, which could cause dramatic fluctuations
in idle RPM. If the nuts on the throttle shaft
are overtightened, it will bind. Obviously
check the throttle cable. But I still bet on
the choke.
Ronald

Ronald. I thought the 77 model year was not testing exempt in California. The Weber is an excellent carb. But, the twin SU set up is better. (This may not apply to the 38/38 sold by Brit Tek. I have not tested that carb.) However, the bottom line is that Anthony has had his car into "a British Specialist about four times and they have not fixed the problem". This either indicates the carb is junk or the "specialist" does not know what they are doing. A second opinion may be called for. Or, more information from Anthony. Such as, "Did they tell you your carb was junk and needed to be replaced?". Any piece of machinery has a finite rebuild life. Once that is exceeded, there is not a great deal which can be done. Recently, some friends of mine worked on a car for a fellow who needed to get it through emissions. The owner did not want to spend the money on a Weber conversion (acceptable here in Arizona), but wanted the Z-S to be reworked to pass. It took months of work, hundreds of dollars, etc. to get the Z-S to pass. Cost more to tune the Z-S than a Weber conversion would have cost. Weber makes a good carb in their 32/36 DGV series. Not a great carb, like the SU, but a good, easily tuned carb which will provide excellent service for a number of the "hobbyist drivers". The old quip, "If MG needed an Italian carb, Churchill would have worked a deal with Mussolini" may have, an probably was, true when the MG came with SU carbs. If he had known that MG would have been stuck with a Z-S, we can well belive "Winnie" would have struck the deal. He was a very practical man.

Part of this, I must admit, is my own ego talking (writing). I have never been able to tune a Z-S carb, on one of my cars, to pass emissions. However, when the car was taken to professional tuners, they could not either. Ronald's suggestion of an SU HIF-44 might well be the best solution if you must have a Z-S. Only the experts will know the difference and very few experts work for emissions testing companies.

Anthony, I hope you can get the problem sorted. Les
Les Bengtson

Les:
Much appreciate your comments and added insight.
I have sent a separate note to your e-mail address on your suggestion.
Cheers!
Richard Standish

Anthony,

I hesitate to add a comment behind Les, because he's sewn it up so well. However, I'd like to reinforce one thing he mentioned. If your British specialist claims to be able to rebuild a carburetor, but can't solve a high idle, you're overdue for a second opinion.

To that end: I know of two fine British car mechanics in California. Both are in the North, but may be able to recommend someone down your way. The cost of two phone calls is a cheap gamble. They are Doug Jackson at British Automotive in Novato (www.mgbmga.com) and Lawrie Alexander at British Sportscar Centre near Sacramento (www.britcars.com). Both have been quite helpful to me in the past. (Granted my correspondence with Mr. Jackson was brief).

As Les notes, the opinion may come that your carb is done for. This advice should not be ignored. I too believe that a carburetor can be so worn that it's more expensive to refurbish (properly) than to replace.

If you decide to buy a rebuilt unit, trusted sources are Joe Curto and Midel (www.joecurto.com and www.sumidel.com respectively). I would also check with Brit-Tek (www.brittek.com). They'll sell you the same parts as most mail order houses, but their service is top notch. The guy who answers the phones has been building MG's forever and is quite happy to chat technical with you. They try very hard to help you get the right parts for your needs and get them installed correctly. (No relationship there, except as a satisfied customer.)

Best wishes,

Matt K.

Matt Kulka

I had a problem with my 78 somewhat similar to Anthonys' several years ago. In my case, the idle speed screw was missing (there is an idle screw and a "fast idle" screw). I had the car in to a local mg specialist to tweak for passing smog; after several hours (and $$$), they got it through, but it didn't idle consistently. It turns out that they had been setting idle speed using the fast idle screw, which is affected by the choke. I got a replacement idle screw (from moss, they have unusual threads), and the idle settled down nicely. Next time around (2 years later), I spent the tuning money on a new cat converter (old one was a hollow shell) and air pump, and passed without problem. It is rather annoying to have to preserve all this not-very-effective smog gear, when the Weber could probably tune to be just as clean, but that's life in CA.
Tom G

This thread was discussed between 13/01/2003 and 14/01/2003

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