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MG MGB Technical - Weber DCOE or DGV?

I have run the DGV on several cars but I have never been in a car with a DCOE. Has anyone had experience with both carbs? How much power difference is there and how much did the DCOE affect mileage?
Bryan

Bryan:

I have run both and currently have the
dcoe on the GT. Both work well but are very different. The DGV 38/38 idles and runs well, but will feel lethargic compared to the SU carbs, you really have to put your foot into it since the second barrel does not open with the first one.

The DCOE is good for going fast. There has always been a slight hesitation on acceleration and I can not get rid of it. It seems common to the breed. DCOE tunning is a black art of which there are few good practicioners. I run a full mechanical advance distributor, so it does nothing for the mileage.

If I was in the market, I would be looking at the 38/38 DGV version available from Brit tek. The two barrels open at the same time. Use a vacum advance for street use.

Pete
Pete

Twin HS4's
Richard Cranium

What an asstoot observation!
.

, said Mr. Pot...er, Mr. Period.

This question comes up quite a bit. You can search the archive, but in general, it depends what you want from the car.

I'll preface by saying that I have DCOEs, DGVs, and SUs. I use SUs on all my cars now.

Generally, DCOEs are good for speed and DGVs are good for "reliability", meaning in most cases DGVs replace worn out SUs. The owner is then amazed at how much better the car runs with the DGV. SUs in proper tune and condition do everything a DGV does only better. So, for the street, I use SUs.

For cars I have raced (MGAs and 'Bs) I've had modest luck with DCOEs, but have since gone to SUs. I was never able to get a responsive throttle throughout the rev band. Once the the engine winds up, it really is an acceptable carb. The delay feels a lot like turbo lag, if you know what that's like. A DCOE isn't the best DD carb, but I've done it. MPG suffers, but top end power (or at least perceived top end power) is greater with the DCOE. That said, unless you have the engine to take advantage of the extra flow, SUs are enough for most street cars.

My personal choice has been HS4s, but I have HIF4s on my GT and I like them just as much.
Kemper

A pair of SU HS4's are hard to beat for the street
- even with a mildly tweeked engine. I prefer the
HS4's external fuel bowls as it's easier to service
the needle valves than with the HIF4's

I have been running with a 45DCOE for many years
with absolute dependability. Having said that, I do
not recommend it to most folks as setting up the
carb requires a lot of road testing and fiddling with
extra jets, etc.

A DCOE really shines when you add in a mild cam,
slightly higher compression, and mild porting.

Pete: Have you tried swapping to a larger set of
accelerator pump jets in order to stop the slight
hesitation on acceleration you are experiencing?

Don't be timid here. Try stepping up to larger jets by at least 2 sizes. After the hesitation goes away, then try stepping down to a smaller jet by 1 size
(in order to avoid wasting fuel).

If there's no change, then try swapping the fuel
bowl inlet jet with one that has a smaller fuel
exhaust port.



Daniel Wong

I've spent a few years running a DCOE and would second the comments made: it requires heaps of setting up to get right, is tremendous fun once you are there, but only makes sense on a modified engine. Fuel consumption will be damaged not least because it feels and sounds best when driven with the throttle wide. I've no experience with DGVs but FWIW the SUs on my Roadster are great from idle to flat out, and suit the (mostly standard) engine.

Pete, if it's a stumble from idle, I cured that with an extra progression drilling

Kemper, part throttle response (daily driver!) has been much improved since plumbing in some manifold advance

Daniel, what chokes are you using?



Steve Postins

Steve:

Chokes = 34mm
2nd Ventrui = 3.5
Mains = 145
Pumps = 45
Emulsion = F16
Air Correction = 165
Idle = 50F9
Fuel Inlet-Exhaust = 45

I'm running with flat-top pistons, Demi Elgin cam
(installed slightly advanced), ported head,
Special Tuning exhaust manifold.
Daniel Wong

Thanks Daniel. As you know, there are more combinations than a Rubik's cube and I'm always curious as to what people are running.

I've been running this set up for a long time:

Chokes = 36mm
2nd Ventrui = 4.5
Mains = 160
Pumps = 50
Emulsion = F2
Air Correction = 160
Idle = 60F9
Fuel Inlet-Exhaust = 40

The engine's a 1995cc with 9.5:1 cr. Following a useful rolling road session with Peter Burgess I've changed to the following:

Chokes = 38mm
2nd Ventrui = 4.5
Mains = 165
Pumps = 50
Emulsion = F2
Air Correction = 160
Idle = 60F9
Fuel Inlet-Exhaust = 40

It's been a good move on this engine; it's liberated the top end and the car is still fine for town driving.
Steve Postins

Danny.

Might I point out my asking you about some form of tech article on setting up the DCOE for the MGB engine and when its use would be most appropriate? Please consider taking the time. Currently, we are running over 10K "hits" per month on the website and your article on bench bleeding the master cylinder is consitantly in the top ten articles people look at. You trusted me that it would be important to write such an article. I felt it was, and you responded. The facts demonstrate that it is important to a number of people, demonstrated by the number of people reading it, and by the number of other websites which have linked to the article.

You are one of the most competent people I know on setting up the DCOE for the MGB, with all of the multiple factors involved. Thus, please be so kind as to write up a tech article for all of the people interested in this. I will post it, should you desire, or link to whatever website you should post it on.

We have been on this BBS, together, for quite a long time. You have emerged as the individual who has the best expertise on this particular subject. Most of the rest of us are without a clue (clew). Thus, if the knowledge you have available is not absolute, you have a better understanding of what is involved than most of us and, should you be able to, could put this knowledge, both what you know and what needs to be still worked out, in the hands of people who are just learning about performance upgrades.

Please consider this. Thanks. Les
Les Bengtson

Hi, I just finished an engine rebuild on a 1975 MGB, and this is my first experience with these cars. I have the following questions:

1) Could some one clear up for me what DGV ,DCOE, HS4, and SU means?

2) The engine has an intermittent miss in it, and I am considering rebuilding the Zenith/Stromberg carburetor, and would like any tips on what the negatives are on this carburetor?

3) The mechanic that removed the air pump years ago, for some strange reason removed the air regulator off of the side of the carburetor, and blocked off the ports. What effect might this have on the intermittent miss?

4) The engine seems to "dull out" a bit, and the catches up when the engine is rev while leaning over the finder. Could this suggest vacuum advanced problems? I have not yet mapped the advanced as a function of RPM's.

Thanks Layne Wilson

Layne Wilson

QUESTION 1:
DGV - Weber two barrel progressive carburetor. Simple bolt-on and reliable (down draft). Performance not as good as a set of SU's (IMHO).

DCOE - Weber high performance carburetor. Complex and must be tuned by experienced person for proper function (side draft). Overkill for (near) stock street machine (IMHO).

HS-4 - Version of SU carburetor fitted to MGB's from 1963 to 1971.

SU - Skinners Union - At the turn of the last century, there was a shoe company in England called Lilley and Skinner. The Skinner side of the company was one George H. Skinner, a genius of sorts, and in 1905 he was granted a patent on the SU carburetor. He soon teamed with his brother, Thomas Carlyle Skinner to produce a working model. (by Skip Burns)

QUESTION 2:
ZS NEGATIVES - not heavy enough for a boat anchor. I have two B's that came to me with ZS carbs. The first one (on a '76) went in the trash and was replace with a set of HIF-4's. The second one (on a '80) was destroyed in an engine fire and was replaced with a set of HS-4's (AUD135).

QUESTION 3:
I don't have a clue.

QUESTION 4:
Could be the distributor (and/or vacuum advance), but could also be a ZS carburetor problem.
Steve Buchina

Les -

Flattery will get you everywhere.

I've been up to my hip in other projects and so I often
set aside my Weber DOCE for MGB how-to article
in order to stamp out other fires. All I can say is
that I've not forgotten about it and I've had the
article stewing & brewing in my mind for quite
some time.

Adapting a DCOE to an MGB is not a simple bolt-on,
turn-key affair. It requires a bit of home fabrication
and a lot of road testing and experimentation and
patience. Trying to cover all of the bases on a webpage
is going to be daunting and, in fact, will probably require
several webpages in order to make good reading.

Having said that, in the meantime, the good folks at
Teglerizer have a set of excellent webpages that are
very good at explaining most of the basics for getting
started on a Weber DCOE conversion for MGB.

http://www.teglerizer.com/dcoe/webertune.htm
Daniel Wong

Danny. Many thanks. Les
Les Bengtson

Les - Would a DCOE count in Arizona as a "single carb" for emissions purposes?
John Z

I've also run a DCOE on both a street MGB and race MGB for many years. My experience is that you can either set the DCOE up for low and mid range driving with good transition, or set it up for high rev power. You can't do both effectively.
The manifolding for the siamese port head just will not allow adequate flow speed to keep the mixture atomized with the large chokes necessary for top end power.

If you use small chokes as Daniel is doing the DCOE is a very effective daily driver carburetor. It will get good fuel mileage and be very tractable. On my car I went even further to a 33mm choke, which completely removed any hesitation without causing birds to fall out of the sky everytime I had to accelerate. With these chokes, though a pair of SUs will out flow the carb. at top end.

With larger chokes, such as the typical 36mm that comes in the standard issue carb the air flow becomes so slow at idle and tip-in that the mixture does not distribute effectively and typically the engine will only idle on three cylinders. As Daniel noted, bogging under tip-in can be reduced by increasing the pump shot, which allows enough fuel to wander down the manifold and into the engine to get going, although the environmental impact of so much raw fuel is enough to gag a sparrow.

On the other hand the DGV is a much better balanced carburetor for street use as the small primary gives good enough air speed to allow the use of even the silly gooseneck manifolds needed for cars like the 1500 Spitfire and Jaguar. But the progressive nature reduces driving enjoyment as the carb. will feel strangled until the secondary opens and the pump circuit again has to be right on to prevent bogging problems under tip-in.

The non-progressive downdraught has the same inherent problems as the DCOE, but with the added insult of a horrible manifold design. I can not see how the design would give much advantage as the carburetor is not as adjustable as the DCOE and so getting the correct pump shot to cure tip-in bog would be tricky. I have not owned or driven one, so I am open to the argument of experience.

After running both designs of carburetor for many years. I do come to the same conclusion as many others on the forum. MG had it down when they specified a pair of 1 1/2" SUs for the engine. For all around drivability and power there is not a better normally aspirated set up readily available.
I do say readily available, as I have driven a twin Mikuni slider set up that is a definite improvement over the SUs. No bridge, so much larger available venturi and an accelerator pump.

that's my two bits and bit more

Kelvin.
KJ Dodd

John,The anser is no. The DCOE is not legal in Arizona, but the DGV is. Ray
RAY

John. Might be interesting to find out.

Les
Les Bengtson

Kelvin. I am in the process of getting my GT back on the road after floor replacement. Need to get it ready for emissions testing within the next two weeks. Hence, have been driving it a lot both for casual and communting driving. The twin SUs are, as you note, far more responsive than the DGV on my 79 roadster and the GT just seems more fun to drive. Very subjective, I will agree. But, after over a year of not being driven, the GT seems happy to be back on the road.

Les
Les Bengtson

Great discussion! No one has mentioned the 13/4 inch H6S SU carbs. My engine has been rebuilt by APT, V12 cam, 1.5 ratio roller rockers, aluminum head and headers. I have a Weber 30/36 DGV and manifold in the box, but I feel like the DGV won't be enough carb. The DCOE sounds like too much trouble. The H6S in more carb than the H4S's and looks like a direct replacement. Any thoughts?

Mike
M.A. Barrera

I forgot to mention, compression ratio of 9.5:1. Also, I have a DGV 32/36 carb, not 30/36. Engine has not been installed or run yet.

Mike
M.A. Barrera

Mike. The HS-6 carb requires a different intake manifold than the HS-4 does. Hence, it is not a "direct replacement". You would need to purchase a new intake manifold to mount the carbs. I believe that Moss has these available.

I am currently using the Weber DGV 32/36 on two of my cars and on my daughter's car. All of which need to meet emissions testing requirements, which the Weber does.

Steve S. might have some comments on how well an HS-6 carb system would work. Peter Burgess has written a quite excellent book on the MGB engine, "How To Power Tune MGB 4 Cylinder Engines", which contains a lot of good information which may be of value to you. Steve you will need to contact through the BBS, by a posting mentioning him in the title. Peter has a website which you may contact him through. Both have been very good about assisting me in the past and I do not hesitate in recommending you contact them for the best information available.

You should be able to provide them with the bore oversize and whether there has been any additional work done to the head. I am not aware of what a "V12 cam" is except when used on a V-12 engine. Thus, the cam specifications would be appropriate to include if contacting them.

As to the DCOE, my friend, Danny Wong, runs one and he is one of the more knowledgeable people I know about both that carb and things MG. Unfortunately, like most of us, he has to work for a living and does not have as much time to spend here as he might like.

Les
Les Bengtson

Mike,
FWIW, Carl Heideman said at MG2006 that HS4s/HIF4s are good for up to about 150 hp. Based on that, for most motors HS6s would be too much carb.....
Rob Edwards

Has anyone had experience with the weber 34/34 downdraft? Rather recent option for a B, but I haven't been able to find any reports on its use. Thanks.
Bob Johnson

This thread was discussed between 04/10/2006 and 23/10/2006

MG MGB Technical index

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