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MG MGB Technical - MGB WEBER

I am thinking of upgrading the twin su's on my 1972 mgb gt to a twin choke 40dcoe weber carb. Has anybody fitted a 40 weber to their car?? Are there any problems with fitting this carb to the car?? Also does it make the car less driveable at low speed or cause difficulties at idling? How about fuel consumption?

Thanks
Chris
c durnford

If you are not at all familiar with the Weber DCOE
and what they entail to get them properly dialed in,
then I would recommend against swapping out
the twin SU's.

Bolting up the carb and manifold to the engine is
no problem....it's the subsequent tuning and testing and minor fabrication, and modifying and recurving the dizzy...and retesting....and buying
jets and tubes and retesting and...well...you get
my point.

When properly tuned the twin SU's are nice and
snappy and are rather hard to beat.

Swapping to a DCOE requires a bit of tuning skills
and patience.
Daniel Wong

hi. I had (note the word had) a side-weber and replaced it back to su's. everything daniel said is true. they are great for racing, but for everyday use stick with the su's.
mike

General consensus is that the Weber is not an "upgrade" without extensive other mods to improve breathing - compression, cam, head work, high performance header (and it is hard to beat the flow of the stock CI header).

On the other hand - your stock SU's maybe getting old and tired. Due for a rebuild, rebush, etc. etc.?

Racing MGs did get side draft Webers or one size larger SUs - but also benefitted from the other stuff, too.

One of the most common fallacies (in life, not just MG ownership) is to take an old, worn out part and perform some sort of "hot set up" fix and end up doubly screwed. The "new thing" isn't really that good - especially when matched up with the environment - and you've passed up the opportunity to just put it back to where you should be and where the original specs and requirements were.

Not to say some stuff isn't helpful, but it needs to be carefully considered.

Sort of like dumping a wife for a girlfriend 20 years younger than you are.
John Z

I run a 45 DCOE on our rally GT. As suggested above it does need setting up properly and the only way to do that is on a rolling road. The best one for that down south UK is Peter Baldwin at Marshalls near Royston in Cambs. Once set up then they are more stable that SUs I find.

Problems with DCOE is the fuel consumption, less than 20 mpg at times and lower torque in mid range, depending on cam etc. We run a Peter Burgess head with a Piper 285 cam and get max. power around 5,200 but plenty of power from 2,000 upwards.

I reverted to SUs on our Roadster with a similar spec engine as it is more tractable for running around and the fuel consumption is less.
Graham Gilmore

I fiddled throught the array of Weber, finally throwing up my hands and the "Outlaw" into the round file. Thet are just not MG friendly and too much fiddle for me. Vic
vem myers

Our GT had a Dellorto twin choke carb and it made the car go. It has only doe a few miles and is for sale. Offers?
Bob Marshall

Chris-
When it comes to the subject of carburetion, many people tend to opt for items that they perceive as being exotic, such as the Weber DCOE or the Dellorto DHLA. The Dellorto DHLA atomizes fuel to a much finer degree than the Weber DCOE does, producing more efficient combustion and better fuel economy than the Weber DCOE. However, the finely atomized fuel droplets combust more quickly, forcing the use of a richer fuel-air mixture. The fuel thus takes up a greater volume in the induction tract than the larger droplets produced by the Weber, displacing air and making for an effectively smaller fuel-air charge, and thus less power output. However, due to its production of a finely atomized fuel charge, combustion becomes more efficient, creating higher fuel economy and less pollution. Conversely, the less-finely atomized fuel produced by the Weber DCOE design requires a more generous supply of oxygen in order to combust properly. This is the reason for that carburetor’s reputation for being sensitive to changes in altitude. If you switch from a Dellorto DHLA carburettor to a Weber DCOE carburettor, you should easily get a 5% increase in power output across the entire powerband.

The use of the Weber DCOE 45 carburettor on street MGBs came about as a result of their use on the factory team’s racers. This fact, of course, produced a “monkey see, monkey do” mentality amongst those seeking more power for their street MGBs. Why did the factory race team choose the Weber over the tried-and-true SUs? It has to do with the design differences between the two. The SU is a Variable Venturi type, which makes for smooth although slightly slow throttle response and excellent fuel economy. The Weber DCOE 45, on the other hand, is a Fixed Venturi type. It has the advantage of having an injector pump to shoot raw gasoline into the venturi when the throttle opens rapidly and thus makes for very fast throttle response. This was a definite advantage on the racetrack, so that is part of the reason why the factory race team chose it over the SU. Remember that on a racetrack, smoothness and economy must be subordinate to responsiveness, as its responsiveness that makes aggressive driving possible. Victory is what counts on the racetrack, and nothing else will substitute.

This fast throttle response produces the illusion of more power and so purchasers of this unit tend to experience what Psychologists call the “Halo Effect”: they have paid out the big money, sweated the installation, spent more money to convert their ignition system to a centrifugal advance distributor (Weber carburettors do not have provision for a vacuum takeoff for working with vacuum advance ignition systems: read the fine print!) and so they are already predisposed to feel the power increase even before they drive. When the quick throttle response creates the illusion of more power, they become like religious converts! In reality, all other factors being equal, there is no worthwhile difference between them in terms of power output on the dynamometer readouts unless a radical camshaft is being used.

Should you decide to use the Weber DCOE carburettor, you would be well advised to use a Soft Mount kit to protect it from the effects of vibration (APT Part # SMW-45). At the point when the engine creates its greatest levels of harmonic vibration, fuel can froth in the floatbowl, causing the fuel-air mixture to run lean, imperiling your valves and piston crowns. Be warned that under no circumstances should the Weber DCOE carburetor be bolted solid to intake manifold. When installed, the rubber O-rings of the soft mount kit should be compressed between the intake manifold and the carburettor by aircraft-type Nyloc nuts only to the point of providing an airtight seal. Both the carburettor and its intake manifold should then be held in place by the simple system of an anchor plate, a strut, and an anchor nut that braces it to the side of the engine block. The resulting trapezoidal mounting system was reliable enough for the factory racing tem to adopt it.

Be advised that neither the Weber nor the Oselli intake manifolds have a balance tube to modulate pressure fluctuations between the two intake tracts, which is necessary to prevent “robbing”. What appears to a takeoff nipple for this purpose on the right side runner of the manifold is in fact provided to allow the fitting of a vacuum-operated servo mechanism for the braking system. The unmodulated pressure fluctuation, which is aggravated in the siamesed intake tracts by the uneven breathing resulting from the 180° opposed throws of the crankshaft, is the reason that these unbalanced intake manifolds have no provision for a suitable vacuum advance takeoff. While there is a vacuum takeoff fitting (Special Tuning Part # AEH 793) on the factory’s Special Tuning intake manifold (Special Tuning Part # AEH 772), it is for a power brake mechanism, not a vacuum advance control mechanism. The advance plate in a vacuum advance distributor would be rattling back and forth so violently that consistent ignition timing would be all but impossible to achieve. This in turn forces the use of a pure centrifugal advance distributor. If you decide to use either of these intake manifolds, expect poor part-throttle response, high engine temperatures, a tendency to burn valves, a tendency to preignition under heavy loads, decreased fuel economy, and a ragged idle. On the other hand, the Cannon 801 intake manifold has provision for the installation of a primitive balance tube.

There is, however, a considerable difference between the Weber and the SU in the process of setting them up. The SU has only one fuel-metering needle and one fuel jet, so you can modify its metering in your driveway. The Weber, on the other hand, has a multiple choice of replaceable main and auxiliary venturi sizes, six jets (starter air correction jet, starter jet, idle jet, main jet, accelerator pump jet, and air correction jet), plus an emulsifier tube! As Peter Burgess rightly points out in his book, carburettors are rarely properly set up as delivered (but people rip a Weber out of its package and slap it on their engines in sheer ignorance of this fact). This multiplicity of jets and venturi sizes does, however, make it almost infinitely adaptable, even to practically any exotic camshaft lobe profile, and this is another reason why the factory racing team used them. They could more easily tailor the engine’s performance characteristics to the type of track that they were about to race on. However, unless you are using a radical camshaft, have access to a dynamometer, and you really understand how a carburettor works, take my advice and use the 1½” SU! Its 130 Cubic Feet per Minute airflow capacity is quite adequate for the majority of streetable small-bore B Series engines, plus it can readily be modified to increase its air flow capacity.

There is, however, a considerable difference between the Weber and the SU in the process of setting them up. The SU has only one fuel-metering needle and one fuel jet, so you can modify its metering in your driveway. The Weber, on the other hand, has a multiple choice of replaceable main and auxiliary venturi sizes, six jets (starter air correction jet, starter jet, idle jet, main jet, accelerator pump jet, and air correction jet), plus an emulsifier tube! As Peter Burgess rightly points out in his book, carburettors are rarely properly set up as delivered (but people rip a Weber out of its package and slap it on their engines in sheer ignorance of this fact). This multiplicity of jets and venturi sizes does, however, make it almost infinitely adaptable, even to practically any exotic camshaft lobe profile, and this is another reason why the factory racing team used them. They could more easily tailor the engine’s performance characteristics to the type of track that they were about to race on. However, unless you are using a radical camshaft, have access to a dynamometer, and you really understand how a carburettor works, take my advice and use the 1½” SU! Its 130 Cubic Feet per Minute airflow capacity is quite adequate for the majority of streetable small-bore B Series engines, plus it can readily be modified to increase its air flow capacity.
Steve S.

steve.. thanks. while i was only browsing the entries here, my thoughts of changing to a weber in the belief of symplicity,have now been changed. excellent response with detailed descriptions. (time to rebuild the su's.)

Again.thankyou.

Cactus

I attended a talk by Gerry Brown & Ron Gammons in the early 80s where their comments about 1 1/2 inch SUs was all favourable. They took questions about bigger SUs (1 3/4 inch) and Webers and said that for road applications twin 1 1/2 inch SUs were perfectly good enough for the majority of MGB engines. They said they would not put 1 3/4 inch SUs on a tuned car and would stay with 1 1/2 inch SUs until quite a wild state of tune when they would opt for a Weber.

At the time they were really into T Racing but were dealing with a number of MGB racers.

Things have moved on somewhat, but changing the carbs is still probably the last thing to do after the Head, Manifiolds, Distributor, Cam, etc. etc. etc.

John.
Yotkshire, UK
John Prewer

This thread was discussed between 02/08/2007 and 06/08/2007

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