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MG MGB Technical - MGB & Webber Carb

I have been lucky enough to find in the trunk of a recently purchased MGA, a 52-45DCOE Webber carb, and a brand new K&N air filter for same.

I am led to believe this carb fits an MGB.
Since I also have a '77 MGB from New Mexico with the original ZS carb and cat convertor, I thought I might utilyse the Webber.

Can anyone tell me what (and where to get) intake and exhaust manifold to use?

Is there anything else I need to consider, such as timing?

Any and all help will be appreciated.

JR
JR Ross

This year I put about 15,000 miles on a 1977 MGB that has 8 3/4:1 pistons,a 45DCOE, a header and a Crane cam. The carb was set up by someone who knows how (not me) and delivered great response and good fuel economy (28+ on highway). I totally enjoyed driving the car, but sold it to buy a Mini pick-up truck. However, I also once owned a 1970 MGB with a fresh engine and an "out of the box set up to run on an MGB" 45 DCOE kit bought from a major British car parts vendor that was a total pig because a DCOE is never ready "out of the box". I couldn't get the car to run satisfactorily until I stuck a set of SU HIF's on it and, voila, it purred and ran like a champ. My advice is free and worth much less, but here it is.... Unless you have higher than original compression and a proper camshaft to take advantage of additional carburation capacity, stick with your Z/S or install a single HIF6 from a Marina or better yet, use an intake/exhaust set-up from a pre-1975 MGB. If you must use the DCOE, try Moss, Vicky Brit, etc or the classifieds for an intake manifold and use a stock early exhaust manifold. Good luck with your car.
David
David

Anything else to consider? Read on-

The use of the Weber DCOE 45 carburetor 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 to 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 Progressive 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 race track, so that's part of the reason why the factory race team chose it over the SU. Remember that on a race track, smoothness and economy must be subordinate to responsiveness, as it's responsiveness that makes aggressive driving possible. Victory is what counts on the track, 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've paid out the big money, sweated the installation, spent more money to convert their ignition system to a centrifugal advance distributor (Weber carburetors don't have provision for a vacuum takeoff for working with vacuum-advance ignition systems: read the fine print!) and so they're already predisposed to feel the power increase even before they drive. When the quick throttle response creates the illusion of more power, they're 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.
Unfortunately, the Weber's intake manifold imposes a major drawback: In order to facilitate the mounting of an aircleaner with adequate flow capabilities, its 9.5cm length is short. This shortness forces the use of a very curvaceous path between the carburetor and the intake ports, which in turn causes the fuel charge to be biased towards the ports for the outer cylinders (#1 & #4). The result is that the outer cylinders (#1 & #4) tend to run richer while the inner cylinders (#2 & #3) tend to run leaner, the differential between the two increasing with engine speed due to the greater inertia of the fuel. The Weber 13cm swan-necked intake manifold, or the similar one offered by Oselli, will reduce this tendency while producing more low-rpm and midrange power, but to fit an efficient aircleaner you will need to rework the inner body panel with a soft mallet. This was never a problem for the factory race team, but many private owners will take exception to the idea of hammering away at their engine compartments. Be advised that neither the Weber nor the Oselli intake manifolds have a balance tube to reduce pressure fluctuations between the two intake tracts which is necessary to prevent "robbing." This pressure fluctuation, which is aggravated in the individual intake tracts by the uneven breathing resulting from the 180 degree opposed throws of the crankshaft, is the reason that these manifolds have no provision for a vacuum advance takeoff. 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 is turn forces the use of a pure centrifugal advance distributor. Expect poor part-throttle response, high engine temperatures, a tendency to burn valves, and a ragged idle. Should you decide to use this carburetor, you would be well advised to use a Soft Mount kit to protect it from the effects of vibration (APT Part # SMW-45).
There is, however, a considerable difference between the Weber and the SU in the process of setting them up. The SU has only one needle and one jet, so you can modify it in your driveway. The Weber, on the other hand, has a multiple choice of replaceable 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, carburetors 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're using a radical camshaft, have access to a dynamometer, and you really understand how a carburetor works, take my advice and use the 1 1/2" SU! The bigger 1 3/4" SUs might make for a bit more power at high RPM, but unless you're mounting them on a 1950cc engine with ported heads, you'll get it at the price of less low RPM power (which is where a street engine spends most of its operating life), a lumpy, vibrating idle, and difficult cold-weather starting. If you do choose to use them on the aforementioned type of engine, mount them on the Special Tuning intake manifold available from Burlen Fuel Systems. Don't buy SUs from an aftermarket outlet. Cut out the money-grubbing middlemen and have Mr. Burgess get them direct from the factory and set them up to fit his headwork, or buy them yourself at http://www.burlen.co.uk and follow his instructions on which needle and jet combination to use. Although more difficult to set up and more expensive than the SU HS4, the SU HIF4 is easier to adjust and gives somewhat better mid- and high-RPM performance. This can be improved by retrofitting the throttle disks from the earlier HS4. Just be sure to refit the phenolic spacers and heatshield when you install them or the fuel will percolate in the floatbowls, causing the engine to run lean and all but refuse to restart after being parked for a while when hot. Be aware that the heatshields used with the HS4 and HIF4 carburetors are not interchangeable.
There's another reason to use the SU: aesthetics. They look right, especially when used with K&N or stock aircleaners. The sidedraft Weber DCOE 45 looks as though it's been adapted and, due to clearance problems, changing the aircleaner element is no fun at all.
Steve S.

Sell the Weber, buy an SU setup.
william fox

Sell the Weber, buy an SU setup. There's a reason it was in that trunk.
william fox

JR,

The DCOE you describe can be fitted to and therefore jetted to suit literally hundreds of cars in various states of tune.
Everything from Alfa Romeo to Ford Escort and lots of others in between.

Since you have , I assume, no knowledge of the set up of the Weber my suggestion is to have it fitted and then have the car tuned on a rolling road dynomometer by someone who knows MGB's and Weber carbs.
This will not be cheap.

Doing it by trial and error can be a long frustrating and expensive affair at best.

Personally I have fitted DCOE Weber carbs to an MGB and found no difference in day to day running over the standard SU and the dyno reports backed upo the "seat of the pants" impressions.

I know other people have found improvements but the details are again usually personal opinion and conjecture not hard numbers.See the comments about the "Halo effect" above.

Your choice since it is your car but if it were mine I would sell the Weber and use the proceeds to do something beneficial.

Cheers, Pete.

Peter Thomas

JR, fwiw, I run a 73B in Ontario. Came with a weber carb. Runs great. No problems and lots of fun.
Stu Cameron
S Cameron

As with most things, there is more than one way to skin a cat. The question is, how much are you willing to pay to accomplish a cat skinning? Even a good SU setup can cost a few coins when done by a good shop on a dyno, or hours and $$ spent searching for the right needles on your own.

A dyno is probably the best way to know the reality of what is happening with your particular engine as so many variables exist with timing, compression, porting, iduction flow, exhaust flow, the cam chosen, etc, etc. If you follow a formula - using this carb, that cam, this other dizzy, etc, you perhaps can get pretty close to what someone else got. But if you make your own choices, you will inevitably skew the mix one way or another and in that case, the only way to really know what you what you have done is to see it in numbers that reflect a final result.

The things that Steve has pointed out are well documented and certainly something to consider. But just know that the dynamics of any set of modifications are affected by every choice, whether for an SU or a Weber setup.
Bob Muenchausen

JR- My seat of pants under "Aluminum X-flow". Vic
vem myers

Steve's right. I had been sucked in by the sidedraft hype, but the reality isn't as rosy.
First, the bad news:
They ARE a pain to setup. If you have a good manifold, clearance is EXTREMELY tight. And a 45 is going to have to be VERY choked down (like to 36) to work.
Now for the good news:
Hopefully, if it worked on the A, you're already in the ballpark as far as jets.
Second, you can fit a balance tube, and pull manifold vacuum from it. Does you no good if you're dizzy wants port vacuum, but still, it's not hard. Drill two holes, run tube.
Finally, IF it's all in place and setup properly, and IF you've worked out the ancillary issues (clearance, dizzy, etc) and IF the motor is up to the increased carb, (cam, breathing, and on down the line), then the DCOE really CAN put out more power while remaining pretty streetable.
Getting through all the IFs is the problem. If a DCOE hadn't come on mine, I'd run SUs. IF I find a good set cheap, I'll probably switch.
Baxter

Your case sounds familiar: My Sidedraft came in the trunk of a friend's MGA. He had no use for it so we traded. The Carb cost 450 Cnd to rebuild. Another 500 to get set-up and air filtered. This would have bought a new SU set-up. It would also have been less of a hassle.

The results? Well, since I was after a vintage racer look, I think the Sidedraft can not be beat for "Cool Factor". SUs with trumpets also look the part, but I wanted an air filter. The carb was set up by someone who knew what he was doing. Once warmed up, the car idles just fine. However, until it is warm, the thing pops and spits like crazy. I atribute this to the fact that the long manifold condenses fuel until the whole thing comes up to operating temp.

If you decide to go with the Weber, I highly recomend the services of Carburator Rebuilders, in TO. Regadless of what you send in, the results are like brand new. They will charge around 500, a new Weber is around 750 Cnd, so you have a choice to make.

You need a manifold and that is going to cost a few hundred. Your air filter may or may not work with your selected manifold. More $$$.

One more thing; the sidedraft makes a LOT of noise. You either love it or hate it.

Pete
Pete

My 74 originally had HIF's that had an erratic idle. I found out last summer that they had 2 different pistons. Either the previous owner mixed them up or there was a mistake in the manufacture. Anyway, I was able to borrow a 45DCOE complete with manifold that had been set up for a 69 MGBGT. The swap was straight forward and fit under the bonnet/hood with just enough clearance.
Through the summer and fall it ran fine, but I use it for a daily driver, and when the temperature got below 40 degrees I started noticing problems and increased oil consumption. It was set up so that the crankcase vent from the pushrod cover just went through holes into the bottom of the air cleaner without any pcv valve. Apparently the engine started sucking oil out of the crankcase and into the Weber, causing the problems.
About 3 weeks ago, I got my Christmas present, a brand new set of AUD465's. No more oil consumption, better drivability on the highway, better gas mileage--the new SU's were one of the best investments I've ever made on the car.
Ken T
Ken Thompson

The Weber DCOE is NOT a progressive type carb. An example of a progressive type carb would be the DGAV.
Daniel

You don't need to choke down a 45 DCOE to 36mm chokes to make it work. I run a 45DCOE on a 1300 A-series that is fully streetable with 40mm chokes.

To get a 45 DCOE to produce good bottom end power with chokes bigger than 36mm you need Full Radius inlet rams from TWM (if you are in the USA, ITG if you are in the UK) and some patience to get it properly calibrated.
Daniel

It isn't a big deal that you can't run vacuum advance on the distributor with a DCOE. All the vacuum advance does is improve gas mileage.

The DCOE can be noisier than other carbs especially when you floor the throttle.
Daniel1312@aol.com

Daniel-
By golly, you're right. The Weber DGAV is the progressive one. Thanks. Have you found a way to mount effective aircleaners on full-radius inlet rams? If so, which ones are you using?
Steve S.

Hearing all this I am now shelving any hint of a thought in my head towards replacing my SUs.
They have been as fickle as a woman, but I can see now the good vs. bad and understand it more.
All the "expert friends" that tell me to get rid of my SUs and put something else in simply don't know how to tune SUs.
I don't know either, but I'm lucky to have a mechanic that has the SU knack.
David

Daniel Facts are a little scarce in these parts! What Daniel says is correct as usual, the dcoe is a superb carb but does require indepth knowledge to set up correctly, but once set up it can be forgotten. The SUs on the other hand are very easy to set but do tend to have problems with fuel heights and needle valves. The Weber is capable of delivering more power although this would be hardly noticable on a road car, but I use one because they look good,sound good and maintenance free so I can forget about it. With regard to why the factory used them, they are a better carb, why then did they not use them on road cars? simple cost!
Bob

Peter Burgess' "How to powertune MGB 4 cylinder engines" states that "the single carb should be at least a 45, with the 48s for rally and up to 50 for race use". He has a reasonable amount of info on the subject. From personal experience I found them to be temperamental! Mind you that was multiples on larger engines trying to get balance. I would still give it a try if I found one in the boot!
Chris Moore

David,
That one statment about most people not knowing how to tune them sums it up nicely.
SU's PROPERLY SET UP are as good as just about anything else and one heap better than something just bolted in place with fingers crossed.

By the way it would be worth asking / bribing your mechanic to show you how to just in case they leave or you do.

Cheers, Pete
Peter Thomas

I suggest you DO NOT follow advice to use 40mm venturi in a Weber 45 DCOE - the bottom end performance , and throttle response, is likely to be negligible and the fuel consumption appalling, and the car is likely to be an absolute cow to drive

Over some 30 years of trying(note the word) 45 DCOEs with MGBs I have never had any success using larger than 36 mm venturi and if you study the Weber bible by Passini which reproduces the Weber venturi selection diagrams you will see why - for an 1800cc engine using one choke per cylinder ( which effectively the B does even though it has siamesed ports )having a power peak ( not maximum rpm) of say 5500 rpm will require about a 36mm venturi - 40 would not be required until the peak power was at about 6500 rpm which for a road engine would be very unusual indeed

Further, for road use 2 SUs nearly always give both better, and much much nicer , results, with good low and mid range performance and economy, power at peak within about 2-3 % of even the best DCOE setup .The only thing you miss is the noise of the Weber which resembles a gobbling camel !

Christopher Storey
chris

I think Chris has the sound about right. You do have to love mechanical moaning if you run a Weber.

Pete
Pete

Chris, I am lucky my mechanic is very friendly and doesn't mind explaining and even teaching me a thing or two. He has pointed out many times that SUs are designed around technology which is almost 100 years old but is still as simple and close to perfect as you can get for the job carbies are supposed to do.
He tells me that atmospheric conditions can even change the way SUs perform to some extent. Mainly his opinion is that they need to be set almost perfectly or you notice even the slightest problem.
I fully intend on getting the final tools needed so I can start doing my own timing and tuning adjustments.
David

Chris, you need to read my post again to see the context of the advice.

Also just because you can't get 40 chokes to work doesn't mean it isn't possible. I am in the West Midlands and you are welcome to drop by and have a chug round town in too high a gear with me driving to see what a 45 DCOE on 40mm chokes can achieve with a race cam.

The fact is that the performance and flexibility I have now with the 40 chokes are way in excess of what I first started out with when using 34mm chokes.

I don't think John Passini wrote the DCOE bible -he waffles a lot. Anyone would learn a lot more by speaking to David Anton at www.aptfast.com or any reputable chassis dyno man like Peter Baldwin.
Chris

This thread was discussed between 24/12/2002 and 30/12/2002

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