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MG MGB Technical - Downdraught Webers

Has anyone had any experience of fitting downdraught Webers to MGBs?
How do they compare to the side draught Webers?
Are they actually any good? I'm interested to hear all experiences!

Obviously I understand with the downdraughts the air and fuel has to travel down and then along whereas with the sideraught its just straight through the side.

The downdraught kits appear to be available from 2-300 brand new (albeit delivered from America) whereas the sidedraught kits are around 600.

This is the kind of kit I'm talking about:
K Goldup


A friend bought that carb. from this seller and it is a fine substitution for an old stromberg. But if you are looking for a lot more performance; it is not the case.


Jean G.
Jean Guy Catford

All depends on you final application. There's no doubt, almost any carb could be adapted to fit and work. Manufacturers ultimately want cost benefits and after service efficiency. Really one cannot beat the SU for simplicity and ease of tuning. Sure one can look critically and say for ultimate performance on a track, perhaps a side draft Weber may be best but now you sway from simplicity, you have all the jets and emulsion tubes to deal with. A down draft weber will work fine if set up, but count the number of parts in it in comparison to the su? If you want the car to jusy run, you have easy access to any other carb and it's set up ready to go, sure go ahead, is it any better? I don't know. Mike
J.M. Doust

I have run the Weber DGV carb on three rubber bumper MGBs because it is considered (by our emissions testing people) to be a direct replacement for the Z-S carb. But, I would never install one on my 68 GT as the twin SU set up is decidedly superior in performance. The DGV uses a progressive throttle linkage and the second barrel does not open up until the throttle is depressed about 2/3rds of its travel. Fuel economy may be better with the Weber, but I have not been able to prove that with my cars. Performance, with the Weber's primary venturi being smaller than the HS-4's 1.5" is considerably different. The SUs seem to be much more responsive than the Weber at anything other than full throttle acceleration.

Les Bengtson

The only advantage I can see of going down the Weber route, is if you want to flow significantly more air for a balanced, high revving, ported and gas flowed, larger capacity engine. Unless you have such an engine stick to S U's, AND even if you had one just bolt on 1 3/4" S U's.
Allan Reeling

many moons ago (24 years!) we fitted a brnnd new downdraught weber on a friends MGB just to see what the difference made 1 bhp more than the 1 1/2" sus with K&Ns, it felt crap until we put a couple of extra springs on when it felt like the SUs and drove quite nicely, not to be confused with DCOE sidedraught performance carbies!

P Burgess

They are for different applicarions. DCOEs are the twin sidedraughts with both butterflies moving together. These are really for racing and yes one 45DCOE flows more than 2 HS4 SUs. Nailthe car away from the lights though and all you will get is a cough and splutter as air flow drops to 0 over the jets.
The road carbs are fixed jet sequential butterfly devices and I think they are offered for classic cars now as the "Last Man Standing" in carb making. They are owned by Fiat and I suspect their continued existence owes more to the Italian love of cars than the business case. Webers are terrific carbs of their type, but the constant depression SU was the pinnacle of design for road use. Modern sports bikes use a dual butterfly design which is reminiscent of how an SU works.
Stan Best

If you are building an RB driver the DD weber and an early cast iron exhaust is the way to go, cheap to buy brand new, easy to install and it works well. I've done a bunch of them and I've had few problems

I have had both the Webber DGV and twin SU's. Have SU at the moment. Both carbs when set up perfrom well and you wouldnt notice the difference unless you looked under the bonnet.

I have the twin SU's at the moment for originality purposes and because my Webber developed a flat spot that need some work. I found the SU's easier to sort myself. Webers are can be tricky to sort and you need a good mechanic to do it. I personally think recon SU's are easier to tune and sort for a DIY. If your Webber isnt plug and play you will need a machanic to properly jet it.

A J Ogilvie

I agree with you about fine tuning...with the prgressive secondary choke it is a little finnicky getting the fuelling correct, needs a rolling road IMHO. SUs work well over a wide AF range and can be 'home tweaked' by pulling the choke on a little whilst driving to see the effect.

P Burgess

I think Classic Morotsports ran a conparision feature last year on Weber vs SU. Same car with both carbs set up by experienced techs. The bottom line was that the performance and gas milage etc. were comparable and the deciding factor was did you prefer the classic look of SU's or not.

Ed Emery

Interesting to know, For now I'll stick with the SUs. Possibly 1 3/4s if I do get hold of the ones I've been offered.

Thanks for the info.
K Goldup

This thread was discussed between 29/03/2011 and 02/04/2011

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