Welcome to our Site for MG, Triumph and Austin-Healey Car Information.


MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - 50mm Weber

Anyone running a 50mm sidedraught on their big capacity B engine. ?????

Not at this time but when I was an agent for Dellorto carbs several years ago we very succesfully fitted 48mm carbs to MGBs. The trick is not to use too large a choke and utilise the added venturi effect. Jim
jim soutar

Hi Jim
Yep I've had a 45mm Weber with 40 chokes and then 48 Del.with 42's but although the 48 is fine it doesn't quite have the same top end power as 2" SU's and I want all I can have. Unfortunatley the car is a real pig to drive at lower speeds with the SU's and terrible to get started cold as there is no choke on them at all (ex-Jag)and very little manifold help either Where as the Dellorto is a fantastic performer but lacks at the peaky end of things. It's fine up to about 6500 but just flattens off a tiddle reaching 7000 which doesn't happen with the SU's The mixture is fine but I feel that the carb just won't flow enough and want to get this 50mm weber working. It has 46mm chokes which are probably a tiddle large but the dyno will sort that.
My reason for posting was to see what manifold length anyone was running for a starting point just to get a head start but I guess it"s back to the rubber hoses and long dyno sessions

Thanks anyway for your interest Willy

Willy -
I say that neither the SU nor the Weber nor Dellortos are set up right. And I doubt very seriously if bigger ones will fix that.
What sort of Jag 2" SU don't have chokes - most if not all are HD I think, and those do. The old thermos don't, but I've not met a 2" version. Maybe a late 4.2 saloon?
Or fit the thermo aux start!
You might want to go to MGExperience/Motorsports and post your question/read up on what the guys who really know are saying about it. Near 200 bhp flywheel, 8000rpm, and when SCCA said they had to go back from 2" SU to 1 3/4 power went up.

FR Millmore

The SU's are from a series 2 xj6 sedan.
Don't really want to go down the SU road now that I've got hold of this big webber.
I've found that to use the SU's there is a need to run a balance tube in the manifold and that just makes the thing too cranky to drive down low with the cam that has to be used to get the thing working up in the revs.
Without the tube and with a weber/delorto the lower revs are a lot more user friendly
I will sort it out just want to talk to anyone that has run one but it looks like I'm alone
I'll report back with the results Willy

Hi William have you tried the old sidedraught weber style SUs? Best of /worst of both worlds? We tried a pair years ago (1988) on a TR4 that had to use SUs for the class...uproar we caused!! We had much more bhp with the pair of webers when we tried those on.
I have yet to see large sus make more bhp than large webers. The act of lifting the piston in the su uses energy from the incoming air, this must reduce the amount of air available to the engine whereas the weber utilises the aux venturi using less energy to signal the main jet. As you say lower rpm performance goes out the window with big sus. maybe if someone reinvented the SU with a Keihin or Mikuni oval bore then we would get better part fuelling?

P Burgess

Just to give a little more info, the Le Mans TRS Triumph had these special carbs.
I attach a picture from a MINI website.


P Burgess

Hi Peter
I must admit I have never even heard of the Weber/su let alone had a picture to look at -- Thanks I'll have a read up about them.
Basically I've decided I just have to be able to get this 50 working It's got me, It just has to be done.

I think my manifold has been a fair bit too long when I had the dellorto on but even still it would have been lucky to be any more than 2" longer than the 2"SU manifold I'd been using so probably not the cause of the problem

I really want to sort it out - It's a killer engine when it's going well. It's a full 2300cc and as cranky as a snake. Unfortunatley I sold the car with a mild 1950cc engine in it to a guy and am now working on another bodyshell so it will be a while till I can get it on the rollers , very frustrating all up.
Out of interest here's a pic of the old car, as you can see by the roll bar the new owner is a bit taller than me ------- Willy


Well I suppose that with 2300 cc you might be getting into an area where you would need large enough main venturis to be able to make the 50 DCOE run, but it would surpise me if you NEEDED that large. I'd think the 48 with suitable venturis would serve quite nicely.

The DU6 SUs are not known as Weber-SUs. They are a double barrelled SU made for the FPF Climax engine and used on the Le Mans Triumph TRS twin cams. I've owned a pair and played with them but they are really just a curiosity and even if someone could find AND afford a set, I wouldn't recomend them for serious racing. They played with these on the prototype MGA Twin Cams.

If you want to be different, and rules allow, why not use fule injection? They ran TJ mechanical injection in the old days and a modern set up with electronics would be a nice tunable way to go.
Bill Spohn

Hi Bill

I reckon Willy called em Weber/SUs cos they look a lot like Webers, not because Weber made them, which as you know they did not.

We were fortunate to have 6 pairs to play with on the TR4. The owner of the TR4 was enjoying winding up the opposition and the sponsor spent a long time pre-internet tracking down the carbs.

We felt one port per SU was too much and the carbs suffered from piston flutter at certain rpms. They were not a match for a pair of 45 Webers with long intake manifolds.

The biggest problem to me with injection is overcoming the charge robbing properties of Siamese port heads and keeping really good atomisation with the very large inlet ports required for Full Race Bs.

P Burgess

Sorry about the Weber/SU bit MY mistake
Bill - Peter
Firstly thanks for your interest
I realise that the 50mm Weber is maybee a tiddle large but I havn't been able to get the top end performance that the 2" SU's gave out of a 45 weber or 48 dellorto but they are better mid range by a fair bit and a lot more user friendly
My hope is that the 50 weber will have the same feel as the others but reclaim the lost top end power
I really like the feel of big throttle butterflies especially when doing hillclimbs It just feels like you can get onto it quicker and more throttle control
It's going to happen
The reason I started this thread was just to see if anyone had found some magic formula to save me a heap of testing but it will be interesting to get it working so it doesn't really worry me
Thanks again Willy

A pic. of the dellorto manifold The engine is solid mounted and the carb is bracketed to the guard to stop it floating about


Might it be worth trying split Webers? With the research we have done on one port/cylinder setups we reckon 18" manifold or none at all!

P Burgess

That 18" Would that be the actual manifold runner length or valve stem to venturi
It seems long but mate if you say it works I don't doubt it for a second
As you can see in that last pic I had the old manifold adjustable but never got it out quite that far.
I'm really looking forward to having it running again
Cheers Willy
11-45pm here I'm off to bed

Hi Willy

The 18 inch was just the manifold.

P Burgess

This thread was discussed between 28/10/2010 and 03/11/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now