Which Carb for MGB?

[ChasR]

At the moment my MGB is running a fairly standard engine atm, with the only mod (barely even that!) is a Falcon Single Box Stainless exhaust system:


That is 2 1" 1/2 SU HIFs I think. I think they are in fine condition, but they are in dire need of rebalancing (atm the front one's doing most of the work, and although the car isn't smoking, it doesn't idle too great. So I was just going to set them up .

But my dad keeps on going on about twin Carbruettor setups being the creation of Satan, reckoning that they ruin MPG, performance, and reliability for most of the time. He wants me to go to a single 1" 3/4 carb setup. Sure it would be easier to setup but I'm worried it would take away my top end power and maybe some of the midrange shove ( I guess I wouldn't use the top end but it's nice to know it can be used .

This then bought up the subject of possibly getting a Webber 45DCOE on it and putting that on. But I've heard they're useless unless you have a Rolling Road handy.

In terms of mods I'd probably also replace the cast manifold and put on an LCB manifold .

The question is which setup do you reckon is best for a standard low mileage 1800 B Series unit and why? .

[Seth]

Standard or nearly standard car - A pair of 1 1/2" SU's every day!

A 45 Weber would only be any advantage on a full race motor and would just reduce your MPG massively on a standard car.

A single 1 3/4 would do as you say I reckon. Reduce top end power a bit and possibly stay in tune a little better. The twins are easy to set up though once you get used to it and you'd need to keepp chack on a single anyway.

Tell your dad that my Oxford did better MPG with a twin 1 1/2" SU equipped standard 1800 engine than with 1600 fitted with a single 1 1/4"

[Paul H]

A single 1 3/4 is too small for 1800cc imho - eg Amazons have twin 1 3/4s!

Spridgets are often fitted with 1 big carb (1 3/4?) instead of the twin 1 1/4s but then A series do have those delightful siamesed inlet ports...

I'd stick with the twin 1 1/2s as Seth suggests. If you're new to setting up & balancing twin carbs, there's a couple of very useful articles (originally written for Amazons but I'm sure applies equally to most twin SU setups) here:

www.vclassics.com/archive/stoichio.htm
www.vclassics.com/archive/sucarbs.htm

I found that these explained everything much more clearly than the Haynes (although that is still needed)

[ChasR]

Cheers for the info, I think my mind is made up on the SUs .

Atm I've been offered a Piper camshaft (can be anything I want really (I was thinking Mild or Fast Road 276). Reckon it's worth getting one or shall I leave it?

Also what do you guys reckon to Luminition ignition systems to replace the Points and Condenser as well as the LCB manifolds?

Cheers for the help so far .

[RetroWarwicK]

if you want a single su carb, could you not use a 2" one (off rover sdi's??)

otherwise stick with the current set up but get them rebuilt (which sound like they need it = -woud explain why you are getting idling problems). ditch the standard filters for KN pancakes with internal ram stacks FTW.


also make sure you fit a heat shield to keep the carbs cooler as they sit over the exhaust.

[Nathan]

When I was in the market for upgrading twin SU's were usually the best option with fast road needles. However I wanted a little more poke and with a better Cam the DCOE was the best option for my setup. the Single carb setup was used in the Marina was it not?

I say if your going to upgrade then upgrade the head, cam and timing gear, as well. All these set to fast road spec will give the B a lot of life, and wont hamper the MGP too much, unless you put the DCOE on . TBH the ammount of money you can spend doing that you could simply drop a SD1 motor in, Yours of course is the rubber bumper model so its basically a straigt swap (Kinda).

If you have a DCOE then yes I would recommend a RR day, but once its setup its setup fella.

Remember you already have the big valve head , so sticking uprated needles in the carbs and setting them up properly will transform the car if you drove it as standard.

[Mr K]

luminition magnatronic = worth every penny, bin the points! far more reliable. i fitted mine, and havent touched it since.

[Seth]

bgtmidget7476 said:

Remember you already have the big valve head , so sticking uprated needles in the carbs and setting them up properly will transform the car if you drove it as standard.

I'm pretty sure they went back to small inlet valves after 74/75?

[Nathan]

could be right seth, but a quick look at the MG archives and my head stamping states its the first of the big valve heads. Would of thought they would continue the big valve for longer than a year?

Chasr post the head stamping up here, usually has 18v at the beginning.

Magnatronic or optronic has to be a must.

[Seth]

My Moss book says early 18V engines (circa '71 I think) were big valve and later ones from '75 went back to smaller valves. It was probably done for emmision reasons or something. Same was true for Marina lumps. Engine number might not count for much now as things could have been swapped around.

Maph2, standard MGB air filters already have neat little "stub stacks" in the form of a thick radiused base

[Nathan]

your right seth, just looked at the archives.

I have a spare Big valve head at home I am sure of it, possible small upgrade?

In response to you question chasr, the best option woudl be a change of needles in those SU's and probably some better Air filters, you could also go and get the head ported and polished. IMO just a needle change tune and Optronic ignition to start with.

defo loose the cast iron fannymould, its curse word and weighs a ton, there is a place in notts, who will make an alloy version but thats still pants.

Thats the route I went too, but I soon got bored of it and wanted more power.

[Ratdat]

Here's something to think about. Only one cylinder draw in air at a time. With the twin carbs, each cylinder draws air via a 1.5" carb. With a single carb each cylinder draws air via a 1.75" carb. You can calculate wether the single 1.75" carb will have enough flow for your engine at peak RPM by calculation but I have no doubt it would. I've seen a turbo 2.8 fed with a single 2" before.

[RetroWarwicK]

Rover M/T16 conversion? or moss supercharger kit

[bigginger]

General consensus in Mog world is that twins are nothing but a PITA, and the larger single is better

[Seth]

A single 1 3/4 will undoubtedly flow enough air (as mentioned already, Marina's used them on the same engine) but presumably experience and testing showed BMC that a bit more sporty performance could be had by using two smaller carbs. Intake flow on the siamese port engines is complex and makes injecting them very tricky...

Personally if I could fund it my next step would be this one.



HRG crossflow ;D

[Ratdat]

I suspect the use of twin carbs had more to do with tradition than performance. Twin SU's on sporty models goes back well before the days when manufacturers actually got technical on tuning. I would think it's what the typical buyer would expect to see under the bonnet and a single carb might leave them feeling a bit short changed.