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 - Fitting 1 3/4' SU Carbs

Hi, quick bit of research advice if possible from anyone who has fitted the 1 3/4" carbs to a B. Do they use the same throttle linkages and mounting plates as the 1 1/2" carbs? I'm looking at eBay ex-Rover pairs to get the right carbs plus service kit but can't find any references to the necessary linkages. Moss of course offer the full setup, but at over 900 I'd rather do it myself. Thanks in advance!
A Riddett

The linkages were quite different between HS and HIF carbs, if that's any help.
Paul Hunt

Without doing head work there is no real advantage to using the larger carbs.
Bill Spohn

Bill is right. If your engine is in standard form the larger carbs may actually reduce the performance.
Mike Howlett

They use a different manifold and the linkages are shorter - you can cut down the shafts.

They will drop power at mid range rpm and worsen fuel consumption - due to slower gas speed.

The only gain is at the top end so unless you regularly drive at 5000 rpm + then you will be disappointed with the result.

You would be better spending the money on getting the head ported by Peter B and fitting a Piper BP270 or BP285 camshaft.

The 1 1/2 inch SUs are good for up the 140 bhp!
Chris at Octarine Services

Why do you want to fit 13/4" carbs? Only any point , as Chris said, if you are massively increasing output. And that will need lots of head work. and over bore, cam change etc.
Allan Reeling

If you have a nice even idle at 900rpm and no flat spots, they will probably be better than anything bigger second hand.
If they are worn, rebuild, it makes a nice difference, unless they are HIF4s, then I would buy a pair of HS4s and rebuild those.
c cummins

I can endorse what's already been said. I bought a pair of secondhand HS6s on manifold - I think they were fitted as standard to the Austin/Morris 1800S Landcrab. Initially fitted them as is, with floats in the middle, then swapped them over as per MGB arrangement - I think from memory using the HS4 MGB linkages.

As has been said, used more petrol with no performance increase. Got drawn into this by the MGB Special Tuning booklet. Whilst they looked good, I reluctantly replaced them with a nice early set of fixed needle HS4s.

Whilst I hate to say this, because I'm an inveterate tinkerer, it's difficult for performance to beat the nearest you can get to a well set up early spec standard car ie good bores, correctly set tappets, timing and points etc eg I'm sure my '68 non o/d roadster went a lot better than my current o/d '72. Might be distance lending enchantment or better fuel thirty years ago. On the other hand, road tests and anecdote seem to indicate later cars were thriftier.

P A Allen

Maybe it's becuase we're upside down, but my experience with them had been completely the opposite provided that the head has been well worked. (I completely agree with above negative comments if fitted to a standard head)
With a well worked head, fast road cam profile and properly jetted, HS6's give an instantly noticeable and pleasing increase in torque from 2000 RPM upwards and are similar than the HS4's below that, with a perfectly acceptable idle.

As for fitting: HS4 or HIF 4 linkages can be shortened and adapted, while they HS6's will bolt straight on to the standard manifold studs, there being enoghu subsatance in the manifold and spacer blocks to open it out to suit and achieve a very effective set-up.
Paul Walbran

Sorry, timed out on the edit. Last para should be

As for fitting: HS4 or HIF4 linkages can be shortened and adapted, while the HS6's will bolt straight on to the standard manifold studs, using two of the four mounting holes (perfectly adequate). There is enough substance in the manifold and spacer blocks to open it out to suit and achieve a very effective set-up.
Might not look quite as pretty as the off-the-shelf version, but is just as effective and well cheaper.
Paul Walbran

I'm agreeing with Paul on this there's plenty of metal in the manifold to get a good shape through the spacers and well into the manifold
The two spare boltholes on the top of the 1 3/4 carbs can be disguised by making a neat cross bar to fit across connecting the carbs and using it to mount the throttle cable
I like the feel of bigger throttle plates for general driving much better responsive feel than 1 1/2 carbs
Admitedly they may use a tiddle more fuel than the std carbs but thats probably down to being able to get your foot into them better but that is why we're putting them on isn't it.
I do feel that you need a decent free flowing exhaust to get good results though if you are planing a standard small quiet exhaust there is little gain really
cheers willy
William Revit

Maybe it's the upside down thing Willy :-)
Paul Walbran

Or our nice fresh air Paul
William Revit

For what it is worth - I was on an MG club trip in Spain a few years back - one of the others had a roadster with 1950 cc engine and 1 3/4 carbs who reckoned his car was faster than mine with an 1868 cc engine and 1 1/2 carbs - so we found a nice straight bit of country road with a slight uphill incline - I left him well behind.

In theory the 1950 producers more power than the 1868 but the power band is a different shape which influences the driveability.

The smaller engine with smaller carbs was just much more powerful up to around 3000 rpm.
Chris at Octarine Services

It also depends on what the rest of the set-up is like - cam, head spec, compression ratio, exhaust system. I once had a similar view about 1-3/4 carbs, but many years ago a customer insisted I fit them and I was really surprised at the result - idled well, transitioned well right from the word go, and much more torque that the standard carbs from 2000RPM upwards, both on the road and on the RR.

In proportion, it's not much different from 1.5" on the A series, another combination that works well.

Having noted all that, one feature of the larger carbs is that they go a bit flat on full throttle under 2000RPM, but part throttle pulls well - a bit like driving with a Weber. It just needs driving style to be adapted a bit to get the best out of it.
Paul Walbran

the three fastest b's here are on 1 3/4 carbs and then there are the others --- sort of speaks for itself a bit
please excuse my typing i'm running a bit injured at the mo willy

the biggest problem i come ccross with b's is incorrect ignition curves to suit modified enginesb std curves just don't cut it and with 1 3/4 carbs this is magnified in the operation of the vac advance
it's the same with an a series with 1 1/2 or a sidedraught
because the carb is bigger it doesn't pull vac on early enough causing that boggy feel but with plenty of early timing the 1 3/4 carbs appear to perform quite well to me
anyway as they say each to his own

William Revit

My MGB now has 1 3/4 carbs and is probably a damn sight faster, but then is does have 3.6 litres of V8 pulling mixture though them.

I don't argue that properly set up with an engine spec to suit, then 1 3/4s DO work in as much as power delivery at the top end is better - but if you are driving in town or on twisty country roads then the extra low rev torque of the 1 1/2 carbs is a better bet.

However, just slapping a set on an otherwise standard engine, which is what the Leyland tuning book calls Stage 1, is likely to disappoint the owner in terms of performance - they won't even notice the extra 3 or 4 bhp extra at the top end but will notice the bogginess at town driving speeds.
Chris at Octarine Services

off subject
paul we had a triathlon here on saturday and the bike leg came right up the hill from the beach to my place they did 40klm which was six laps up the hill and including the rest of the course around the river i would estimate getting close to half the field were newzealanders i reckon they'll think twice before coming back here again it was cruel
to give you a rough idea of how steep it is
the power poles down the side of the hill look like the top of one pole is about the same level as the bottom of the next
they would have been doing probably 5kph max coming up and 80-90 going down
i heard one girl squealing all the way down first lap but i take my hat off to her as she was one of the very few that kept pedaling all the way to the bottom most of them just coasted down
i ride it every day going to work and it's a fantastic hill you can really generate some amazing pace if you get some speed up early in the top section
in the elite mens race six of them got away from the bunch on the first lap 5aust 1nz and they managed to hold a gap to the angry bunch behind approx 100mtrs for the whole leg a good effort considering the size of the chasing bunch they would definatley have held out till the end of the bike leg but still had the run to do i reckon the old legs were a tiddle rubbery after getting off the bike
William Revit

Willy, sounds a bit like our driveway, 1 in 2.5 :-) I just get tired reading it! I'll stick to pulling my achilles playing football with my sons.

Chris - completely agree about the Leyland stage 1, I think that's the main point of this thread - don't bolt on bigger carbs without doing anything else and expect it to go better, you'll be disappointed.
Paul Walbran

Cheers all,more than happy with the plusses and minuses of the carb, it was the actual fit that was proving interesting! Much obliged for all comments.
A Riddett

This thread was discussed between 23/02/2014 and 04/03/2014

MG MGB Technical index

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