MG-Cars.info

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

Recommendations

Parts

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 Midget and Sprite Technical - SU HS4 carburettors - which one?

Hello everyone, my absence for a while generally corresponds to the fact that the Midget has been going well and I haven't got stuck! However, a question that, from a search of the archives, does not appear to have come up before.

The SU4 carburettors currently on my wife's 1977 Midget are right at the limit of wear before we have to do something about them. Having taken it to Car Tune near Wisbech, which certainly made a huge improvement to the set up as a whole, their verdict was that the carburettors on it at the moment are not correct anyway, so the logical thing to do is find a pair, get them refurbished and then fit them. However, this raises the question of what should actually be on there (and the slightly relevant question of what we currently have, and what's incorrect about them - something to do with lack of a fuel return apparently)?

The MGOC spares shop suggests FZX1258 and FZX1122 being potentially relevant parts, with AUD665 being too early and FZX1327 being too late. Are there any clues as to which of the two it should be (January 1977) and how would I know the difference if I asked someone on Ebay to send me photographs?

Of course, another option would be to work out what is missing/incorrect about the current ones and change only the components whilst getting them rebuilt, but again I'm not sure what is incorrect in the first place.

Any input appreciated.

Thanks

Alec
AG Gunner

Some useful info from the Moss catalogue in the photo below.

Also, M&G International catalogue says FZX1122, Waxstat jet, 1977 on.

Do your carbs have any tags on the float chamber tops?

I wasn't aware of any fuel return on Midget carbs, other than US-emission versions.

There is a seller on Ebay called Spitfiregraveyard. As well as Triumphs, he also seems to break quite a few 1500 Midgets.

https://www.ebay.co.uk/sch/i.html?_trkparms=folent%3Aspitfiregraveyard%7Cfolenttp%3A1&_trksid=p3542580.m47492.l71970&_ssn=spitfiregraveyard


Dave O'Neill 2

Dave, thank you for the information. With better understanding of what to look for, I think I now understand what is there, which looks to be FZX1258 (there is even a tag on the front one, now I know what to look for and where). It looks like these were indeed fitted for the narrow date range in which this particular vehicle was built, so that, plus the very worn condition, suggests they may well be original.

I have added images in case anyone can comment on what might be 'wrong' as if it is trivial and external we will probably get these carbs rebuilt rather than replace them.

Alec





AG Gunner

Close-ups of the front and rear carbs, tag clearly visible.

Anything obviously incorrect?

Thanks

Alec




AG Gunner

Alec. Are you restoring a car for use in car shows? The ones that take away points for anything that is not original and where a factory paint job would cost one competition points? If so, spending a great deal of money to gain very little fits in perfectly in that case.

If, however, you are simply after the enjoyment of owning and driving a classic sporting car, one can be more relaxed in one's approach.

The British never had the volume of automobile production that allowed them produce several different brands of the same item (e.g. there are several different companies who produced distribtors in the US, which in the UK, Lucas was the standard). As a result, the British made a basic model, one that could be adapted to numberous uses, and differentiated them by a "specification number". The specification number told, in the case of the carbies, what column spring was used, what diameter (internal and external) the jet was, what needle was used, and all of the other information that was needed to build up any number of such carbies to that specification, each being useable for the original specification. And, the original specification was designed to work with every engine, built to its own specification, used in that model of vehicle.

But, engines, evern ones built to the same specification, are slightly different from each other. More so back when MGs and Sprites were built, perhaps, than today, but they will all very slightly from each other. And, other parts of the engine system, like carbies and distributors, may well require some adjustment, tuning, and modification if the engine system is to perform at its maximum level of performance. One common thing is to tune the SU carbs by installing a different needle, either because of emissions requirements, the desire to increase performance, or a change in operating altitude which requires a change in the mixture settings.

As soon as you custom, performance tune your carbie, it no longer meets the specification on the tag commonly found attached to a fuel bowl. The listed specification number requires a specific carb needle and you have changed it for one that works better under the operating conditions that you are driving under.

Except in a show car, the specification of the carbs and distributor is a starting place that should work, moderately well, with any engine to the specification it was developed for. But, for best performance, the carb and dizzy need to be custom tuned which, by definition, changes they away from the original specification.

Except in a purely show car, go with something in good condition, that works properly, and has been individualized for your vehicle and driving needs rather than relying on a specification number.

Les
Les Bengtson

Hi Les,

I largely agree (and no, this is not a concours vehicle). However, whilst people have every right to do what they want to their own vehicle within the boundaries of safety, I generally find it easier if everything is kind of how it was. It means that when parts need changing there are no difficult, and potentially expensive, surprises. For example, in the UK it is much easier to obtain metric nuts and bolts than UNF and definitely than BSF, but if you know it should be something and it isn't then the spanners don't fit. In the same way, if the carbs are different from stock then it can make the car work better (good) but it does mean someone has to know this when it comes to any future work. At the moment, anything which is non-standard is just as likely to be user error as a deliberate improvement, so knowing something is non-standard if someone recognises it as such from the photos at least means that if I have to order something online, I know it is going to need more care than just assuming it will fit.

Alec
AG Gunner

Your carb set up looks fairly original to me.
So if they are just tired I would just rebuild them with a full rebuild kit from Burlen - they do specific kits for each carb model.

New jets, needles etc which do wear as they are constantly rubbing with the swing needle design ISTR they have - normal service items.
The main spindle shafts are also included complete with bushes- however to fit the bushes requires reaming or line boring.
However I have found that very often it’s the brass shafts that wear mostly (included in the kits) with much less wear in the alloy body. So just replacing them unless there is excessive wear will make a very significant difference to the engines running.

Richard B.
richard b

If you look on the Burlen website it shows the part numbers for different years of Midget.

https://sucarb.co.uk/catalogsearch/result/?q=MG+midget

You may be able to drill down and work out which parts you would need once you identify your carbs.
P Peters

This thread was discussed between 17/05/2025 and 24/05/2025

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now