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MG Midget and Sprite Technical - Sorting SU Carbs

I have been looking at this (in the book) for some time today using the 'Tuning SU Carburetters' second imprint 1970.

I started on the car this morning by going over the float chambers, replacing the damaged gaskets from when we had them appart to do first start, petrol pipe unions replacing clips where necessasary, and the breather pipe from rocker box hose connections.

All this has made the car start easier and it was not difficult before so all good.

All plugs still show 'sooty' (not oily) so mixture rich and so I am back to setting the carbs.

I have read through the long and tedious routine several times in the aforementioned book and am now aware that while its not difficult it is time consuming. In addition some of the order seams 'strange' that is to say that its like set this and change it later if necessasary. While I understand that carbs are very much a matter of art rather than pure science I want to make sure I am at least reading the correct script.

So is the book I am using the best one to use or is there something better / newer / more pertinent?

(You are all going to say just get on with it arn't you? Now remember it says do this at normal running temp and I'm an ammer chewer and carburetter virgin so will take me a long time and she will be cold by the time I get anywhere near finished ;-) In fact if I start now I reccon it will be the week end before it is finished)
So how important is working on it at normal temperature?
Dave Squire - Notts

Dave, If I understand your question correctly, the answer would be that only the actual final adjustment of the carbs must be done at running temperature and that is done with the engine running anyway so it won't cool off.

What you need to do is get everything cleaned, regasketed as required and reassembled and then with the jets under the carbs screwed down by two full turns (if I remember correctly!)the engine should start but probably not run very well.Now you need to warm it up.
Pull the choke out just enough to give a fast idle, if your choke is working OK of course, until the engine warms up and then adjust the carbs.
First you should use a balancer, or use your ears until the suction hiss is the same, to adjust the idle setting of each carb separately ( you may need to disconnect the linkage that connects the two throttles or you will not be able to adjust the idle for each carb on it's own) and then adjust the jet adjusting nuts under each carb the same amount each time for both carbs until you get a good smoothish idle at the correct idle speed.

Easier said than done, of course!

John
JB Anderson

Dave,

This is fairly simple to follow.

http://www.classiccarhub.co.uk/articles/guide_to_setting_up_tuning_su_carburettors.html
Pete Ottewell

OK John,

So if I start working on it cold today (cos as you say this is a good time to clean and check - I have the kit) and mark it all up as I go along for reassembly at the weekend, I can warm it up for 'tuning' once it is running again. At this point it will be all reassembled and checked over and working correctly. (assuming I put them back together right) Makes sense really.

So I need to overcome my kids hood fear of carburetters now and get on with it.

Thanks.
Dave Squire - Notts

Thanks Peter,

As you can see I would have suggesting meeting before now appart from this giving me gip.

That looks good, looking at it all now.

Cheers,
Dave Squire - Notts

There are also good clear instructions on the Su / Burlen web site and a U tube video by John Twist.
The start point is of course to check valve clearances and points gap before you touch the carbs.
Guy W

Thats the thing you see, my brother in law who drove it 10 years ago into the dry garage says that it was going fine when he put it in. (He has had two strokes since then so.....)

So appart from the stuck number 2 that we had to free up you would expect all to go fine.

The trouble is that I found the points wired wrong before it would start so some things were changed. (the rear dampers were as well so ....)

On the other hand the carbs look completely like they were never touched (there is still a lot of dirt in that thar engine bay and the carbs look pretty original road miles filthy) however the choke does not work on the cams correctly and I have not touched that.

It runs rough as in choked rough so the question is can anything other than the carbs cause that effect? The engine has run for approx 1 and a half to two hours since being originally unstuck and its not really better. The engine starts easy and now sounds OK on start but becomes lumpy and stays lumpy and gets worse if its driven, like it chokes when trying to accelerate. No bad engine noises, when idling it does not change speed but will not run much at all if the choke is out immediately after starting. Has to warm up on the accelerator. (frustrating or what when trying to work on it)

I am rambling again, so will resign myself to sorting carbs out so I know they are correct.

I get like this with cars when I do the work myself until I understand the individual motor.

All comments (about the car or my approach) much appreciated.
Dave Squire - Notts

Dave,
I wasn't saying not to touch the carbs, simply that you should check valve and points gaps are correct first, otherwise you will have difficulty in getting the carbs adjusted correctly as "per book"
Guy W

I know Guy and thanks,
I suppose I have been wondering about it not starting once I have reset them but if everything is working enough now it should.
Good thing is that whilst procrastinating I fixed the leak in the workshop roof, put some reinforcing brackets in it and put some screws in the broke main strip light so it works. No other excuses to not do the carbs now.
Decission made and thanks all. I am going to start the learnin curve from the excellent comments and links and if it doesn't fix it I will know how to do carbs ready for the next time.

Cheers, Dave
Dave Squire - Notts

Dave,

As has been said before storing a car can cause problems to arise.

I sorted a 'B' out where it had not been run for years (well looked after prior to lay up) and the SU's HS types had the jet tubes stuck and wouldnt move - petrol crud had seized them - i.e no choke on one carb.
So yes best to give a good overhaul and clean first.

If you want to check out the choke operation - remove both dashpots, operate the choke and the jets should move down from where previously located and return when choke push in.

Both jet tubes should return to approx the same level.

If you want to retune - as Guy says check everything else is set correct first i.e points/valves etc.

The Burlen instructions explain how to 'zero' both jets by turning up to level with the bridge and down so many flats etc as a starting point to have both carbs the same - from then only move both jets the same amount.

When tuning you have to 'unclamp' the joining throttle shaft so 'air speed' of both carbs can be checked and balance before reclamping.
Is there any local BBS members who could lend an ear ?

R.
richard boobier

I have always found Colourtune useful for working on SUs. If you haven't come across this, it's a glass bodied spark plug that lets you see the ignition taking place and allows you to judge whether its rich or weak. Use in combination with very small lifts of the carb piston using the pin on the underside of the carb body.
As the carbs get old small amounts of wear etc seem to rewrite the rules as to "text book" behaviour and it then gets a bit hit and miss, but at least this device lets you see it.
The trend seems to be bin the twins and put on a single! I have some sympathy with that!
Graeme W

Dave,

I could come over and give you a hand if needed. I have both Colourtune and Carballencer kit I could bring along. it would have to be at the weekend though.
Pete Ottewell

Dave

Just as an aside make sure, especially if the car has been laid up for a while, that it has fresh decent octane fuel in it, setting up carbs with old fuel will not give you a satisfactory result.

Ed
Ed H

Dave,
as a backup to the SU book (and of course the tuning information in the Driver's Handbook) here are some videos that will provide more information and perhaps clarify and confirm what you've picked up already -

(looks like too many links for one post so I’ll split over two posts)

44 Secrets of the SU Carburettor Part 1 of 2 - http://www.youtube.com/watch?v=7GRAcqDySog&feature=plcp

43 Secrets of the SU Part 2 of 2 - http://www.youtube.com/watch?v=60Bj_2cZQnc&feature=plcp

29 SU H-Type Carburetters - http://www.youtube.com/watch?v=GHNlT5yHDxk&feature=plcp
Nigel Atkins

41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

55 Centring the Jet - http://www.youtube.com/watch?v=sxVFCY_1aSA&feature=plcp

147 MG Carburetter Tuning - http://www.youtube.com/watch?v=4nvGLgO6pj0&feature=plcp

35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw&feature=plcp


Nigel Atkins

Nigel,

Hav'nt looked at all your links but
29 and 55 relate to 'H' types and 35 'HIF' types none of which were ever fitted by Leyland to 1500 midgets.

Too much information !

R.
richard boobier

Hi Nigel,
Those John Twist videos are what I was referring to earlier. But I couldn't be bothered at the time to go searching for them on U-tube.

Do you keep a database of these sort of things? It would take me ages to go looking for that many each time. Its about as much as I can manage to add one link to a message!
Guy W

Hi Nigel,

You are correct I need to carry on getting my head around how it works. (rather than just doing it). I usaually get in a strop when I think something is in my head and then realise its not how I thought it was. (Its a strop with myself). My posts sometimes reflect this particularly if I have run out of patience or get exasperated ..... like in this one.

I am slowly assimilating the underlying ideas (and I do need to) so let me just bounce these off you:

When the choke is out the jets go down and let in a lot more fuel constantly independently of the accelerator - that is until the choke is pushed back in - when they should be returned to their 'normal' resting position. (whatever that is for now). This can be seen as the jets lower when the cable is pulled.
The fast idle screws (together with the jet setting nuts) affect the base setting of this.
When the accelerator is pulled the needles rise in the piston assemblies opening the jet more. The vacuum in the assembly together with the damper make the needle movement acceptable to the engines requirements (smoothes out sudden changes that may cause the engine to faulter). This cannot be seen because it is inside the piston/suction chamber body. (can this be seen through the air intake when the filters are off?)
The throttle adjustment screws affect the base setting of the needles in the jets and therefore tick over.

Is that it? (in principle of course).

Cheers, Dave
Dave Squire - Notts

Richard,
I should have added the general principals of the videos apply

as for too much information Dave can pick and chose which if any of the videos he wants to watch, if he watches one and doesn't like the presentation then no doubt he won't bother to look at the others

the SU book covers background info on the carbs too and is from 1970 so does it cover the later carbs exactly(?)

sometimes it's easier to understand a video than written info or to reinforce it

for the info in one place there's the Driver's Handbook

at the risk of sounding (but not trying to be so) snotty how can you know the full contents of the videos without viewing them

I offer all the videos as many cars aren't on their original type of carb(s) and I can't remember which carbs were on which models and who has which carbs on their cars, more than just the OP will be looking at the posts and possibly taking info from it - but it's all just offered not forced on them, take it, leave it :)
Nigel Atkins

Peter,

Thanks for the offer. I need to try and get my head around it so have arranged everything in the workshop (in between workies of course) to start and maybe finish at the weekend (Saturday). I am aiming to clean and if necessasary disasemble the carbs and service them as per SU book. If I am woeful and useless and the car doesn't start when I am finished can I would like to take you up on your kind offer after that?

I tend to not pick things up too well when doing something for the first time with someone else so I need to go at it first. I have the gasket kit from Sussex and have done the float chambers.

Ed,

Thanks for the advice, the fuel is Shell V power and is a couple of weeks old. I think the carbs may have been set to burn crap now I thing about fuel. I think the car went in the garage for the original lay up when there was a fuel crisis and I was burning leaded in my old cavalier at the time cos not many cars could use it and there was always some about. We still had 2 star I think at that point which is a lower octane rating (standard UK fuel is 4 star). Anyways the fuel is getting used and rotated.
Dave Squire - Notts

b*gger as normal whilst composing my post I miss others

Guy,
yes I've just got a Word.doc with those listed on

Dave,
I'm very non-technical and non-mechanical so will leave it to more knowledgeable others to say but I think you're very slightly over combining the vacuum and piston damping

when you have time and are relaxed have a look at videos 43 and 44

I find somethings take ages to drop into place in my brain, after a very long time I suddenly get it but other even simple things I always get the wrong way round no matter how soon or often they are repeated but this also happens with very intelligent and/or educated people so I just accept I'll have to live off my charm and good looks rather than my brain

. . . which is why I live in such humble circumstances :(
Nigel Atkins

Nigel,

The H type carbs (up to late 50's ? MGA etc)require jet centre to be set - HS do not and the later HS types as fitted to later 1500's as Dave's car will have are bias needle also !

HIF are totally different both in that they have integral choke mechanisms and float chambers - neither of these principles apply to HS.

Therefore totally irrelevant.



R.

richard boobier

Dave,
for the fuel IIRC the 1500 like the 1275 was originally for 97 octane fuel, it would be the timing that would be set for the lower fuel (probably 95) but if it ran well on 95 then it would also run well on 97,98,99

I use Tesco Momentum 99 in my car as often as I can and find it no different to V-power other than usually being less expensive
Nigel Atkins

sorry Richard as someone who knows little about carbs and having seen the videos I disagree, the basic information and carbs in bits is useful background information to those that don't fully know or understand the carbs

I agree if you know about the carbs it is not useful but if that the case you wouldn't want to view the info

however I'll look at 29 and 55 again and remove them from the list if they add nothing as I now have a new month's viewing allowance and I've not looked at them for a long time
Nigel Atkins

Richard,
yeap I take your point, my apologies, 55 adds nothing I'll remove it from the list

but 29 gives the possibilities of mismatched parts and clearly states it's for H types so whilst not entirely suited does provide useful info on checking things match so I'll keep that in and the carb tuning videos offer loads of general advice plus I expect most on the BBS will soon realise that their carbs are not exactly the same as those in the videos and I add something about using them as the general basics rather than specific

I must admit I had to put the list up again after losing a load of documents before so the list wasn't sectioned out like before

(almost) all comments gratefully received and acted on as part of our policy of continued improvement, or something like that

anyway thanks for pointing it out
Nigel Atkins

Dave, no problem just shout. Good Luck.
Pete Ottewell

Thats great Nigel, spot on, I am understanding that fine.
I knew it was simples but without the description of what causes what the lack of moving parts made it difficult to work out how it worked.
I think charm and good looks are a good way of living and surviving in this world by the way. People rely on academic one up manship and their 'superior intelligence' for their basis of social relationships. This is irrelevent to their peers in everyday neighbourly life and then they wonder why life is so difficult. ..........
I will be on with it tomorrow night.
Anyways, Thanks loads, Dave
Dave Squire - Notts

well another reason I post links is because I don't really understand the subject and even when I do I've lost what little power I had with verbal and written communication

what gets me the most is that no one challenges the charm and good looks bit - those that know me usually at least snigger or sneer :)
Nigel Atkins

Charm is what they do with snakes, Nigel. Or its something to do with Pendle.
Guy W

well ! I've never been compared to a snake before

although, as Terry Wogan used to say I've the figure of a racing snake

. . . just after eating a Spacehopper
Nigel Atkins

Nigel,

No need to apologise - I just think to much information that is not focused confuses - it does me anyway.

Dave,
I think your grasp on SU's is either better than youare letting on - or a fast learner !

R.
richard boobier

I just collected all the J T vids on carbs without checking them again so that was my mistake, last 2 or 3 months I've been closer to my monthly limit than usual so I get the phones calls to go fibre despite still having 20% capacity left one time - I get fed up of telling them how Maggie wouldn't let BT go fibre a lifetime ago because it wouldn't help her friends and supporters get even richer even quicker - I've not had the same one ring me twice
Nigel Atkins

Richard, this has been going on a long time. I started the carburetter thinking when the engine had just been unstuck back in the summer. (I was only a few days away from on the road back then you know). I have been dreading and thinking about carburetters on and off since then and working hard to not have to do this. Well maybe just putting it off and reading bits on and off. So I have been trying to order my thoughts ever since. (not so smart eh?)
However it helps me if I have an understanding of how stuff works to have the confidence to begin the practical stuff. (Its a way of me reviewing like I am as I type this, it puts my thoughts into order). Once the thoughts are in order (a la Nige) I can do it usually. Underlying premise is important as wrong premise = cock up. However quite confident now.

As for definitions of snakes re Terry Wogan shape I am definately not a snake. Much too far gone to be that slim.

Pendle? come on you lot you know name dropping is my weakest suite. So OK I will look a right chump again I know - I give in - who or what is Pendle?
Dave Squire - Notts

Dave,
Guy's comments were for me after what I put in my post - I think there was a rogue snake reported in Pendle but I might have it wrong as Cumbria is a land far from me here in sunny (yesterday) Northants, now Moorhouse's Pendle's Witches Brew did occasionally make it down here and sold out very fast and I've got A Nutter's signature so I have been up there a couple of times, even had lunch at the nuclear place
Nigel Atkins

Dave,

I think it has something to do with Witches, They're quite funny up north you know....
Pete Ottewell

this is the best book I've ever found for SU carbs building it has a lot of good detailed info

its also easy to understand....and a great price its an invaluable book

prop

http://www.amazon.com/gp/aw/d/1855202557/ref=mp_s_a_1?qid=1357168732&sr=8-2&pi=SL75
Prop and the Blackhole Midget

Thanks Prop, I found it in GB money as well.

Thanks all, I will now start on the doing.

Cheers.
Dave Squire - Notts

Dave

Junk the twin SUs and fit a HIF44. Balancing problems solved and several advantages to boot!

Cheers

Mark
Mark O

I have to disagree mark...women dont get near as horny with one carb as they do 2
Prop and the Blackhole Midget

This thread was discussed between 01/01/2013 and 03/01/2013

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.