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MG Midget and Sprite Technical - Mixture and Carb set up

Hello guys,

I've had a grumble rumble when I turn the engine off... sounds like the engine may be preigniting.

So, I had the spark plugs out - lots and lots of carbon build up - suggesting its running to rich - could lead to the preignition.

I repainted my dashboard this weekend, and had to disconnect the choke cable to remove it, so when I put it back I noticed the choke was partially open when knob was fully home - which could go some of the way to explaining why it was running rich.

Alternatively - my carbs need adjusting. Any tips or ideas? This is my worst nightmare!

C


C L Carter

Hi C
Adjust the choke cable first so that the choke shut off completely. I have a little slack in my cable when the knob is pushed fully in.
Did you have any issues befroe removing the choke cable and dashboard? If not then the issue is likely to be just the engine running with the choke on.

How about a picture of the painted dash. We all like a picture.
Good luck
Dave
Dave Brown

Get the choke cable adjusted / working 100% with jets returning fully upwards.

Do not adjust the carbs.

Run the car and determine if still rich... lumpy engine, black smoke from exhaust, smell of petrol from pipe when hot, black plugs... these are all a sign of running rich.

Use the colour of the plugs to determine if one or both carbs running rich; if rich, weaken the carb by turning jet nut 2 flats upwards. Re-test.

I'm a big believer in adjusting one thing at a time, and seeing the expected change step-by-step...

I like to see the plugs have off-white ceramic around the electrode and a little carbon around the body.

BTW - the plug in the pic is probably a little rich - from the Kseries; I've since experimented with slightly weaker low-throttle running - but gives the idea. Would expect the brown ceramic to be lighter, but maybe not brilliant white.

Anthony Cutler

Hi chaps, thanks for your responses. I want to do the adjustment in the daylight... so will wait till the weekend to get stuck into this. Do you use any special testing equipment to adjust?

Yes Dave, had problems before took the dash off. So could well be the choke. Will address this first.

Re image: See this page for before and after. I'm documenting what I do to the car in this Facebook group: https://www.facebook.com/profile.php?id=222406136#!/pages/Midgely/300732513272669?sk=photos
C L Carter

No special tools needed to adjust SU's

Listen to Ant
Do one thing at a time
Onno Könemann

Whilst no special tools are needed, I find it very much easier with an SU spanner as there is not a lot of room to work (at least that is the case on mine). I also use a carb balancer.
Where about in Middlesex are you? Would be happy to lend them and give you a hand if we are nearby.
I had a similar problem. No matter how much I adjusted my carbs, they remained too rich. Problem was that the choke was not releasing fully due to internals and in the end refurbishment solved it.

One thing at a time as others have said is the best advice.
Graham M V

Graham, thats great. I live in Vauxhall, but do most of my work at a garage in South Kensington. Would be great to borrow carb kit and expertise at some point... Where are you?

C
C L Carter

Vauxhall and South Kensington were not in Middlesex the last time I looked! Unfortunately I am near Watford so thats not really round the corner!

This is the sort of spanner you may find helpful http://www.sucarb.co.uk/productDetail.aspx?id=28081 though check with them it is the right size for your carbs.

But as it is so easy to do, I would first check, that the choke is releasing completely when the knob is pushed in. It could be failing to release completely either because of a problem with the cable, or with the carbs themselves. You should see this quite easily if you pull the choke knob out and then push it back in. Check at the carbs that the choke lever has fully returned and that you cant move it further back with your hand. As Dave has said, the choke cable should have slack in it.
If all is well, then when adjusting the mixture to make them less rich, you should try just a very small amount at a time. Be patient,
I have a colour tune kit too, but can't get on with it so I do like Anthony says, and take it for a short run and check the plugs.
Graham M V

Don't get spooked about twin carb tuning - it is very simple if you go about it the right way. The main point is, before touching the carbs, check everything else is right. Adjusting the carbs gets a unfair bad reputation because many people ignore this very basic point and try to tune the carbs to get rid of a fault that's somewhere else.

I have commented further on my website at http://www.mgparts.co.nz/twincarbs.html
Alternatively, the more comprehensive revised version for my site is in draft form at http://www.mgparts.netprofess.com/advice/technical-notes/twin-carburettors/
Paul Walbran

Thanks guys - will check that over on Sat and give some new plugs a test run.

Graham, lets not get technical, I think the Middlesex was added automatically when i registered my address, but seeing as Middlesex doesn't actually exist anymore lets all have a beer instead!

I have inlaws in Edgeware, so maybe I might be able to bring the car over your way at somepoint. Still, I'm going to play with the choke first... never know... may fix the whole problem there.

Cheers
Christian
C L Carter

Christian,
certainly checking the choke could resolve it for you

I'm also a big believer in cleaning and lubricating, which most servicing is all about so I'm a big believer in servicing too

as you're not going hands on until the weekend you might find the following of use as research and information

they not Spridget specific but the principles remain the same

general adjustment, engine not running
http://www.youtube.com/user/Universitymotorsltd#p/u/27/4nvGLgO6pj0


general adjustment, engine running
http://www.youtube.com/user/Universitymotorsltd#p/u/140/ASeMfXfjNpw


matching pistons
http://www.youtube.com/user/Universitymotorsltd#p/u/134/IfU47Oqq9wA


how a SU works part one
http://www.youtube.com/user/Universitymotorsltd#p/u/131/7GRAcqDySog


how a SU works part two
http://www.youtube.com/user/Universitymotorsltd#p/u/132/60Bj_2cZQnc


I’d be derelict in my duty if I didn’t remind you that carb setting and tuning along with much, much more information is available in the owners Handbooks and that you should get a paper copy relevant to your model - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

also you can, if you want to, add your vehicle profile to view to help and speed up future answers to questions
N Atkins

Christian,
Keep it simple, dont make the mistake of tinkering with everything in one go as that can be disasterous. And dont ask me how I know!
Graham M V

This thread was discussed between 01/11/2011 and 02/11/2011

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