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MG Midget and Sprite Technical - Carb trouble

I'm finally trying to get my midget on the road again after restoration. Carbs are giving me some trouble though. As long as the choke is pulled (even just) the engine runs very well. But I had to turn the jets one full turn (6 flats) down to get it idling at 1000 rpm w/o choke. The rear carb will then "spit" (misfire?), rev up and fall back to about 1000 rpm. Throttle seems slightly unresponsive.

This is what I have already done:

- Valve clearance checked and adjusted
- No apparant vacuum leak
- distributor thoroughly cleaned
- new spark plugs, HT leads, distributor cap, red rotor arm
- timing adjusted with strobe light

Could the float chamber be the culprit? I have those floats that cannot be adjusted, and the clearance is less than what is specified in the Haynes manual. Would a floated carb cause the symptoms described above?


A Damm

Hi Andi
Have you checked the quality of the fuel from your tank
My MG sat for a few week and all the %!#$%! settled
and contaminated the carbs

forget the Haynes buy or look at the Driver's Handbook, Haynes for repairs Handbook for prevention -

just my thoughts:

as it runs well with choke it suggests a fuel problem

you could also try checking that the pistons are matched -

check rear carb bowl needle and seat float moves freely and supply pipe to carb and fixing

throttle pedal and cable have free movement

tappets, CB points, plugs, timing, mixture in that order

when you cleaned the dissy did you lubed and oil it too

check out these two John twist videos for principles of setting carbs

Nigel Atkins

I have a brand new tank, in-line fuel filter and the petrol is less than four weeks old. So, this is rather unlikely.
A Damm

Thanks for all the info Nigel. I'll check that the pistons match tomorrow. I do have a funny feeling that the front piston is lifted higher than the rear one.
A Damm

sorry Andi I missed your name before

other thoughts;

damper oil in suction chambers

be wary of new rubber in any parts and hoses as it can be rubbish I found new Goodyear 6mm fits better to the carbs and does crack or perrish in a few months

set up of linkages or return springs
Nigel Atkins

sorry I was still typing while you posted

if one's in front of the other also check damper oil and which way round the internal spring is fitted and if it's the same in both carbs

even though it's not directly for your car this video also might help with other things to check/look for and give you more background info -

Nigel Atkins

Another culpret....would be a clogged hose from the float chamber to the bottom of the carb....aka the jet tube

Becareful...if that hose is old, the tubing can be brittle

Prop and the Blackhole Midget

Thanks for all the ideas - I did a piston match test yesterday and that seemed alright. I still don't get the rear carb working right. Whatever I do, the rear carb seems to be lean. I can turn down the jet adjusting nut all the way and the engine speed will still fall (almost to the point that the engine stalls) when I lift the piston 1 mm. If I understand the carb setting procedure right, this suggests a lean mixture. Hence, I have a feeling that the carb simply doesn't get enough fuel.
I already thought of what Prop suggested, and will remove, check and clean the jet tube today (it's new though). I'll post the result.

On the good side - I did take the Midget on the road for the first time in 1.5 years yesterday afternoon. Great feeling. And even though the carb wasn't perfectly tuned, it was still good fun driving it.

A Damm

Andi, it might be a worthwhile just to check the needles are the same in each carb, as you don't say if you owned it prior to the restoration and there's no guessing what some people get up to,
Graham P

+1 on carb needles

and check rear float bowl needle and seat set

and fuel pipe to rear carb for internal breaking down and blocking and security of fixing of pipe to carb
Nigel Atkins

If the fuel pipe from the bottom of the float bowl to the jet is the nylon type (covered by a spring) be sure that it is not damaged (previously kinked).
Trevor Jessie

I'm finally onto something here. Jet tube was free and no kinks. I then checked the length of the inner springs and the needles. One of the inner springs is 138 mm while the other one is 131 mm. More importantly for my problem, the needles are indeed of a different length (I did not own the car before restoration).
They are both GG needles and their overall length is the same allowing for some tolerance (41.41 and 41.14 mm). But the length of the needle seat is different - 10.5 and 11.24 mm. So, if I install them with the needle seat flush with the piston, there is a considerable difference in length, which should explain my problems.

I'm going to order new internal springs and needles. I would appreciate your thoughts on the type of needles though. According to the little tag my carbs are AUD344 - used in (some) Midgets from 1969-71 (carb bodies AUC870, AUC871, Suction chambers AUC1357, pistons AUC1432).
Obviously, the previous owner must have changed the setup as my car is a '73. The SU catalogue suggests AUD1468 needles. A little search on google yields that they are usually sold for the Mini. Can I use these or should I go for AN or AAC needles? I'll be using pancake air filters.

Thanks for all the support - it is greatly appreciated.

A Damm

sorry I should have put when piston matching to layout the components and check them, I thought it was in the video but I see I'm wrong

before you order springs and needles - if you have the old thin pancake filters these are not the best to use, if you still have the saucepan filters those with regularly cleaned or replaced paper filters would be better (or K&N filters inside)

if you mean you have K&N pancake filters then depending on the use of stub stacks too could mean you need a richer needle than with sausepan or thin pancake

the springs are normally 'blue' but some stray into use of 'red' IIRC

I also wonder if one carb takes a fixed needle and the other a floating needle ?

don't know if this is any use -

but I'd wait as there are experts that often post here
Nigel Atkins

ETA: a photo of your carbs and engine bay may also pick up other matters carb related or not
Nigel Atkins

Thanks for all your help Nigel! I should have had the idea to check the needles myself but I was naive enough to assume that they'd be the same. The car came with pancake filters but I wouldn't mind using saucepan ones if the car runs better. I might get a chance to post a picture tomorrow.
A Damm

Check that there is not an old o-ring in the bottom of the float bowl where the jet tube goes - that can get you. The o-ring should be well back on the end of the tube when you install it, and make sure the tube is pushed home before you screw the nut in.

Needles are set with the shoulder flush with the piston face, and the small end of the needle is not in play on these carbs - some needles are 1/4" or more shorter. The measurements you gave are not significant. What is significant is any wear on the needles, usually at stations 1 & 2, or the first 1/4" from the shoulder - you will not be able to set them if they have wear, and especially different amounts of wear.

You could also measure the needles at precise points along the length to check they are in fact the same, as the markings indicate - it has happened! Easy - set your dial calipers to some gap in the needle range, and drop the needles in side by side - both needles should go down the same amount. Differences will be very obvious. To check they are what they say they are, look up the measurements by station in needle charts available from many sources. And PLEASE, measure in INCHES!

Also check that both jets are .090" diameter - a big jet will really send you screaming. And look carefully for wear - the jet hole will be not round.

Nigel- floating needles have different pistons, and you need special adaptors to use normally fixed needles in them.

All that said, big differences in how one of a pair responds is generally because the throttles are not synchronised correctly, and the engine is running mostly on one carb at idle. You cannot set mixture until the throttles are opening the same amount.

You must loosen the interconnect shafts between carbs, then set the idle screws, then tighten the interconnect levers when all is done so that both carbs open the same amount - which you can see by both pistons rising together and the same height - assuming all four cylinders have the same compression etc.
I start with the idle screws 1 1/4 turn in from zero and jets 10 flats down from flush with the bridge. Be certain the fast idle screws are clear of the cams when you set the main idles. This normally gives slightly fast idle so it is easy to start, but you then need to back the idle screws out a tad. Mixture will be closer than most are after some "expert" tunes them, if all else is correct.

FR Millmore

Thanks FRM - your expertise shows how little I know. I checked the needle diameter and they both are the same at different points. However, one needle is bent - the tip of the needle is off centre. When rolling the needle seat flat against the side of the calipers, the tip of the needle will almost touch the calipers in one position and when turned 180 degrees clear them by about 0.040''. So I think I will have to go for new needles - the question is which ones.
A Damm

FRM should be able to help with needle suggestions

these videos will help with more info

44 Secrets of the SU Carburettor Part 1 of 2 -

43 Secrets of the SU Carburettor Part 2 of 2 -

55 Centring the Jet -

cheers, another thing to try to remember as I didn't know you could get adaptors for the needles, I was thinking as well as the needle being a different type that the piston or carb might be different, hence the photos

I thought it might also lead onto engine breathing as has happended before with previous owners mixing and mismatching
Nigel Atkins

Update - I have just spoken to Burlen Fuel Systems and they were very supportive in cross-checking the part numbers. I do have an earlier carb (that is not correct for the year). However, the suction chambers and pistons are the correct ones for that carb and Burlen assured me that this carb can be used on a 1275 Midget. They suggested using blue internal springs and AN needles. I hope that springs and needles arrive before the weekend, and I'll post the result.

A Damm

as your vehicle profile not up I'll go on my very poor memory, (and try not to mox your car up with Mark's) I think yours is a 1972 so as is in the owners (Driver's) Handbook - standard needles AN and spring light blue

these are for standard filters, still not sure which pancake filiters you have but if they're the thin mesh and foam ones they're not the best to let the air in - perhaps a photo
Nigel Atkins

sorry about this - I haven't got around to taking a photo yet - stuck here at my desk. I have a '73 Midget.

The trouble I had was that for my earlier type carbs (AUD344) the SU catalogue did not suggest AN but GY (AUD1468). Burlen recommended to go with the AN needles though.

As to the air filters. The car came with the thin mesh and foam filters, but I will eventually go for original saucepan filters when I can source a good second hand pair. I will be taking the car to Germany at the end of October (in the back of a lorry together with my '93 Honda Transalp and furniture). The German TÜV (MOT) is usually quite particular when it comes to non standard air filters.

A Damm

sorry my poor memory Andi

you'll want to get the whole car set up and running well before you go away then

I'd guess the GY needles where for the AUD344 carbs as fitted to the 1300 (saloon) cars -

I've no idea of the diffrence between these and other models of carb but as long as they have the breather pipes I expect they match in very easily

I'm sure you'll find things much better withn the new needles and springs - see one of those John Twist videos for which whay the springs fit

don't forget to sort rest of car, 36k-mile service/check up and get the (owners) Driver's Handbook

regularly driving the car will help keep the car reliable and you'll learn what the car is capable of and how to drive the car to getthe best out of it

good luck

Nigel Atkins

Just a quick update - put in new needles and springs today and this did the trick. I still have to do some fine tuning but the car runs way better than before. I get it to idle at around 800 rpm and it revs up well. No need to mention that the "spitting" has disappeared as well. On the downside, my alternator seems to have given up its ghost now.

A Damm

Well done Andi

but astill get those photos up especially of the engine bay as more help and even better running may be available

lack of photos can can cause premature alternator failure :)

put up another thread about your alternature or look in Archives or recent threads as there may be help for your alternator too

ETA: oh, you were putting up a new thread whilst I was typing
Nigel Atkins

Centre your needles! 10 minute job - which I incidentally failed to do, intitially causing very similar symptoms to yourself. Also ensure same needles, same height, same rest position in jets etc etc before centering.

Yes, everything mentioned so far is excellent advice, I would add a good squirt of carb cleaner does not do any harm...
Mark O

This thread was discussed between 16/08/2012 and 28/08/2012

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