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MG Midget and Sprite Technical - Uneven tickover

1275 A series; MkIII; Pirahna ignition; LCB; servo - otherwise standard.

I've searched the archives using variations of this thread title but drawn blanks.
When cold it runs fine but once at operating temp the tickover is all over the place from 500-1000 rpm with frequent stalling the result.
Started with ignition and had a look at the 'red' rotor arm which was a bit worn as was the distributor cap so both were replaced with Distributor Doctor items.
I've upped the tickover speed (without success) but am now thinking on the petrol side or an air leak possibly?
Suggestions for areas to start checking gratefully received.
Jeremy Tickle

Are both carbs fuel mictures correct? Do both pistons rise together?

Could be air leak, but does it idle fine with choke at cold, but idles roughly when choke pushed in as warms up, maybe the choke is sticking on and not relasing at the carb end?

Just my rsmbling 2p there!
R Williams

It could be the dreaded A series inlet manifold gasket, very thin above and below the port and fails all to easily causing just this symptom.
Paul Walbran

Worn carb spindles??
Bob Beaumont

I was going to suggest worn carb spindles, as these certainly make adjusting to get a consistent tickover almost impossible. Relatively easy to test though, by smearing something like siloicon, or grease over the end of the spindle to seal any air gaps there.

If this does temporarily solve the problem then you may well get a more prermanant cure by just fitting new butterfly spindles without necesarily re-bushing the carb body.
Guyw

What both guy and paul said
prop

Agree - look at the dual carbs.

2 things resulted in a rock steady idle even on my very cammy 1380...

Aldon yellow Distributor replacing the worn original (the vacuum advance plate wobbled all over the shop and was even worse on a recon unit)

Replacing 40 year old 180K mile dual HS2s (even with a rebuild and new spindles) with younger HIF44.

Dean Smith ('73 RWA)

Thanks for all the replies - plenty to check there.
In between posting and coming back to the forum I've had a quick dabble.
It seems thaat the pistons don't fall together, one doesn't fully descend but that's remedied by 'wriggling' the jet assembly. Is there a way of fixing the jet assembly in a certain position, I'm assuming (always dangerous) there is? Would this cause the erratic tickpver?
Cheers.
Jeremy Tickle

I did what Dean did and replaced my twins with an HIF44. Better response, a little bit more power and lovely even idle.

If the needles are of the fixed type, you will have to centre the jet until the piston falls onto the bridge with an audible click. The sprung (wobbly) type needles shouldn't have that problem.
There is a little book called "Tuning SU carburettors"(if it's still available), which will tell you all you need to know about carb adjustment. There are other more modern books you can buy though, which include later carbs,but that one, being quite old, deals with our type of carbs.
Bernie Higginson

Or download the advisory SU tuning guides direct from the Burlen web site, FOC.
Guyw

Bernie,
good to meet you on the MASC stand at Tatton on Sunday and thanks they're the fixed type so I'll need to centre them - I remember being shown how to do that when about 18 - a long time ago!
Guy, thanks for the tip. Have been on the Burlen site, some good info there and as you say FOC.
If that don't cure it then there's plenty here for me to check out :) Thanks for all the helpful responses.
Jeremy Tickle

Worn out springs on the distributor + worn spindle shafts on the carbs. I've seen plenty of rebuilt engines where the ancillaries were ignored. Everything has to be up to snuff for a good running engine.
S

Something I've never done but isn't there a spray you can use around suspect areas of air leaks which causes the engine to speed up if if ingests the vapour through a leaky spindle, manifold leak and so on?
Graeme Williams

Any volatile spray will do that - carb cleaner, releasing fluid, damp start, etc. If there is an air leak anywhere in the induction system where there shouldn't be, the volatile fluid is drawn in, enrichens the mixture and the engine momentarily speeds up. Its a very quick and easy way of finding small induction leaks.
Guyw

Over the last few weeks, I've discovered exactly how much tappet clearance/setting effects tickover speed and smooth running- far more than I'd have thought possible.

Badly set/ badly worn tappets stopped my engine ticking over below 1200 rpm. They were worn on the shaft, and much too big a gap. Adjusting them allowed the tickover speed to lower to near normal, and replacing the whole assembly has made a world of difference.

It is also a quick and easy check/adjustment- before worrying about carb jets and air leaks, etc.
Andy Collins

I've seen plenty of rebuilt engines where the ancillaries were ignored

steven... if you cant see it or its considard black art then it dosnt count

haha

prop.
prop

Hmmm....I must be king of the black arts then....that's where I always find the faults...but like you I've had many a sleepless night trying to figure out what's gone wrong. Great feeling to solve those riddles isn't it!
S

This thread was discussed between 09/06/2016 and 10/06/2016

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