I have some SU Carby issues that I need some help on. 
Engine is an 1100 + 60", flat top pistons, mild cam, LCB, 12G206 head and HS6 on Minispares manifold. 
It has an issue with erratic idle speed and mixture, sometimes it is 1000 RPM and happy, mixture is 14:1, the next set of lights it is 1100 RPM and mixture is 8:1, after that it is 700 RPM and mixture 26:1.When it goes to 26:1 it gets rough and erratic, so I believe the Air fuel ratio meter,(Wide band O2) a touch of the throttle will lift the revs, and it will settle down and be happy and go back to 14:1.It seems like the bottom of the needle is too lean, and just a bit up it is too rich. 
What wild needle have I got to create this, well a BDL at the moment, but anything that is close has the same problem. I have tried using a Red and Blue spring, same issue. Sometimes you get a slight change in revs, and the idle becomes erratic and you will see the AFR meter swing from 8:1 to over 26:1 and back again in a second, with engine note changes that make you believe that it is true. 
Also (and maybe related) when driving it has a persistent flat spot just off idle, the AFR goes to 26:1 or higher, and putting your foot down more will get rid of it, or waiting a bit it will sort itself out. The only way I can get rid of it is going to a much richer needle so the lean spot is rich enough that it does not worry, it, but it makes it way way too rich and you can smell it really rich and see it on the AFR meter at 8 or 9:1. Dashpot oil is 20W50 engine oil. 
I thought it might be a sticky Dashpot, I have checked it, done the drop test, and it all moves smooth and like it should. 
The throttle shaft is new, the body has been rebushed, I can feel no movement in the shaft and pressure on the shaft makes no difference to the idle. The little stopper on the bottom of the Dashpot is there and I can't remember what is measured but it was correct when I looked up what it should be. 
I have been making changes and scratching my head for years now and I can't figure it out. 

Any ideas greatly appreciated. 

Stewart.

Hi Stewart
I had a similar problem with the idle changes on a HS4 which I finally tracked to the return spring and acc cable. The spring was a little too strong and the cable was sticky in its sleeve.
The flatspot will likely be needle related, a rolling road session with a good operator will reveal all sorts of remedies.

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Cheers Dave.

No doubt you've checked 100 times, but it sounds like there is an air leak...

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did I tell you that I won a trophy?

Try an OA6 needle

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

BDL is the same profile as OA6 but is a swing needle

Thanks Ken.

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

Thanks for the replies and suggestions.
Sorry it has taken me so long to get back to you, life gets in the way sometimes.

I ended up putting a slightly stronger return spring on the carby, that seemed to make it a little more consistent.
I used various sprays to try and find an air leak, as it does act like one, but found nothing.

I found the info from ACDodd on this page to be helpful :
https://www.theminiforum.co.uk/forums/t ... rbs/page-2

He mentions the fast drop piston mod, I found that online in MG/264 Service Memorandum for the MGA.
I did this mod to the damper piston. Which means with a heavier oil it does not flat spot on gearchange.

Also read about how 20 weight oil is not 20W50 like we have now.

In the end went for a BDL needle as the OA6 had a few lean spots in it, and a heavier oil in the damper. The result is pretty good for numbers, and it drives well.
The idle still hunts around, but keeping the idle mixture a little rich (high 13's)means it is no problem.
So I am happy with how it all drives, but perhaps this is why fuel injection was invented !
My last daily drive car with a carby was sold about 20 years ago, so maybe I have forgotten in that time that a carby is not perfect like injection is.


Stewart.

The varying idle may be distributor / timing related rather then carb.

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Cheers Dave.

Had thoughts about ignition too, changed for other reasons to a CSI electronic distributor, no difference, also disconnected the vacuum line to the distributor , no difference again.
Very much ground hog day.......