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MG MGB Technical - idle speed keeps changing

I am having trouble setting the idle speed. I adjust the screw and get an idle speed I like, but as soon as I drive it, the idle speed changes. Any ideas?
Robert sewill

This could be a number of things the most likely is that the throttle cable inner wire is binding in the outer. Remove and apply a liberal amount of grease and then reassemble and try again. The second possibility is that the throttle butterflies are not ptoperly centred in the venturis. You will need to remove the carbs to adjust this but it is just a matter of slackening the butterfly screws backing off the throttle adjustment screw and then closing it fully to centralise the butterfly discs. Thirdly it is possible that the carbs are not in good alignment and to cure this you will need to slacken both off and adjust their positioning on the manifold until they move perfectly freely.
Iain MacKintosh

With SUs it is almost certain to be worn spindles, what carbs are you using?
c cummins

As above, but in addition, test the movement of the pistons in the chambers....They must be tested individually, to see if they drop all the way down each time....If the chambers are worn, or the jet isn't perfectly centered, the piston will not fall back to the correct position each time.
E.B. Wesson

Thanks guys for the help. I have one su carb. I have an appointment with my MG mechanic for this Fri. I'm not much of a mechanic myself. Once again, thanks
Robert sewill

Robert- It's helpful to know the year, etc. of the car we're spitballing. One SU? Prolly not. Probably a single Zenith Stromberg on a 76-80 smog car. All sorts of tweaks were done to these unfortunate years, and most were a problem right outta the chutes. If Colorado doesn't have restrictive Smog Statutes like California, you might consider a rebuilt set of SU with manfolds to brighten your ride and your smile. On the ZS carbs, I've found time and again, micro cracks in the diaphragm causing idle to ebb and surge. If so in your case, it's a fairly straight forward, ez fix that won't cost an arm and a leg. Cheers, Vic
vem myers

If you have the Zenith-Stromberg and you have any leaks in the vacuum lines or a leaky valvecover gasket it will be just about impossible to get a good "tune" on the carb. Sticky choke adjustment needle can also do this. This mixture needle is part of the choke mechanism and requires choke disassembly.

Most do not like the ZS carb. I finally got mine functioning well.... Now all I have to do is leave it alone... but where is the fun in that!
C.R. Tyrell

On MGBs with real carbs (i.e. twin SUs) a quick test is to go under the bonnet with the engien idling and press down on the ends of the throttle shaft attachments. If there is significant wear, this will change the air gap where the leaks are enough to change the idle speed.
Bill Spohn

That's usually done to see if the cable or butterflies are hanging up, and is always a good test in this situation regardless of the cause. There must be a little free play in the cable to make sure they can always rest fully on the screws.

A 'fixed' vacuum leak should always be able to be compensated for on the idle screws if everything else is correct, a 'variable' leak less so.
PaulH Solihull

Hey Robert...SU or ZS? Vem
vem myers

This summer I had the same issue with my 77 roadster which is running on a points distributor and a Weber carb. An ever changing idle evolved over a period of a few weeks into lousy acceleration and really poor performance.

The problem turned out to be a failing diaphragm in the distributor's vacuum advance. I believe the symptoms grew worse as the diaphragm deteriorated. A trip to Advance Distributors for a rebuild/recurve took are of all these problems.
Brian Denis

This thread was discussed between 21/10/2012 and 19/11/2012

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