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MG MGA - Ideling Questions

After the car has been driven for a while I've notice that the idle speed seems to drop down. Its hard to tell how much because the tach doesn't seem terribly accurate at low rpm.

But it definately seems rougher, enough that I feel like I have to feather the gas pedal to prevent stalling....it won't actually stall just feels that way. But most notably when it gets like this the ignition lamp goes on.

Any thoughts?

Thanks,

Tysen
Tysen

Hi Tysen. While this is not the only cause of the problem you describe it is probably the most likely one: This problem is often due to wear in the carburettor body, where the throttle shafts sit. Wear in the throttle shaft area allows excessive air into the carbs. This is essentially a vacuum leak, and can play havoc with the idle speed and also affects fuel economy. The cure is to have the carb bodies drilled out and carb throttle shaft bushings installed. New throttle shafts might also be necessary if they are worn out. If you shake the throttle shafts every which way, and there is more than a whisper of play in any direction, your carbs need to be rebushed and repaired as described above. Good luck! Glenn. PS Rebushing the carbs is NOT a job for an amateur. It requires special equipment.
Glenn

Tysen,
If its idling that slow the reason for the ignition light is that the generator output is insufficent at the low RPM. As for the idle itself, I'd check for vacuum leaks, especially around the carburetor shafts. To check for leaks, spray a little engine starting fluid at suspected areas and see if the engine revs up.
Regards,
GTF
G T Foster

I see Glen beat me to the keyboard! Sorry to be redundant.
GTF
G T Foster

I have found that in many carbs the major problem was cured by just fitting new shafts. The wear on the shaft is much greater that that in the body of the carb.
Sandy
conrad sanders

All true, but don't get too upset about a bit of wear in the throttle shafts. This is one of those "truths" that is way overblown and used as an excuse for everything including the state of the world, but it is not usually the cause of significant trouble. There are a lot of cars that "should" have new ones, but don't because the owner doesn't have the inclination or cash to fix it; some of these cars were mine, and I have the knowledge and tools. The leakage is only important at idle, and the only real effect is that a correct setting of the mixture at idle will give a slightly rich mixture everywhere else. So, if there is say .010 slop in the shafts, set the mixture at idle about 1/2 flat lean - and most people can't get it that close anyhow - it will idle OK, save fuel at idle, and be fine everywhere else. It may take a bit more choke for cold starts; The only other effect is that on some cars, the throttle will sometimes come to rest at slightly different positions, leading to a slightly variable idle speed, but it will be random, small, and not temperature related.
The symptoms you describe - slower idle and rough when hot - are typical of a somewhat rich mixture. Its fine when cool, too rich when hot. It probably wants to be a flat or two leaner.
I know you just got it, get some books and read up on adjusting SUs. Get v familar with it and come back for further help - Happy MGA-ing!
FRM
FR Millmore

For now, I'd just turn up the idle a bit. And study the wondrous SU.
Tom

Tysen,

If you want to study the construction and tuning of SU's go to this site:

http://www.tr-register.com.au/Files/sugeneral.html

You need to read it all, even the section on single SU's, as some information is not repeated in the twin SU section.

Mick

Mick Anderson

Mine was doing the same--slightly leaning the mixture eliminated the problem. Five minute fix.
R. L Carleen

This thread was discussed between 30/07/2005 and 01/08/2005

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