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MG MGA - idle speed

Just recently, after a long drive the engine idle has increase to about 1500-2000. Makes slowing down harder! At a stop I can bring it down with the clutch but it resumes again once going. Cold idle is fine (after short warm up) Return spring is OK.
Any suggestions?
Art Pearse

Vacuum leak?

A hose off, broken split connector. Gasket. Try spraying around the the inlet manifold with something suitable whilst at high idle and see what happens.
Nigel Atkins

Art. You note that the cold idle is fine. Is this cold idle with the choke pulled out or not? If the choke is not pulled out, while the car warms up, it means that your idle speed is being set by using the hot idle adjustment when the engine is cold. Hence, the idle will increase somewhat significantly when the engine is fully warmed up.

The choke both moves the jets downwards to enrichen the mixture and moves the throttle plates to a more open position to increase the cold idle speed. As the choke is opened, the first movement opens the throttle slightly even before the jet is moved downwards. As the choke is pushed inwards (less choke), the jets move upwards fully before the quick idle cam is fully disengaged.

So, Nigel has some useful ideas which may well help to determine what the problem is, but will not do anything if the problem is improper set up. Set your hot idle adjustment with the car hot, set the cold idle adjustment with the engine cold and the jets not starting to drop, then see if you really have a carbie problem.

Les
Les Bengtson

Hi Les. The "cold idle" I described is after a couple of minutes on choke then high idle then pushed way in. So it is on the normal throttle stop. When driving I get the normal engine braking. Now when it gets hot after say 30+mins driving, I start losing engine braking because it wants to idle faster.
I'm going to check all the manifold and carb nuts for tightness first.
I don't think the normal high idle mechanism is the problem.
It is also starting to run on a bit when switching off.
Art
Art Pearse

When you get this high idle condtion pull over and check that your throttle shafts are not stuck ie returning to the the stop on the idle screw.
Mike
Mike Ellsmore

I had similar a few years ago although not such a high idle rpm. I sorted it by increasing the spring tension on the ends of the throttle butterfly shafts.

Steve
Steve Gyles

Thanks for all your comments. I'm trying to fathom the influence of temperature!
Art Pearse

Art
If the idle mixture is too lean, either by adjustment or an air leak, the result will be the hotter it gets the faster it'll idle

willy
William Revit

Problem fixed!!
The front float bowl had rotated on its mounting so that the bowl was rubbing on the front torsion spring.
So the throttle shaft was sticky.
Art Pearse

Art. Glad you got it sorted. Les
Les Bengtson

Well done Art. I guess I was half way there with the shaft!

I found my shaft, when hot, was not always closing that final couple of thou. I would give it a push, feel it move a tadge and the idle rpm would drop down to the correct speed. Apart from increasing the spring tension at the end of the shafts I gave the bushings a squirt of lube. Been good ever since.

Steve
Steve Gyles

OK, I thought I had fixed the problem, but it has come back, occasionally. It happens only when the motor is hot, and not always. Never when cold.
If I raise the revs using the throttle, it will try to idle at 1500-2000. However, if I bring the revs down by braking in gear, it will stay low when I declutch. I can get it to idle as low as 500. Touch the throttle and it stays high! So this is quite mysterious. Is it possible I have a sticky piston in one or more carbs?
Art Pearse

As I don't know any more than the basics I'm all for going back to, and always starting at, the basics.

As much of servicing, maintenance and repairs are mainly just about cleaning and lubricating that what I'd do, check everything, and if required take it apart, and clean and lubricate.

Last year I had an intermittent sticking jet on one carb that I thought I'd cured when I had the carbs apart and cleaning and lubing them but it turned out the jet was still intermittently sticking in use on the car but I was too slow to realise or find this. Willy was pulling his hair out (assuming he had any) because he couldn't understand how I couldn't use the lift pins to tune the carbs but part of that (but not all) was this Sod's Law stealth sticking jet.

I had the carbs complete with linkages on and off the car so many times I became quite adapt and speedy at doing it.

There's been a couple of threads recently where going back to the basics and rechecking what's already been double-check, or not checked because "it won't be that", has resolved the issue.

It echoes what Les put earlier in that if there are issues with the set up then these will cause other issues.

The great fun of it is the intermittent bit, even immediately stopping to check can be too late or alter the circumstances.

As you put about brake to accelorator pedal use difference, if next time it happens you pull the choke cable instead of pushing the accelerator it might point to either choke or accelerator side of controls and linkages.

Which also makes me think of something like accelerator cable or pedal or linkages that might be sticking, frayed, kinked, worn or even just in need of clean and lubricate.
Nigel Atkins

Torsion springs on the throttle shafts should be wound up fairly tight to bring the throttles smartly closed. There should also be a tension spring on the throttle arm from an oval washer on the cable end downward to a bracket that extends from the lower carburetor mounting bolt. Three springs please.

On most MGA (later production) there should also be a tension spring on the throttle pedal trunnion in the passenger compartment at the input end of the throttle cable. You should find details of all these things on my web site.
http://MGAguru.com/mgtech/
Barney Gaylord

Art
Have you pulled over when you got this high idle condition and opened the bonnet and checked with your hand if the ithrottle shaft is sticking, is there any gap under the idle adjustment screws (try with a feeler gauge)?
Mike
Mike Ellsmore

This thread was discussed between 19/06/2021 and 13/07/2021

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