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MG MGA - Idle speed increases

When I start my car of a morning (10 C or 50F outside) and it warms up enough to have the choke pushed fully in it idles at 800 RPM. After the water temp. gets to around 180F the idle speed increases to 1000 RPM. Then after a long run (100 kms) down the highway the idle speed then sits around 1200 RPM (engine temp. 180-185) and stays there for the rest of the day. When I take it out the next week the same happens over again!
Help please.
Mike
Mike Ellsmore

Mike. You need to adjust your carb linkage properly. You should have the first portion of the choke cable pull out operating the fast idle cam on the mechanism, but not dropping the jets. The jets should be dropped, enriching the mixture, with the last half of the cable being pulled out.

In operation, you pull the choke cable out completely and start the car. Then, you allow the cable to move forwards, raising the jets, until you get best cold idle. As the engine warms, and it will only take a mile or two, you reduce the choke to less than half way out, leaving the jets fully up and the cold idle (fast idle) cam engaged to keep the not fully warmed up engine running. When the engine is fully warmed up you would see about 1,600 rpm in this position. So, as the idle speed increases, you allow the choke cable to retract to keep the idle speed at about 1,000 to 1,200 rpms.

When the engine is at operating temperature, the choke is fully in and the carbs are idling on the hot idle mechanism which should, at that time, be adjusted to about 800 rpm (as your engine will allow).

Les
Les Bengtson

Les,
I might not have explained my problem properly - linkages are set up okay (I think)- after engine has warmed up enough that the choke can be pushed back in and the engine doesn't stall when idling (jets are right up, high idle cam is clear of the adjuster screw and linkages are all slack) idle is initially around 800 RPM but gradually increases as engine gets warmer - to around 1000 RPM at 185 deg F - then when I go for a long high speed run and then let the car idle the idle speed has crept up to 1200 RPM. Is there anything that would make the idle speed creep up like this?
Regards
Mike
Mike Ellsmore

Mike, I had this problem too. I tried tuning the ignition and carbs to improve matters, but even when this was as good as I could get it, nothing really worked. I eventually rebuilt the carbs, and found a variety of faults - listed in a thread a couple of months back. This has dramtically improved the idle characteristics, and the smooth running of the engine. Apart from the faults listed, I think the carbs were becoming slightly leaner as a result of a long run, and this has the effect of speeding up the idle. Also, for some reason I think the the throttles were not fully retuning to their stops when thoroughly warmed up (possible due to heat expansion). All of this was corrected during the carb rebuild. But I spent a lot of time on this problem and I know exactly how frustrating it can be - good luck with your investigations, and let us know what you eventually find. Adrian B.
A Bennett

Mike

Rest assured you are not alone. I have frequently found the same. I found that blipping the throttle occasionally brought the idle back down, but I always felt there was 'stiction' in the system when fully hot. I cured my problem using good old engineering techniques - installing a stronger jet lever return spring!

Steve
Steve Gyles

I seem to recall a similar problem on my TD, which I eventually put down to one of the throttle spindle springs pushing the butterfly valve against the carb body as the valve closed, and sticking slightly open. Check for signs of rubbing inside the carb, where the spindle goes through. The previous owner, from New Zealand, had had the carbs rebuilt using non standard springs. When I rebuilt them using the correct springs, and allowing sufficient clearance for the butterfly valve to return without fouling the body, the problem stopped.
H BUCKELDEE

Mike. Have you checked the throttle plates for loosening? As some of the others have mentioned, they need to be tightly attached to the rods and adjusted so that they will fully close. I have seen these loosen up and cause the type of problem you describe.

If the car, when fully warmed up, idles at 800 rpm, it should continue to idle within 100 rpm of that after a long run. Certainly going up to 1200 rpm indicates that something is causing a bind in the system which is keeping the throttle plates from moving back to their original, 800 rpm, position.

In addition to the throttle plates being slightly loose, the linkage can bind as it is being returned to the idle position, as can the throttle cable. This type of thing is difficult to diagnose over the internet as it is an interactive process between the investigator and the carb system. Use the ideas we can give to you and pay attention to how the system is acting as you do your checks.

Can you force the throttles further closed and drop the idle lower when it reaches 1200 rpm? If so, it may be a cable sticking, the pedal sticking, or the linkage binding--or it can be the throttle plates shifting slightly and not allowing the throttle to close fully to the idle position. Pull the air cleaners, open the throttle, examine the throttle plates for tightness (you may want to remove the pistons and suction chambers to make this easier). If you remove the pistons and suction chambers, make sure they are throughly clean and one of the pistons cannot stick in a slightly upwards position. Are the springs the same length, diameter and made from the same diameter wire?

This should get you started. With more information, we can focus more intensely on what may be the actual problem.

Les
Les Bengtson

Have you put your foot under the pedal and pulled backwards. If the revs drop the fault is in the cables and levers not returning.
Just a simple test.
David
DJ Lake

David. Your test works with a mechanical linkage--i.e. the pedal directly connects to the mechanical linkage which, in turn, has a direct connection to the carb throttle.

The MGs use a cable that fits into a slot in the pedal. If you were to "pull backwards" the pedal is not capable of forcing the cable upwards, through the outer sheath, both because it is a stranded wire cable and because there only a stop on the lower end of the cable. The cable pulls the pedal up, but the pedal cannot force the cable in any direction except downwards. Have an assistant hold the throttle open, then move the pedal to the upwards/backwards position and observe that the cable dangles down below the pedal.

Les
Les Bengtson

Thanks Guys,
I will investigate this weekend and report back - I normally notice this problem when I am well into a day trip and promise myself I will look into it when I get home but then when I get home I find a Swan's game is on the TV (Australian Rules football team to you northerners) or the Navigator has other jobs lined up, so the high idle gets put off for another day!
Mike
Mike Ellsmore

I had a similar problem after I had rebuilt my carbs. It seemed to be caused by setting the adjustment too lean. When I made it a bit richer, it went away.
Dan Smithers

I currently have a similar problem. I notice it when idling at stop lights. Idle stays around 1200. I find that if I quickly pull and release the choke, the idle drops down to where it should be. I've tried lubing the jets with graphite oil, but it hasn't helped.

M.D.
'57 Coupe
M. D.

I have a similar issue.

Here are a couple of thoughts that occurred to me

1) does the engine warming thin the oil in the dashpots allowing the needle to rise more?

2) does the engine warming increase any air leakage around worn shafts?

Just a few thoughts.
T McCarthy

I don't believe the oil viscosity in the dampers has anything to do with it - it is only there to slow down the rate of rise of the piston (richen the mixture) when accelerating - at idle it should be stationary.
Mike
Mike Ellsmore

This thread was discussed between 25/05/2008 and 31/05/2008

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This thread is from the archive. The Live MG MGA BBS is active now.