Idle speed suddenly too high

[mach2andy]

Having not used the car for a week or so, I took it for a run today. Once the choke was fully in, I found that the idle is suddenly 400 rpm higher than it was previously. This was apparent both with the engine luke warm, and hot. I've had a good look at the choke mechanism and it all appears to be as it should, and there is nothing jamming the pedal or throttle linkage. The car is fitted with standard UK spec SU's.

Anybody on here experienced this before?

Thanks in advance

Andy

[Monkeyfinger]

Andy - seeing as my tacho is playing up, are you sure the idle speed is actually higher or could it be the tacho reading high? However, I'm sure your ears won't be deceiving you...

[MarekH]

I found that the idle is suddenly 400 rpm higher than it was previously....
Anybody on here experienced this before?

You have a vacuum leak somewhere. Air is bypassing the throttle.

kind regards
Marek

[RICHOT]

Sounds like a vacuum leak to me as well.

This has happened to me in the distant past with my XK120.

Try testing for a leak by passing a piece of small bore plastic tubing along the main inlet/cylinder head gasket (and possible the ones from carbs to manifold), whilst holding the other end of the tubing to ones ear.
I found a slight hiss coming from the main gasket, underneath the manifold.

Happy hunting!!

[mgcjag]

Hi Andy.....in your last post you were haveing stating problems if the car was left sitting for a while..... (you never did post an update to your problem)......when was the last time that the car idled at your "correct" speed.....have you worked on the car since then?.....Steve

[mach2andy]

I now don't leave the car for longer than a week without running it, but I suspect that if I did, it would still be difficult to start because I've not been able to find anything obviously wrong. Having thought about this new issue for a while, and your suggestions of an air leak, I think my next port of call will be to check to see if all three butterflies are definitely fully closed as they should be with no throttle demand. I have had my hands in and around the carburettor linkages quite a bit, so maybe I've disturbed something that wasn't fully tight?

[bitsobrits]

Is the vacuum line and it's rubber connectors to the distributor sound?

[christopher storey]

Is the vacuum line and it's rubber connectors to the distributor sound?

This would not account for an air leak sufficient to raise the idle by 400 rpm. It would make little or no difference , and if anythin would lower the speed because of a lack of vacuum advance

[mach2andy]

It's been too cold to fanny around in my garage checking to see if one of the throttle plates is slightly open at idle. As I mentioned, I was wondering if I've disturbed something to cause this sudden change in idle speed. But I did start the car yesterday and ran it up to full temp, and lo and behold the engine now settles at the correct idle speed. It's a mysteron!

[mgcjag]

Possible sticking lever/mechanism....lube may help....Steve

[Rich70]

I’ve previously found and confirmed an inlet gasket leak by sparingly (small quick squirts) spraying Easy Start around suspect leak areas while the engine is running. The idle will quickly ramp up when you spray it in the area of a vacuum leak.

[mach2andy]

Problem solved. I found that there was a crack in the copper advance/retard pipe which would have been causing a slight air leak at the front carb. But I think the main issue was that the centre carb fast idle stop was maladjusted causing the throttle plate to be held slightly open with the choke fully in. I had been fiddling around with these to try to improve the car's cold starting. I found various references to what the gap should be be between the stop and the arm varying between 2 thou and 15 thou, so in the end I reset them all to 5 thou which is the thinnest cranked feeler gauge I have. And hey presto, having spent some time rebalancing the carbs, the engine now idles at c. 800 rpm & runs as smooth as a gravy sandwich again.

[RICHOT]

Good result.

Cheers,
Richard