6 cylinder idling fast when hot

Morning all, I've got a 1979 6 cylinder petrol and over the last 2 months or so it's started idling fast when the engine is warm, it doesn't bother me much as I don't spend much time sat idling when I'm out in it but I should probably find out why it's doing it.

It starts up fine every time and idles roughly where I expect it to (by ear, no rev counter) and drives fine but when the engine warms to it's normal working temp the idle speed increases to maybe twice (pure guesswork, it sounds like how it idles when the choke is fully out when cold) what they should be and are steady at that rpm. If I engage a gear and with the footbrake applied slowly lift the clutch to biting point and then disengage the clutch again the revs drop to where they should be for maybe 2 seconds before creeping back up to a steady fast idle again.

So over to you landyzone, where do you recon the problem lies? Answers on a postcard please.

sounds like a worn butterfly valve on your carb, when warm engines run better anyway it could be that with it all warmed up there is enough extra fuel getting in to increase the revs a bit try turning the mixture screw just a tiny tiny bit to see if that helps your cause any good luck

The exhaust does smell like it's running a little rich, I'll have a play with it at some point, thank you.

Check that the throttle is closing properly when hot, then check the mixture, though it sounds like it may be the ign timing out which causes the rev's to pick up slightly which increases the vacuum which in turn advances the timing which increases the revs. Check with a timing light and see if the timing advances when hot

Thanks for that owl, I'll borrow a strobe from a mate at some point and have a look.

Today I set the inlet and exhaust valve clearances as per the green bible, rubbed the rotor arm and the distributor cap over with some fine emery paper as there was a little scorching, changed the points for new ones and set the points gap again as per the big green book. I've not touched the carb or the ignition timing. I took the landy out this evening and got it nice and warm and didn't have any issues at all with a rising/fast idle, presumably this isn't just coincidence so can somebody please explain why what I've done would have stopped the issue I was having? Cheers.

Perhaps the mechanical advance was sticking and you have freed it of somewhat when you were messing with the rotor arm ?