Idling too fast

[Babord]

With my engine warm, the idle speed has suddenly started running high: approx 1400 RPM, sometimes higher when out driving. It was fine last week, so I thought it may just be a problem with the accelerator cable or choke cables, but all seem to be ok. The idle screws are not quite touching their stops, so adjusting will not make a difference. I can push the top of the rear linkage approx 2-3 mm and this will reduce the speed below 1000RPM (I guess this is where the idle screws are set), but can't see why it does not return to that setting on its own. I can't see anything obviously wrong with the linkages, but then again I am not really sure what I am looking for! Any ideas on what I can check or try?

btw, I needed to top up the dashpot oil this morning, which I had to also do about a month ago. And I noted some black residue and water coming out the exhaust this morning while warming up, and which I have not noticed before. Not sure if they are useful indicators or not.

Any help appreciated!

[Lingen]

Babord.
I think the black water, (and possibly steam?) now coming out of the exhaust is just a sign of the time of year. My stag has started to do it again with the colder air and high humidity.
Your idle screws should both be should be around 1.5 turns in after touching their stops, so something is wrong. I suggest you temporarily disconnect one end of each link rod, and see if that helps. If you still can't get a low tick over, then air is getting in from somewhere. Has one of the crankcase breather hoses split? Is the felt washer in place at the top of the dipstick?Or perhaps the carb pedestal is loose on the inlet manifold, or the "O" ring there needs replacing.
Mike.

[Babord]

A bit more test data, in case it helps:

After accelerating and releasing the accelerator pedal slowly, idle speed is actually 2000 RPM!
If I then blip the throttle, it returns to about 1800 RPM.
If I then push the top of the rear linkage rod it drops to below 1000RPM (but I have to hold the rod there with pressure). When I release the rod, idle holds at about 1400 RPM

[Lingen]

I forgot to mention the dashpot oil. This will be happening because the small O ring on the needle adjuster has failed. I would get them replaced on both carburettors while you are at it.
Mike.

[fussydave]

I had this happen once. It turned out to be because I had revved the engine using the throttle linkage under the bonnet and this had caused the throttle cable to go slack and dislodged the outer sheath of the throttle cable from its seat so that it sat on the edge of its seat thus holding the throttle slightly open - hope that makes sense!

Dave

[jbuckl]

Ensure the throttle discs are controlling the idle.
do this by disconnecting the ball joints. Take it from there.

[Babord]

Thank you to everyone for the help.
I started with the accelerator cable check - was seated properly.
I disconnected the rear rod and the RH idle screw dropped to the stop, but the LH did not. The bolt against the LH choke cam was preventing a return to the idle stop. So I took the airbox off and moved the bolt in a bit (slightly away from the cam) and now the car is idling nicely between 500-600 RPM. I am confused as to why this suddenly happened though? Certainly the bolt could not have moved as the locknut was tight. Anyway, for the time being I am hoping it is not just a temporary fix.

Oh, and I am pleased the black water is nothing to worry about. It does stop as soon as it is warmed up.

Thanks again all. Off for a drive now!