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Ok, so I have had for some months an idle which will not drop (once warm) below 1500 rpm
While riding running like a top, great acceleration and runs fine, but bothers me that there is a problem I don't understand.
Had a look at the vac lines, nothing obviously cracked, all looks good
Butterfly valve plates, one slightly loose, corrected and still the same
Throttle cable free play, checked, OK
Recently cleaned airbox, no issues
Runs same with airbox completely off.
Fast idle screw is wound right back, so not in the mix.
Have not moved the idle stop before, would idle slower last year with the adjuster all the way out & in prev. Years the idle could be adjusted on the idle screw.
what else to check?

I've just had my van (VW Transporter T4) put on a dyno and given a remap (don't laugh...) One of the first things that Wayne (dyno-man) did was to check that all sensors were in spec, as if they are reading false (or not at all), then the whole setup can be skewed very easily.

He found that my coolant temp sensor was OK when fully cold and OK when fully hot, but in between was reading the coolant temp as -31 degrees C...! So when the coolant temp rose to the area where the sensor was faulty, the ECU was responding as if it was a freezing cold morning (in Siberia), boosting the idle and richening the mixture.

I'm not saying your fault is exactly the same, but if a coolant temp sensor is mis-reading, the result could be very similar...

I know you checked the vac lines, but imo money is on a vac leak somewhere can't help much more than that.

Either one of the butterflies is not seating right ,dirt or foreign body holding it open or throttle body connecting rubber loose/cracked.

I presume you have not changed ECU chip as some so called upgrade chips can have the affect of increasing tickover.