As the title says, I changed out the rocker shafts and rockers for some second-hand but good ones (although all the rockers have die numbers that suggest they should have been replaced, the hardened pushrod seats haven't sunk). Everything's back together, and on startup it revs to about 1800rpm when cold and idles at about 900-1000rpm when warm.

The fact that it does eventually screw down the idle suggests that the idle stepper is working. It does the same on gas and petrol, with a little hunting on gas as it screws the mixture up and down (it's always done this). The throttle is returning to its stop, and there don't appear to be any manifold leaks.

Could it just be that the engine isn't auto-adapting to having its driver's side bank of cylinders having working valves? There's only about a quarter tank of fuel in, and I know that Martyuk has said that they disable auto-adaption below a certain level.

Reset the adaptive values.. If you happened to turn the ignition on to spin the engine over with it disconnected, it will have remembered the lower (i.e. zero) voltage as a closed throttle so it now thinks the throttle is part way open. From what I've found it seems that the signal from the TPS makes the idle air valve open to increase the revs on the transition from idle to open throttle rather than just relying on the airflow through the butterfly. As a temporary measure, you can enlarge the holes for the TPS mounting screws slightly and turn the TPS a smidgeon clockwise.

I think that it was OB who mentioned about adaptions now changing below 1/4 tank... want me anyway.

I personally am unsure of whether it does our not on a GEMS, but can't see why it would stop adapting. The bosch ones don't, they'll sul adapt when fuel level is low.

I'd just hook it up and clear them to see if that sorts it as gilbertd mentions

Martyuk wrote:

I think that it was OB who mentioned about adaptions now changing below 1/4 tank... want me anyway. >
I personally am unsure of whether it does our not on a GEMS, but can't see why it would stop adapting. The bosch ones don't, they'll sul adapt when fuel level is low.>

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Yeah- it was me. Came out of a conversation I was having with Mark Adams about something else a year ago:
He said:
Note that GEMS will not adapt the fuelling if there is less than quarter of a tank of fuel remaining. It does this to prevent false adaptation occurring due to fuel surge when the level is low. Any large corrections will remain until the vehicle is properly filled with fuel (at least half a tank). GEMS shuts itself down approximately 20 seconds after the ignition has been turned off.

I suspect that only applies to fuel trims rather than the idle voltage from the TPS. Although mine rarely has more than a quarter tank of petrol in it, the values still alter and I know for definite that turning the ignition on with the TPS disconnected results in a high idle as I've done it.......

So will it adapt itself back down or do I need to get a shot of some diagnostics to reset it?

It didn't seem to when I did the same thing. I opened up the holes on the TPS and turned it to bring the idle down. It was like that for the best part of two years until I bought the Nanocom and was able to reset the values.