I've just had another tuning session with the engine mounted in the plane. I can get it to idle quite well. Then when I throttle up, it will not return to the idle. Revs reduce to something above idle. If I adjust the idle screw, it will idle down and then stop.

edit;
Two hours later:
I switched back to the original carburettor again after checking the pop off pressure.
It is running the best it ever has now with both needles fully closed! It is idling down nicely and taking throttle ok. Just running a bit rich at all throttle openings and revs.
I will have to bend the metering lever down more and give that another try.

Edit#2
Bending the lever did no good at all - again. I've been in this position before, and I'm going around in circles.
There is only one thing that can now be causing the above and that is the metering needle spring is too weak. (I did do the pop test and it seemed to check out within the specifications). However, I will stretch it 3 mm, replace it and try again next opportunity.

Another problem with the plane/engine combination is becoming evident. One of the batteries is running down in a rather short time. The elevators are vibrating at an alarming rate and at a large magnitude; I suspect that this is working the servos hard and draining the batteries. And the battery drain is only one consideration, needless to say; I hope the servos will take the punishment.