In my earlier post today I should have added that :-

1/ The idle needle on these engines is very very sensitve. It MUST be set correctly and 10 degees rotation can have a dramatic effect. By comparison OS (and some other) engines are quite tolerant of slightly rich idle settings and generally their idle srews are not unduly sensitive - within 1/2 turn is OK.

2/ The idle needle must be AS LEAN AS POSSIBLE. Starting from the .002 to .004 gap mentioned in my earlier post, run the engine and tune the main needle for full power. Allow the engine to idle at under 2000rpm for at least 30 seconds then open the throttle rapidly (at about servo speed). If the engine dies with no smoke it requires richening. If it coughs, hesitates and smokes it needs leaning.
Keep leaning the idle needle untill you have gone too far i.e. it will not accelerate away after a 30 second idle. Now richen the idle needle a few degrees at a time untill the engine accelerates quickly when the throttle is opened. Now adjust the main needle again for full throttle performance and CHECK THE IDLE NEEDLE AGAIN. Remember that the idle needle WILL effect the position of the main needle for full throttle performance.
Keep repeating this untill you arrive at the leanest possible idle needle position that will allow instant acceleration to the best full throttle performance you can get.
When it is dead right, the engine should accelerate cleanly after prolonged idleing every time. Closing the idle needle even a few degrees should make it too lean -i.e. it should just die when the throttle is opened.

The really confusing and undesirabe characteristic of the design of these carbs is that an incorrectly set IDLE MIXTURE will effect mid range and full throttle performance dramatically and yet the engine may well idle really nicely!!!