I'm reading this thread with great interest. My plugs are always sooty, I was going to do the air screws today if it stops raining, because the figure I had in mind for the air screws was 1 3/4 turns out. I don't know where I got that figure from though. I was thinking only yesterday, at the traffic lights, tickover at 2000, this is why the japs fitted 2 throttle cables; to make the revs die down a bit. This variable tickover may or may not be related to the rich mixture, we shall see.