Ok that makes sense It sounds as though you hit the sweet spot that your bike likes. Ride and enjoy. I'm glad you reached a conclusion.The more tank fulls you run through it the better it is likely to get. Cheers.🍻Coachconner - I think you misunderstood my mixture settings I quoted. They are only 1/4 turn difference between them. I was quoting the range which seems best on each side.
Yesterday I tried more combinations of mixture settings and choke as well as trying a new coil (just to eliminate it from the list), and it made no difference. So old one back in.
Also installed new air filter (K & N high flow verses the Uni-filter high flow old one) and that made no difference either. I have left the new filter in place.
I tried increasing the idle, but that made no difference.
End result was that with the mixture screws at 3 1/4 and 3 with full choke or 1/2 choke it starts, runs and keeps running and the engine "sounds stronger" with this setting.
No black smoke; only steam and invisible gases!
Starting technique seems to be once it fires and starts to run, I open the throttle just a little to make sure it keeps running, and slowly reduce the choke to maybe 1/4 and it will idle at 900rpm whilst warming up.
I warmed the engine right up and re-balanced the carbs, which weren't very far out and also adjusted the idle.
I then rode the bike around in a small circle outside the garage and it pulled very strongly from standstill with no hesitation. Still no black smoke.