As always, many thanks for the replies and interest!
To be clear, the standard setup on the VHBZ carbs is a 4mm cut-away and I have changed it to 5mm. This makes it significantly leaner at idle and at small throttle openings, but makes no significant difference beyond about quarter throttle. I don’t know why I didn’t think to try this sooner, especially as I had the #50 slides in my box of bits anyway. I’m no expert, but I’m guessing this is due to modern fuel being more volatile and having a higher calorific content. Having said that, I can’t explain why some people have no problems with the standard setup! My general impression with the Dell Ortos is that they can be very fussy about precise jetting, but once sorted, they can perform very well indeed and I think I have finally arrived at that point! I have to say that I’m dead pleased with the fuelling and the plugs are now a sensible colour instead of black and sooty.
My next project on this bike is to do something about the over-complex and leaky breather arrangement. The soft plastic parts are perished and cracked, I did some temporary repairs by sticking on bits of old inner tube with rubber solution, but it’s leaking again and I need to decide whether to buy a new airbox assembly or take a different route. I’ll be sure to report back here! In the meantime it’s running really well and I suppose the leaky breather arrangement is keeping it from going rusty!
Joe.