optimal warm up time

[JohnL]

yeah mate black smoke sounds like a rotor for around 5min then after that runs perfect again>?

"Rotor"?

Automatic chokes tend to be less than reliable and generally not a good thing (even when they're working as intended they tend to take too long to lean out the mixture, at least in my experience). Choked carbs tend to liberally wash down the bore walls with petrol (more so than EFI on cold cycle), which washes the oil film away (causing ring / piston / bore wear) as well as diluting the oil with fuel.

You might want to look into converting the choke to manual operation? It might still(?) be possible to buy cable kits to do this from auto accesories stores.