Carb issues, but not Norton sorry!

Mikuni 34mm round slide on a 500 velo,
From a quick rev the engine will not return quickly to a tick over, the engine as a very slow return to idle... over richen the pilot improves it 100% ,but is much to rich..black smoke and heavy tick over.
weaken off the pilot and it's back to the lazy return to idle after a blip.
Any one had this with mikuni on there Nortons?
This happens when the engine is hot and choke off. nothing on the internet to suggest a cure..

Tried lifting the needle a notch - or winding it up a thead or 6. ?
Leaness may not entirely be in the pilot stage, midrange is important too...

The title here is "other", so a Velo is probably included in that !

We need a bit more information. Was it running OK beforehand or has the carb just been fitted? Does it have generic jetting or are the jets and float height specified for the Velo?

Are you absolutely certain you don't have an air leak?

Is the slide returning fully against the throttle stop after you crack open the throttle? See if you can push the slide down with your finger, through the bellmouth, and note if the idle slows. If so, it is not returning fully against the stop and could be that the cable doesn't have enough slack.

Could you be adjusting the mixture screw while the throttle stop screw is turned in too far? Having you tried backing off the throttle stop a little and then adjusting the mixture screw?