I don't know about a concentric but I fitted a Mikuni TM flat slide carb to a 400cc B31.
Starting was easy, I could kick start it wearing thin soled trainers. A proper starter system instead of a finger soaked in petrol, It idled perfectly, the throttle response was instantaneous with no fluffing or stumbling and the carburation was clean at WOT (86MPH in top gear)
When Amal make a carb as good as a Mikuni I will fit one