if the plug is sooty, and youve not got a restrictive exhaust or clogged air filter, then id guess that the main jet was too big rather than the air screw or needle position being wrong.
having said that, i cant see there being a problem going for a slide carb, but just dont go too big. remember that the cv carb will be a few mm bigger than the equivalent optimum slide carb. so if its currently running a 34mm cv then the slide carb you need would probably be 28-30mm. i bought a chinese mikuni copy for my bsa, it transformed the way it runs, so theres no probleem with the quality of them. the originals are probably made in china now anyway!