[Darren69]

I suspect pilot jets and or pilot galleries have muck in them causing the bad low speed running, and wonder about the float heights or needle valves causing the sooting up of the plugs? The carbs or not overflowing, it is not smoking from the exhaust, but the ends of the silencers is quite sooty, as you would expect I suppose.

That would be my guess also. Has it just started playing up when it was ok before and unchanged? You say its got flat slide carbs not cv ones? Then it has been upgraded, it's a good upgrade over the standard Mikuni CV's