Bandit 600 wont start without choke and backfires when started, any ideas?

Just bought bandit 600 from someone who had it left for 1 year and now i have put fresh petrol in and it wont start without choke even when hot, occasionaly backfires and will not do more than 70mph, all sparks work and have cleaned out all main jets but one spark plug seems a lot blacker than all rest and sounds like its only running on 3 cylinders. throttle is not responsive, either on or off, contantly stalls at junctions, surely if it the idle jet blocked then it would still run well when you open it up but this is not the case, please help as this is my first big bike and not going well so far.