My Virago 250 was running very unevenly, losing power, surging etc. I took out the spark plugs and saw the electrodes were covered in soot. I thoroughly cleaned the plug tips with a wire brush, ran the bike round the block and saw soot build up again. I recleaned the plugs and adjusted the fuel mix screw on the carburettor – which needed to be screwed in a fair amount. I adjusted the screw by screwing in until the bike was just about to cut out on idle then backed off the adjustment screw about ¼ turn. I have run the bike for a week and it was running uneven today. There is some carbon build-up on the plugs, but not as bad as before. I have put new plugs in as they were cheap and will adjust the fuel-air mix again. What do I do if the carbon builds up but the bike is about to cut out when I set the mix to as lean as possible? Could there be some other cause for the carbon buildup on the plugs?