Try using spark plugs that do not have a built in resistor in them. I had these after mechanic rebuilt my engine and it bloody terrible trying to start it, USED to almost pull my hair out. Try using NGK SPARK PLUGS: BP 7ES, type,they work real well. Mine also had fresh fuel, spark, but because the leads were also resistor leads they when combined with the other type resistor plugs were causing to much resistance and seemed to be suppressing spark when they were screwed back into cylinder head. Share this tip about using non resistor plugs with rest of the XS 850 world will you.