I have had my 2001 twin spark for a month or two now. I bought an f650 because of their excellent reputation for fuel economy, i am doing a lot of running around at the moment and wanted something mega cheap. I knew the single spark would be less efficient than the twin spark but i'm only getting between 56mpg and 63mpg gentle riding. not as amazing as i was hoping. anyway what i'm wondering is how easy it would be to convert to twin spark and what would be involved? new head or machining the old one, would it need remapping, etc. etc. ?