you say on tick over its ok but does it rev up under no load. if not sounds like a fuel problem, maybe mixture or blockage. remove carb and strip it down and check for dirt or blocked jets. there will be info on internet showing removal process. be careful as jets are made of brass which is a soft metal. if you remove any mixture screws etc count the turns it takes to remove them so you can refit the same number.

xs11
September 2015