does it misfire on idle and under load.are the plugs correct type, is the plug gap set right. have you got a good spark on all plugs.check all pipe work on fuel system for air leaks,blockages or perishing,even very small nicks can cause big probs.look for any air leaks around the inlet manifold,throttle body etc.may be able to hear air leak under bonnet.
xs11, February 2010