This could be down to a few things, I had the same problems with the 1 I got that had been stood up for 8 months, I started with the carbs! strip them down get them cleaned out and reblanced, crappy old fuel can be the cuse, and maybe check the exup valve is free and the motor that runs it is working ok! if it´s not any of them it could be time for main valves tappits and shims to be looked at, a engine serivce really, pends on miles it´s got on it at lest 30 to 32k mark I´d book it in, your find it will make the world of diff, to the power and running of the bike! good luck hope that helps Drew