Why has my zip 172 got lazy?? what can i do to get it back to what it was like?

I have a zip 172 and when it was set up it was mint good throttle responce, but now after like over a year it has got lazy the throttle responce is shocking and doesnt rev as high. I want to try and improve it for scooter shootout in york this friday 15th and 16th Sep, too late to book it for a setup. Is there any other things it could be like belt?? square rollers??

 

have you changed the piston rings since owning it? have you serviced the carb ie stripped it down cleaned it and put it back together?

 

as said, new rings, clean air filter, gap plug, check carb settings and rollers for square. Make sure brakes arent binding

 

Easiest thing to check is the rollers. When did you last switch them?