how can i fix my sons suzuki lt 80 quad?

hi my son has a lt80 quad it has been ok up till now.It starts and runs fine but has seemed to of lost power and seems to be worse when cold.Its like its running on choke but dont think it is as when pull choke on the engine wants to fade out like it should.I have tried carb,reeds etc.I have also put pot and piston when bought bike and has been wicked till now im beat any ideas would be great thank you