any mods done to it if so it needs to be jetted. where on the throttle is it sputtering. idle to 1/8 is pilot, air screw will tell you what size you need. 1/8 to 3/4 is needle, 3/4 to wot is main jet.
hit http://www.yfzcentral.com/ up they can help you a lot more. and maybe tell you what size jets to use.