Hi all I know this is a late post to this but I had the same prob with my Jinlun 125-12, turned out it was the rubber pipes going to the sucktion valve on the fuel line, they had perrished and cracked causing a lack of fuel, I replaced pipes and repositioned them to hold them away from the engine block, (heat speeds up the perishing), and all is fine. Hope this helps.
SilverD
May 2011