I had similar problems with my Lifan 250. The bike is very good and runs very well in warm weather. After the first service I had problems with running in the cold (less than 5C). Because this is rare here it was difficult to replicate with dealer. The plugs would foul after running 12-15K and the bike run very poorly at low speed in these conditions. Fast running was fine. I have adjusted the pilot mixture so the screw is only open about 3/4 to 1 turn from closed. I haven't had any problems since but it also hasn't been cold enough to really test it. In every other respect the bike is great and cruises well at 100Kph with good acceleration. I did also find low grade fuel was no good very quickly. I run on 95 Octane. Good luck.