I am running a FMF pipe and silencer on my Blaster. I am also running it with a Twin Air air filter. In the summer time I change the main jet to a 260 and put the needle on the 4th notch. It was jetted almost perfect. Now, it's getting cooler when I ride, about 40-50 degrees. The guy at Yamaha said to put a 280 in it. I put the 280 in it and check the jetting and it seems like its running way lean from the plug chop. Why would it be running lean? I think it would be running on the rich side. Do you have to rejet for cold weather?

 

Yes.......you do need to rejet for cold weather if you want it to run Perfect......
and you would need to jet up a size or two......depending on the humidity and the temp change......