ok i have an 08 400ex, bought er brand new a couple of weeks ago. i thought itd be a bright idea to get a pipe since the stock one just sounds terrible. i bought a two brothers racing pipe. its come with a 172 main jet, got it all installed, took the air box lid off ( the instructions with the pipe recommended). it runs fine and idles fine, but when i rev up the motor there is white smoke blowing out of the exhaust. we did have a lil trouble getting the carb back together correctly but now it is back together the right way, runs fine and revs fine. is a lil hesitant from an idle to full throttle all at once, but other than that it runs just fine, cept it just smokes when you let off the throttle after revving, any help at all would be appreciative.

 

If the pipe is still brand new, you will get a little smoke and possibly a smell for a bit until the "new" burns off. If it idles and revs fine, and doesn't backfire on decel, you should be good to go.