I have a 2003 400ex with no air box lid white brothers e series exhaust cdi box and stock carb with FMF jet kit I forget what size jet I put in it but I rode my bike today and when I would get on it in the bottom end it would break up however in the mid to top end it was perfect. Any input or tips would be awesome!

 

Hows your air/fuel mix screw. Its needs to be about 2 1/2 turns out.

 

I believe thats where I have it set unless it moved

 

screw it all the way in. Then start riding it. Turn it out 1/4 turns at a time till it runs better. if it never runs better put it at factory position and clean carb very well.

 

Jetting goes by throttle position, and not rpm. When it stumbles...whats the throttle position? But most likly its lean on the idlejet/mixture screw.

 

Its when I goose it so it would be full throttle