I am the pround owner of 2 400 EX's and after 6 monthes of riding decided it was time to tune them up a little, hoping to maybe give it a little crisper throtle response. I bought a White Brothers E series slip-on, K & N filter with a prefilter and will be using the stock air box with lid. My question is what would be a good starting point on jetting. I have heard recommendations of 150 to 165 for the main jet, I think my stock main jet is a 146, 3rd groove on the needle jet and 2 1/2 to 3 1/2 turns on the fuel air mixture. Does this make sense to anyone. Thanks for any possable input.

 

keihin 152 mainjet, 3 turns out on fuel skrew, and stock needle clip position (3rd groove)

the stock mainjet is a keihin 148