I just bought a K&N Highflow air filter and a slip on exhaust for my 2000 Honda 400ex. Right now my carb has a Keihin 38 pilot jet and a Keihin 148 main jet, and I need new jets so it doesn't run lean. Any suggestions on what sizes I should run now for the jetting? Thanks for the help.

 

Old school way is to first try it with the stock jets. If it doesn't hesitate on take off,then at high speed kill the machine and check the color of the plug. If whitish then you need to jump the main jet up a size or so. Plus you may need to richen the fuel air mixture,plus experiment with different slide needle positions if hesitates off bottom end. Not a cut and dry method on jetting, you have to experiment a little. Some may not even need rejetting and will run ok with stock jetting with just a little tweaking,but I would have a couple sizes over on the main jets handy,and a 40 on the pilot if needed. OPT

 

Dont run it with the stock jets and k&n. I did that and almost cooked the motor. It was so dam hot she was smoking...... They are jetted lean stock. A 40 or 42 pilot will help with cold starts. A 152 main will be close. Adjust the fuel/air screw 2.5 turns out from seated, Then play with it until is where you want it. When adjusting if you turn the fuel/air screw out more than 3 turns to get it to run good that means you need to up your main 1 size.

 

Thanks for the help, I'll go to the dealership tomorrow and see if they have the jets I need.

 

A good place to order them online is Jets R Us mikuni, keihin, hitachi, holley, jets and carb parts