I have a White Brother's E-Series full system exhaust, and a WB's rev box on my '99 400EX. I was thinking about putting a K&N air filter on it as well. Assuming that I put the air filter on, what jets should I run? I am running stock jetting currently and to be honest nothing is wrong with my setup. I just wanted to extract that little extra bit of power from my engine. My elevation is 740ft here in southwest MI. Does anyone know where I could find a chart or something that shows which jets to use at given elevations? For you riders that have similar setups...what do you run? Does anyone know the stock horsepower of an EX and the horsepower with the mods I have? Please let me know and thanks for your help.

 

Sorry I can't help you with the jetting but I can tell you that the stock hp for a 400ex is somewhere around 26 or 27

 

I have two 400ex's with the "E" series a K&N running no airbox lid and a 175 main jet. This works very well for me at sea level. Plugs look great after 50 hours.