the manual on the 400ex says its jetted lean from factory. i want to fix this problem but not sure where to start. the stock main jet is a keihin 148. i put in a keihin 150 mainjet but this did not seem to do anything and possibly a loss of power but i can't tell. my bike is completely stock. should i try to modify my air box and my stock exhaust. i know someone tried to modify their stock exhaust but i don't know the link it is posted on. maybe i need to leave the stock mainjet in and just adjust the air/fuel skrew on the carb. i just want to get the little "extra" out of it that i should be getting. thanks for the help

 

get k+n air filter and dynojet kit with a good pipe.Jet kit will tell you what main to use and what needle setting.

 

i was told a few people modified their stock exhaust on the 400ex. i think they drilled holes behind the spark arrestor. did anyone see any gains from doing this?

 

<< get k+n air filter and dynojet kit with a good pipe.Jet kit will tell you what main to use and what needle setting. >>



Not a good idea, jet kits just suggest what it should be, chances are you are not at the same exact elevation and you may have different things done to your EX to cause a different jet to be needed.


Just changing your main jet won't total help either, your main jet only controls from 3/4 throttle to full. Your Needle jet Controls from 1/4 to 3/4. Just adjusting your fuel mixture screw is totally useless too, that only controls from nothing to 1/8 throttle. and your pilot jet controls from 1/8 to 1/4.

On a stock 400ex you should bump up to like a 42 pilot jet to help it start better. Also if it's running lean go up maybe 1 size main jet and adjust your needle clip position. Moving the clip up will lean your motor out more..moving it don will do just the opposite

 

i now have a modified air box lid with 3 one inch holes drilled in it. i also have the 150 main jet in. right now i am in the process of trying a 40 pilot jet. i won't have the pilot until later this week though. any other suggestions would be great!!