I just purchased a FMF Powercore 4 slip on for my 400ex. I havent installed it yet because I dont have any other jets to put in. The bike is stock and I rideclose to or at sea level. FMF recommends a 150-155 main jet. What size jet do I need to use and will it hurt the bike running the pipe before the jet is changed?

 

i would nt ride with out jetting it.....i used the 150 with fullpipe and airfilter..also went with a 42 pipe jet..didnt change the neelde jet

 

lid off prob go with 155. DO NOT RIDE UNTIL JETTING IS CHANGED. It will run WAY to hot.

 

do you really get anything out of taking the lid off..........not being an a$$ just asking

 

O ya you do. Try it yourself, you can feel the difference. Especially after the exhaust.

 

Thanks for the info, I ordered the FMF kit.. Is just the main jet all that I need to do or should I also do the needle jet also?

 

The needle should be fine. On the fuel screw you'll need to screw it all the way in and back it out 3 full turns. You can try it without doing it but, you'll probably get backfire or a lot of popping on deceleration.

 

Thanks for the advice, I put the 155 Main jet in and tried it over the weekend. What a difference in the bike. I didnt adjust the air/fuel but got no backfire or popping on deceleration. I left the airbox lid on but picked up some of those UNI vents for the lid..I dont like the idea of no lid in the dusty areas I ride.

 

Great! Glad to help. Have fun with her.