I need some help on jetting. I have the following mods, white rev bos, k&n filter open box and DG exhaust. I am presently running a 152 main but the bike doesn't seem to have power under load. Runs great on flats but running sand it seems under powered. I know the 400 is a better trail bike than sand but it seemed to run better stock.
Thanks

PS
DG exhaust is crap.

 

what size pilot jet are you running in it and what is the needle set on...It also seems like it is running lean because of the jet size, try putting the airbox lid back on and see if that makes a difference and after riding it for a while it shouldn't be white, the plug should be a tan color...I think you are running lean and will need to get bigger jets...because on a 2004(not sure what yr your bike is) it says that the main is about a 148 and slow jet is a 38...hopefully some1 will chime in who has a 400ex and could tell you close to the jets you will need to run, if not buy some bigger jets and see which ones make the bike run better.

 

My 2004 400ex has a K&N air filter with the lid removed and a FMF Powercore exhaust. I changed the main jet to a 170 and left everything else alone. The quad runs great. One way to check your jetting is to get the quad to operating temperature and while ridding it fairly slow, shift it into second gear. Then just roll on the throttle until you are wide open. If the quad doesn't stumble or blow smoke, then your jetting is pretty good. I hope this helps you out !!!

 

thanks, I'll try the 170 jet and see what happens.