I have a 2000 Honda 400ex. It seems to be bogging on acceleration and backfire's on the deceleration. I have a Yoshimura RS-3 Comp Full Exhaust on the bike with a K&N Filter with a modified air box. The bike still has the stock jets in it. Is the jetting my problem? If so, what size jet should I put in it? Do I change the main jet, pilot jet and adjust the needle? Please let me know what you guys think.
Thanks in advance.

 

You need a bigger main jet. I had a Yoshi slip on, K&N, and few vented holes in my airbox, and a 165 main jet put me spot on.

 

Thanks QuadFather. You think a 165 is too big? I have been reading these posts and it seems as if I should go with a 155. Do you know what size the stock main jet is?
Thanks for the info.

 

If I remember correctly, the stock jet is a 148. I could be wrong, though. You might want to get a couple different sizes, and see which one your bike likes the best. The same exact quads won't always run the same with the same jets in them.

 

I'll give it a shot, thanks!!!!!!!!!!

 

i would try around a keihin 158-160 mainjet, 40 or 42 pilot jet, and stock needle clip position in the warm months and move the clip down one position in the colder months.

 

cals400ex recommendations seem to be about right and im would give them a try.Generally speaking if you are not having starting or midrange problems i would leave the pilot and needle jet alone and change the mains and see what happens.