I live in Maine and I went riding yesterday with my new toy 2001 400ex with weisco piston and full fmf exhaust the last owner said it would rip and get. Well after riding around my flat yard with the kids I was very impressed I bring it to trail and open it up, well try to atleast. The wheeler went all day but would not let me give full power it would bog down in every gear past 1/4 push on throttle. It started fine it idles great and everything works just wondering why it would bog down so much? My buddy had a 250ex and he was blowing by me it was a sad moment it won't even wheelie it bogs down as soon as I push the throttle. Any help would be great.

 

Did you open up or modify the airbox or filter in any way? Many folks think that removing restrictions on the air intake will improve performance, but it really does the opposite. If the air intake and filter are stock then you have a carb issue from what your saying. Was the carb jets changed after the engine mods were made? You have some reaserch to do!

 

The air box is not modified it only has the k&n filter and full fmf powerboat exhaust. I have not pulled the carb out yet but it seems like its a jet issue

 

With the K&N filter plus exhaust mods....you definetly need to rejet and recalibrate the carb. Sounds like to lean and it should be with the mods you describe.