Just bought a 2003 400ex last month and I love it, but I'm ready to do some modifications to it. I want to keep the reliability of it.so what are some cheap mods that will give me more power? without having to use a big bore kit. Thanks a lot in advance

 

New exhaust and rejet, possibly better air filter. Re gearing it may help move power where you need it too.

 

What exhaust do u recommend? And what air filter? I want top of the line products plz. Thank u very much for all ur knowledge.

 

Uni for the filter. Just about any exhaust. I like hmf, but I have never had a 400ex either.

 

The Honda forums suggest:
1) Uni filter
2) FULL HMF exhaust
3) Air box mod - (I used to drill 1" holes) - just don't go river wading :P

 

I owned a 2001 400ex I could run with the 450r go get a 15 tooth front sprocket and can't remember the rear it was the smallest size eBay has. A hmf comp series exhaust uni air filter and to really wake the Beast up go with the stage 2 hot cam no engine work needed other than stage 2 jet kit you will see a huge difference oh and stage 2 Rev box. Hope this helps you

 

For the air box mod I cut out a section in the center about 3"x3" works great and use a good air filter oil on the uni

 

You could run an aftermarket airbox lid. The gearing changes would help you run with a 450 on top end, but seems like it would really kill your bottom end, acceleration, and trail riding ability.

 

Full hmf system and Rev box are a must if setting up a 400ex. My 01 ran like a beast when I first added these. I then added the 440 kit and alot of other mods but right off the bat I recommend those two things.

 

It started leaking oil today. Right where the head gasket should be, on the left side.. might be a warped head from getting to hot being a 416 and all. Or could it be something different.. the guy I bought it from said he just put all brand new parts in it.. guess I'll find out , because at the very least I will have to change the head gasket.. hell it might not even be a 416.. any advice on this situation? Thanks for all the help