I have had a 2000 400ex for about 6 months.Im tearing it apart for a winter project and rebuilding most of it.as far as the motor goes its strong but smoke a little and dumps a lot of oil through the exhaust but has not lost a bit of power,thinking its the valves.but ill find out when it gets torn down. my first question is that the stock 400ex a arms have the collars at each side and a metal sleve inside a rubber bushing.all the kits i see have the collars in delrin or another material with a SS sleve,does this require the old rubber to be cut out? or does it go in over top like stock ones. juster wondering because mine are so far gone that the rubber is junk inside.also i noticed my swing arm bushing were just 2 sleves showed together and aftermarket ones have needle bearings.

kinda new to getting this in depth with fixing them,usually i would just go buy a new one but with the price of them now,ill stick with fixing it.been the best and most reliable bike i have ever owned.liked it so much i bought the woman an 02 400ex and its like brand new,cant complain at all.

 

I have been looking at the same kits. As I understand it the kits come with bearings, seals, shafts, and bushings. You should be able to remove all of the parts if you get the full kit.
It does sound like you have issues with the valves. You should take a close look at the seals, guides as well as mic. out the valves and lap them.

 

some kits have the delrin bushin with a metal collar to replace the stock bearing set ups.a lot more durable than stock imo.youll need to remove the bearing retainer on each side of the bearing so you can tap them out.repeat this process 8 times to do both side upper and lower...lol