Questions On Yamaha WR400 Modifications
I have probably 4,000 miles on my 99' and it is still as sharp and crisp as the day I bought it new. 
It is a Canadian import so it was not necessary to remove the throttle stop or grey wire and I have never considered adjusting the cam timing to YZ specs, it hits plenty hard as is. I ride alot of tight and technical so I changed out the countershaft sproket from a 14T to a 13T and it works perfect. I am running a White Bros. E-series exhaust with stock jetting and have never complained about performance however, I feel the exhaust is a little loud. The steel valves in the 400's are far less maintenance intensive, I have never had to adjust them or have any top end work done.
I changed the oil every third ride and the filter every third oil change. You can buy the stainless steel filter that you can clean and re-use for about $70.00 US.
I actually liked my 400 more than my 450 as far as ease of maintenance and certain performance aspects. I still own the 400 and will actually race it from time to time at the closed course events where it's tighter and more MX oriented.
Good Luck 
You searched and could not find anything? There are many posts on the various must do's. try searching under "free mods". I love my 400 and also would not consider changing it to YZ timing. Having ridden a YZ426, the hit can cause more problems than it is worth in tight and loose single track. This bike has abundant power as is. Maintenance is also "relatively" minimal, absent the religious oil changes, air filter cleaning and much less frequent valve check. A bit on the heaby side, but still the best bike I have owned.
ive got a e-series on mine, sounds tough as nuts, but is loud.
i pulled my head off yesterday and found the valves to be fine, there was slight carbon build up but they were not worn at all
change the oil heaps, and keep the filter clean. they're good bikes
Ok thnaks for the replies! I was under the impression they were titainium conponants in the valve train.
Honda, there are titanium parts in the WR vavle trains, just not in the 400s. The 400s got stainless and last much longer from what I hear. Titanum came available starting in the 426s i believe.
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Ok, I did some searching but found nothing
, I need to find out, the modifcation that people do to the cam timing, which exhaust people are using with best results, I hear the YZ400 exhaust is one of the best? Also, about hours of these bikes, how many hours use are you looking at before a top end? What kind of service interval times? Oil and oil filters after how many rides?
Many thanks these questions are for a friend looking to buy a used WR400 model, anything to note to look for as a weakness or defect in these models?
Kind Regards
Honda0001
(Soon to be a 2006 Yamaha YZ250-F rider
)