Top-end rebuild - long term maintenance

My '99 WR has seen a lot of action the last 4 years in quite varied locations and terrain. I never had a major problem with it until this last year. I sucked some muddy water into the engine and had to do some repair work. In the process I had a new top end, new valves, and gaskets put in. I should have just done it myself, because it cost about $700 at the dealer. Other than that craziness, its been reliable as can be. I change the oil every 5-6 rides, bleed the brakes (They get mushy pretty often) and change normal wearable stuff. I've had the same bars (O'neil Aluminum) since the stock ones pretzeled the first ride. The Acerbis Rallypros have a lot to do with that I think. Had to replace a couple of front spokes, after a nasty header into an orroyo in AZ. Its been just reliable as can be.


This weekend I tore the top end down on my 98' WR400. It has run flawlessly in the years I have had it. I have noticed a little smoke at startup and low compression when the motor is cold, as if one of the valves is sticking. It ran great once warm. My bike has around 3,000-4,000 miles on it, mostly desert miles. It has had two chains and 6-7 new rear tires over it life. I have checked the valve clearance two time in the past (once to set the EX cam timing to YZ spec). They are still with-in factory specification, but slowly working there way tighter. The only other engine service I have done, is replace the water pump seals and shaft last year, it was weeping slightly. I try to change the oil every 2-3 rides, but I have let it go longer some times.

The piston, cylinder, cams, valve lifters, and valves were all well within factory specs. I bead blasted the piston crown, combustion chamber, and valves. The left and right intake valves had a good amount of carbon build up, the others looked fine. The wrist pin had a bit of scoring on it, it was a little bit alarming, so I replaced it. Lapped the valves, installed new OEM rings, and new wrist pin. Re-assembled the motor with new gaskets, torqued every thing to factory specifications. It starts/runs good, does not seem to have the ocational low compression during start-up.

I am curious to hear what major long term maintenance other owners have done to earlier, well used YZ/WR 400-426. I am very impressed with the reliability this bike has given me. Its almost XR/DR reliability with the performance of a moto-crosser. If it was YZ/CR/RM/KX 250 2-stroke, it would be on its 3rd top end by this time.

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