I agree w/ FMX. My riding buddy has a 01 stock YZ250F and he's on it ALL the time when we ride MX which is always. 2 weeks ago he melted the piston to the cylinder. Havent heard much other then that yet. Let us know whats goin on!
Gee, you start a new thread and then do some work on your bike and look what happens to the thread!
Well, the post mortem's complete. The prognosis: The crank has fractured, clean through the crankpin. The big end bearing assy was effectively holding the two halves of the crankpin in place. With its new two piece crank, this WR250F was consequently the easiest engine ever to split the cases on! It's possible that the crack may have started at the oil feed hole.
Surprisingly, not much other damage has occured. Of course, the crank is toast, and the rod has contacted the crankwheels as they closed up though BDC. Also, the flywheel, while no doubt "gyroscoping" with the support of only one main bearing as it spun down during the emergency shutdown, lightly kissed a couple of the magneto coil cores. I'll replace the main bearings just to be safe. Otherwise, there is no damage and a minimum of metallic schrapnel to clean up.
To my admittedly untrained eye, it looks like a component defect. This is 600 mile easy single track trail-ridden WR250F that has rarely if ever clipped the rev limiter.Every other component in the engine looked like it was brand new.
Although the WRs are sold new with no warranty here, I intend to have a serious fireside chat with my local Yamaha rep about this.
Good news: The local dealer and Yamaha itself has taken this issue extremely seriously and has offered to replace all affected parts free of charge. This type of break is according to them very unusual. Yamaha will be running analyses on the broken parts.
As Kiwimike points out, my bike is a Euro model. That means it comes stock with YZ tank/seat combo, the "grey wire mod" already done by the factory and a one piece all stainless muffler system. Cam timing is set to "WR timing" as with the US spec machines.
I "YZ timed" my bike and fitted the muffler off my stock YZ400F to it. Otherwise, the bike is stock. Interestingly, the YZ timing mod reduces the need to rev the daylights out of the bike - on many offroad tracks, a gear higher is possible with YZ timing over WR timing.
All four-stroke YZs and WRs, regardless of country spec, have rev limiters to the best of my knowledge.
As I wait for the new main bearings to arrive from Japan, I spoke yesterday to the local Yamaha Motor Company tech rep about this problem.
In our country (pop. 3.5 million), four WR250Fs have had crankpin fractures like mine. Extrapolated to US proportions, that suggests something in the order of 300 WRs failing in the US, if bike population and people population are proportional (which they probably aren't, but I digress . . .).
Interestingly, no crankpin failures have occurred on YZ250Fs in this country. I understand that this may be at variance to what is happening in the US and other countries, based on one or two reports of YZ250F crank failures on various message boards.
The four WRs have all failed relatively early in their lives - my 600 mile failure is typical. This suggests that higher mileage bikes that have survived a good thrashing are probably not going to fail.
This WR-but-not-YZ crank failure phenomenon has got the technicians looking at things like crank harmonic vibration and the effect of the much heavier WR flywheel on crank reliability. Suffice to say that, according to the tech rep, the technical guys are still working on the problem.
Yamaha has accepted full responsibility and will cover WR crankpin fractures for as long as it takes, according to Yamaha. I have been impressed with their response to this problem so far.
I know of one YZ250F crank failure. Actually a rod failure, broken rod punch a hole in the crankcase. Yamaha did pay for the repair. Yamaha has been pretty good about covering some repairs if deemed premature failure. It also helps to have a good dealer and a service manager that has good relations with the Yamaha rep.