RE: 2008 YZ250f Center intake valve - broken lifter bucket.
I have seen all kinds of valve problems with the yz250f. I have quite a bit of experience with this model including rebuilding heads. I have just seen a new-to-me problem and wondered if anyone else has seen this before. What has happened in this particular case is that the center intake valve lifter bucket has cracked across its face and broken completely away from the skirt of the bucket. Only the valve lifter bucket was broken. Also, the lifter bore was damaged slightly as a result of the bucket being forced down by the camshaft at an angle when the bucket broke.
To get a better mental picture of what happened, when I first removed the camshaft on the intake side, I noticed a straight crack completely across the top of the lifter bucket looking like a 1/3 slice had been broken off the bucket face. Not down the center but a pretty straight break dividing off about 1/3 of the bucket face. The smaller piece also appeared to be tight up against the crack and appeared to drop slightly below the flush line of the bucket face as you proceeded toward the bore wall.
Looking at the other side of the head, the center valve was stuck open and it appeared that there was no damage whatsoever to the valve or piston. Note: The motor was idling when it just died. It could not be re-started. It was then brought to me and I started my diagnosis.
The bucket could be pushed down and the valve would also go down (or open further) but neither could be returned, i.e. pushed back to the starting point. With some pretty good (but not so heavy as to bend the valve stem) tapping on the valve from the cylinder side, the two pieces of the lifter bucket face sprang out of the bore and the rest of the valve assembly stayed intact. Here's the interesting part, just the head of the lifter bucket broke off, leaving the entire skirt of the bucket in the bore stuck below the slight gouge made in the bore wall. So, when I tapped it out, the broken bucket head pieces sprang out and left the rest of the bucket in the bore. Another interesting note is that the valve, spring seat, spring, spring retainer, and keepers were all intact and appeared to be in good working order. When the broken bucket head came out, the valve popped shut and I was able to push it open and it would return closed, under spring pressure, appearing to operate normally. Only the bucket was broken with some gouging of the bore wall from the off-kilter piece of the bucket head.
Now that you have a pretty good idea of what I found, the question is, has anyone seen anything like this before? If so, do you have any idea what could have caused this? My theory is that the lifter bucket itself was defective or fatigued and slowly cracked and broke under normal operating conditions since the last valve adjustment last fall. The motor has has approximately 15 hours on it since that adjustment.
I am only searching for more input to help me either confirm my theory or to help me become aware of other ideas of what may have gone wrong so that I may fully understand this problem if I see it in the future. No one at the local Yamaha shop or anyone else I've talked to locally has seen this before. It may be as simple as my theory suggests. I just want to make sure.
Thank you in advance for any constructive input you may give.