Vibration problems on '00 yz426f

The vibrations on my bike are pretty bad. As I warm it up on the kick stand and rev the engine, I can see the bike shake. I tried to find the source, but had no luck. The only thing I did find was that the couter balancer on the flywheel sides rotates freely. I took the engine cover off on the clutch side and found all the gears were connecting but balancer shaft rotated freely. The gear on the balancer shaft was connected to another gear behind the oil filter. I was able to turn both gears by hand. I thought there should be no movement unless the crank is turned.


puzzled rider

The woodruf key is sheared. This is a, somewhat, comon problem on the early yamaha 4-strokes. Fix it before you run it anymore.

The parts of the key should still be in the keyways

Well, you found your problem. The key that holds the balancer drive gear to the crank has sheared, and the nut that holds that and the primary gear in place is loose. This condition should have been preceded by a rattling noise you weren't familiar with.

Remove the nut and the gear, inspect the crank and gear for damage resulting from it spinning free, and replace the key and whatever else you need to. Note that the balancer must be timed to the crank when reassembling. There are marks on both gears.

Thanks for the replies. Would there be a permanent fix to the woodruf key sheering off, or should I just replace it ever so often to keep it from sheering?

Keeping the bolt tight is the best prevention. To that end, I'd use red LocTite on it. Some have said that they have found that the stock key, which is a square key, rather than a Woodruff type, did not fit as tightly in its groove as they felt it should. Their solution was to take a piece of square key stock from a hardware store and file it to fit better. That seems reasonable to me, and if I had this problem, that's probably what I would do as well.

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