Yz 426 Jumped a tooth?

Oh no, old blue!!!

Attempted to go riding today. Started the bike up to warm it. Rode about 100 feet in first gear and.....thunk! At about 1/4 throttle the engine abruptly stopped and the kickstarter was seezed. Took it home and looked under the valve cover. It appears the timing chain may have skipped teeth? Well, whatever the cause was it's most definitely not in time anymore.

The motor spins normally by hand through about 70% of its full set of strokes but hits a very very defined dead spot in which the motor rotation completely stops. I have been extremely careful not to force anything.

Well hell....now what? Obviously a tear down is immanent, but what gives? This bike has been running stellar lately. Timing chain only has about 30 hours on it.

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Took the head off. Looks like the center Intake cam journal has been rubbed down pretty hard by the cam. It may be hard to tell from the pictures, but being that the corners have been squared I can't even remove the two of the three shim buckets with a magnet.

After I pulled the head off I was able to get a visual on the piston, cylinder, and valve faces. They all look fantastic. The motor spun nicely with the head off.

I'm left confused...I recently had the cams off to change a shim. The journals then looked great. Maybe the cam caps weren't tightened evenly? (I'm usually very careful about that). Should I be concerned about the bottom end of the motor? Can I file the corners down or is this something a bike or machine shop can fix? A new head for this bike would be hard for me to stomach

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anybody? Still not sure as to what caused this

It appears you suffered an intake cam seizure.  This can be caused by a variety of things, but if the bike is reasonably well maintained otherwise, it's normally due to foreign material that managed to escape the oil filter, a lack of oil, or, more commonly, incorrect assembly of the cam caps. The head is repairable, BTW.  Contact Engine Dynamics.

 

The caps are very critical about torque and tightening, especially the older engines.  They should be completely seated by hand, never pulled down into place using the bolts, and tightened in at least three sequential steps in the sequence or pattern called out in the manual.  Additionally, I always reduce the tightening torque from the specified 87 in/lb (7.2 ft/lb) to 75 in/lb (6.25 ft/lb) on all of the heads I work on.  It's frankly tight enough, what with the oiled threads that are called for.

Thanks for the response. I don't have a picture handy, but what about reusing the same intake cam? It doesn't visually appear to have any damage whatsoever. Also, any reason to split the cases ?

Send the cam along and EDCo will polish it.

No reason to split.

Awesome. I didn't notice until I called, but they are located in Petaluma CA which is 20 mins from my front door (score!). Gentlemen on the phone was very helpful and is optimistic that it can be repaired for less than 200 dollars. Hopefully crisis averted.

I'm really hoping something like this won't happen again, although the damage could have been much worse. I'd hate to put it back together just to have the same thing occur.

So, then, before you put it back together, you might want to pull the right side cover and clutch off to inspect the oil pumps, replace the oil feed seal to the crank, stuff like that.

 

It was probably incorrect cap torque that did it, but you may as well look.

Guys down at engine dynamics were very helpful. Should be back to normal on Tuesday. Thanks Gray

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