2001 yz 426 engine binding up

Guys im posting this for my buddy he picked this bike up last night with this issue and were unsure what to check first.

 

What would cause my 2001 yz 426f to feel like its catching when i kick it over? It kicks fine several times.. Then.. Bam it stops.. I have to put it in gear and roll it back and fourth to free it up? Thanks!

If it is actually locking (and not just that he can't believe it has that much compression), it could be the rod bearing.  The cage can wear or fail so that the rollers can get skewed on the crank pin and jam.  If that's what it comes down to, you can verify it with the piston off the rod. 

 

Another place to look is under the right side crankcase cover.  All tolled, there's 9 gears in there, and anything floating around loose, or a gear with damaged teeth, could cause that.

+1 on the Rod bearing. Exact same issue on my old KTM 250.....

Yeah its definetly binding we have both had plenty of big bikes with loads of compression. Just so i understand are you talking about wrist pin bearing or lower crank bearing? And thank you for the help we both really appreciate it

The rod bearing is the big end bearing at the crank pin.  The bike doesn't have a wrist pin bearing.

Oh it doesnt? Just a press type fit similar to a car engine?

No, it floats.  Just no bearing.

Oh ok thanks for the info these new thumpers are far ahead of the ones i grew up on.

sounds very similar to what mine is doing... going to start tearing it down tonight

 Thank you very much for the help ...you guys were right on with your diagnosis of my bike! the big end bearing is seized,,, we (charvel5150) and myself have decided to go ahead and rebuild the bottom end. Now the top end is in question! I have a few pics with this post to show the cylinder and piston..The cylinder looks worse than it is..theres 2 single (gouges)if you will, where your fingernail catches..the rest is kind of rough but maybe usable?Now the piston on the other hand is rough..on the exhaust side (fingernail catches all the way across) so my question is will a  new piston be good enough ? if we clean up the bore? can we hone a nikasil plated bore?

 

Thanks in advance..Great site!

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You can hone the bore with a "brush", or "spring/ball" type hone such as those from Brush Research: http://www.thumpertalk.com/topic/399309-new-piston-06yz450f-advice-needed/#entry3565759

 

After you have run the hone for 30 seconds total actual hone time, a usable cylinder should be clean, and not show any significant scoring, nor any "shadows" near the top or bottom of the stroke ring the rings digging as they twist while the piston reverses.  IF it cleans up, then have it measured for wear.  The limit is .002" (.05mm).

 

The piston is just plain worn out.

Thank you! and nice write up for the honing procedure ....We will try this out !

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