06 yz450f with possible engine seize

3 replies to this topic
  • jasonoldman

Posted August 25, 2012 - 10:16 AM


I was riding my yz450 a couple days ago in the sand. It was soft sand dunes and the bike was running warm (not enough to boil out my antifreeze though). Anyways wide open in 3rd gear bike sputters hard then locks up. I am a little concerned that it locked up at such a speed thinking maybe seized. At first kick start is hard and rocking in gear seems to confirm a locked motor. A little more fiddling and i frees but does not feel real smooth through the stroke.

Well I let it cool a minute and it fires up but runs rough and bam stops dead. Well we are in the middle of no where so I let it cool for about half hour push it over a sand hill (real fun even with two of us) but being that it was pretty much flat and downhill I decided to ride it out if it would go. Fired up running a little rough got about a quarter mile bogs and dies. Fire up again had to feather a higher rpm start and cruised back in 3rd gear about half throttle. Felt like it had some power but I did not push it so I dont really know. Ran a little better at the end but kick over still felt a little rough and bike was noisy and clanky sounding.

So I started tearing down today thinking maybe a partial seize of a ring or crank bearing. Exhaust valves were both tight (.13 and .15) intakes were all in spec. There was no galling in the cam journals. Removed the head dont see any galling of the cylinder wall. Turning over with head removed by wrench on flywheel or by kicstart is smooth and easy. So now I am kind of at a loss. Could it still be a crank bearing issue. Possible somthing broke in the clutch of tranny area. Any advice on specifics to look for would be appreciated thanks.

My next step will be draining the oil checking for chunks of metal.

  • jasonoldman

Posted August 25, 2012 - 12:20 PM


Ok well I did not see anymore than normal wear on clutch. It seems as if there is a little play in the crank bearings. From the magneto side I can move the crank from side to side (in line with rotation) and also just a little up and down (just enough to see)
Would you say my crank bearings are shot.

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  • etuke

Posted August 26, 2012 - 07:19 AM


possible the valves were so close that when it was hot you were losing compression and cooled down it ran again,don't think a very small amount of play in the crank mains would cause that problem.Only guessing here,was there any knocking sounds?D you check the timing?

  • KJ790

Posted August 26, 2012 - 07:29 AM


Sounds like it could also be a bad big end bearing on the crank. I had this happen on a 250F, it would seize up, then it would kick over fine and start up once it cooled down. It would run ok for about 1/2 mile, then it would seize up again. It did this 3 times to be before locking for good.

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