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amooney301

YZ426 not starting after top end rebuild - any help?

9 posts in this topic

cam alignment.jpgflywheel mark.jpgI've got a 2001 YZ426f in which I just performed a top-end rebuild. Timing appears to be correct, I have spark and I've got fuel to the carb.

 

Any ideas???

 

 

 

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I'm not totally sure. It came off of another 2001 YZ426. Is there a way I can tell the difference? How will this affect timing?

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I'm not totally sure. It came off of another 2001 YZ426. Is there a way I can tell the difference? How will this affect timing?

Cam lobs are different.. the YZ450 keep both valves closed longer ...thefore more compression and that way the timing is affected.

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This was the same exhaust cam that was in the bike several weeks ago when the bike was running fine right before I tore it down to rebuild.

 

Any specs that I should be aware of for each of the cams? Are there certain measurements in which I can identify which cam I have?

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This was the same exhaust cam that was in the bike several weeks ago when the bike was running fine right before I tore it down to rebuild.

 

 

 

If you are sure of that fact, then it's likely that the cam was made for a 426.  OEM Yamaha 450 cams can be used, but the original timing marks don't work because of differences in the way the head is laid out; valve angles and such.  You should be OK.  Take a picture of the cam lobes from the other side of the head with the timing marks lined up.  Then we can be more certain.

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I believe I found the issue - Seized crank bearing!

 

I was able to get bike started, it ran for approximately a minute then stalled. I was positive that the timing was correct. Disassembled down to the piston and found crank bearing seizing at one point in the crank rotation.

 

Thanks for the help.

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