YZ426 not starting after top end rebuild - any help?


8 replies to this topic
  • amooney301

Posted August 27, 2013 - 08:21 AM

#1

cam alignment.jpg flywheel mark.jpg I'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???

 

 

 



  • brandonvick83

Posted August 27, 2013 - 08:58 AM

#2

Is there compression?

  • amooney301

Posted August 27, 2013 - 09:53 AM

#3

Plenty of compression. Engine turns over similar to before rebuild.



  • grayracer513

Posted August 27, 2013 - 10:11 AM

#4

Is the exhaust cam an aftermarket cam for a 426, or a YZ450 cam?



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  • amooney301

Posted August 27, 2013 - 10:40 AM

#5

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?



  • MrKTMrider

Posted August 27, 2013 - 10:48 AM

#6

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.

  • amooney301

Posted August 27, 2013 - 11:43 AM

#7

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?



  • grayracer513

Posted August 27, 2013 - 05:18 PM

#8

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.



  • amooney301

Posted August 27, 2013 - 07:19 PM

#9

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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