YZ426 not starting after top end rebuild - any help?

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???

 

 

 

Is there compression?

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

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

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?

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.

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?

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.

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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