Posts
6
Joined
3/24/2023
Location
Fairfield, IA
US
Just finishing a top end on my 2010 yz450, getting it all buttoned up and before installing the valve cover, tried to rotate the motor both with the crank bolt and kickstart. The motor will spin freely until the exhaust valves are going to open, then stops. The timing is set correctly with the flywheel mark (the one that looks like an arrow), and the marks on the cam sprockets. I have triple checked the timing, the piston is installed in the correct orientation, the valve clearance is good, the shims are seated correctly. I’m completely out of ideas on what to try and hoping for advice. I did install a new oem cam chain at the sametime as the top end as well. Any help is appreciated
Is the spark plug in? If it is, perhaps you're building compression and the decomp mechanism is not working correctly. Take the plug out and see if it will turn over correctly and inspect everything on the decomp mechanism to make sure it's trying to lift open an exhaust valve.
Plug is out
Piston is in backwards.
New timing chain?
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Piston is correctly installed, it has a new oem timing chain
I put the piston backwards once not thinking of the front intake and it did the same thing. 99.9% sure that is your issue.
I nearly made the piston backwards mistake but did catch it on assembly.
If the piston is in correctly your timing is off. No other condition would cause what you describe
Spun cam?
Timing is off. OEM chain? Length the same?
Take plug out. Put a straw in spark plug hole to verify piston tdc.
Work in reverse. Undoing all that you did, while roating and you'll see what is misaligned.
Forgot to update the situation, maybe it will help someone in the future. Took the bike to a mechanic after running out of ideas. He agreed all was assembled correctly and in spec, disassembled and reassembled, same condition. Gave him the oem piston I took out as a last ditch effort, issue resolved. Ordered new oem piston, bike runs flawlessly. Part number engraved on top of wiseco checks out as stock compression for 2010 YZ450. Piston was sent back to wiseco for inspection, no updates yet.
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