stern tregre

Mar 29, 2017
I recently purchased a 2012 yx250f for cheap with 80+ hours so the bike had no spark so I ran the check on all the electronics I discovered it was the stator so I purchased a new one and installed it while doing so I purchased a top end rebuild, cam chain, and shim kit when putting the bike back to together I was lining the flywheel up to top dead center (TDC) and when doing so I noticed that the piston was not at top it was on the down stroke around 5/8in down (is that normal) so I ignored it and lined the cam sprockets up where the were marked in the manual. After doing so I shimmed the valves according to spec (per manual) and turned the bike over with wrench (from flywheel) and noticed the exhaust valves were hitting the piston so I pulled the cam back off recheck my work and still the same outcome. I moved the exhaust cam one tooth down and advanced the timing so that it would not touch (I know its not correct) put everything back together and bike does not start im stumped in this spot. The only thing I can think of is the crank is bent or Im over looking something in the timing area has anyone ran into this problem or know what im doing wrong. First time rebuilding a yz with dual cams.

Rich Rohrich

Moderator / BioHazard
Jul 27, 1999
It sounds like the flywheel isn't keyed to the crank correctly and is out of phase. Start by pulling the flywheel and inspecting the woodruff key in the end taper, and inspect the groove in the flywheel for damage. Once you are sure those parts are solid, bring the piston to true TDC with both valves closed. Once you are sure you are the piston is really at TDC compare that crank position to the timing marks on the flywheel. It should give you more of an idea where the trouble is.

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