Rebuild problems

New here. I have a yzf250 06. Recently put new wiseco piston and replaced gaskets. And done the shims. Had lots of compression. Done about 10 min rideing at low rpm left it over night. Went to start it next morning kick start neirly went down with weight of my foot. Lost neirly all compression but it started so easy. Switched it off left it 5 min same again no effort in kicking it but starts so easy. Any idias cheers

3 things come mind.

1. Pull the valve cover and check the auto-decomp on the exhaust cam.  Make sure the fly-weight moves freely and check for the location.  With the engine off, the flyweight should be toward the center of the center of axis.  Only reason I mention it is.... this is a compression issue and the auto-decomp affects compression at startup.  (least likely reason for this issue)

2.  Absolute best test for health of a top end is.......... leak-down test.  Take it to a shop for that.  If its the top end you just did, leaking air will show up passing thru the problem area.

3.  Check valve clearance again.  Shim selection at rebuild could affect compression if too tight (most likely cause).  Some guys like to set them up tight, others loose.  I always go loose because they always get tighter as you run it.

Cheers for help. Shims were done when it was put back together and valve clearance still ok.

This is the second time this has happened to this bike. It's the 1st fresh piston it's had. The 1st time I lost the compression I took it apart to find the top compression ring had managed to turn turn round 180 lineing up both compression ring gaps. I have a feeling it has happened again. But don't understand how that can happin.

Thank again.

If the rings are rotating on you, then its time for a new piston AND a new or replated cylinder.  I've heard and worked on 2Ts with pined rings, but these bikes don't need that or the factory would do it.  Only reason its rotating is clearance is out of spec.


But you're right, when the rings line up, the compression goes "out the window".

Cheers it must be cylinder. It's a brand new wiseco piston and rings that have just been put in along with all new gaskets

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