2007 YZ250F no compression after rebuild! need help!

[claytoncox100]

I have rebuilt my bike from the ground up, almost everything is new, ring gap is within spec,  valve clearance is also within spec, i'm lost!

[nokickstandsallowed]

Welcome to TT!

 

Do your cam lobes look like this picture?

 

 

If so, and your valve clearances are in spec and everything is new; then the only thing I can think of is the spark plug isn't in or the head isn't torqued down at all.

Your timing marks on the other side could be lined up but the lobes facing inward would be your problem.

 

Another thing may be that you lined up your marks with the piston at BDC instead of top dead center.

 

Let us know what you find.

[Blauderdale6]

Your valve timing is off when you put it together did you poke your finger through the tensioner hole to see off the marks on the cams lined up with the head?

[Blauderdale6]

And did you make sure the mark on the flywheel was the TDC mark? And you held it there?

[Jeremy250f]

I believe your timing is off 180 degrees

[claytoncox100]

sorry for the late reply, ended up being one valve that was staying open due to one shim being a different size than the rest bike runs perfect now!

[Blauderdale6]

Sometimes it's the little things..... how long did it take you to figure that one out?

[claytoncox100]

well the only thing that hadn't been replaced was the piston rings so i checked them and they were out of spec so after ordering new ones and installing them and still having no compression I started over and thought it all threw i was at it for 20 hours straight and was tired so i never thought it over haha. it's been working perfect now I have 35 hours on the engine already and no issues!