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Yz250f low compression strip down and parts inspection

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Bike in question is a 2004 yz250f with questionable past and often poor maintence history

however it was running well and making very good power, my 2011 kx250f has nothing over it.


however the compression became very low, its alway been low ish.. but to the point where you could turn it over by hand easy enough

and near on impossible to kick start and wouldnt idle, always noticed it breaths abit out the breather as well which seemed to have gotten worse.

in any case figured valve specs were out, but they all came up spot on


so here we are. head looks to be in reasonable shape but the bore is very glazed and there is several noticable ridges towards the top of the bore

some evidence of debries going threw the motor nothing massive though, running slightly on the lean side.

any way pictures sell 1000 words before and after soda blasting.


hoping a diamond hone can be ran threw it and new piston put it.


sorry about the water mark.







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Agree with SUnruh... I assume you are familiar with the process and how to measure.


If I may point you in a direction.... I once had a cam that the auto decomp mechanism was sticking on it and causing issues.

You can check the mechanism by  moving the flywheel weight out and checking to see if it snaps back in without hanging up, or more likely in your case, hangs up as you move it out or doesn't move all the way out to the edge.


I also had another one that the spring loop broke off the end and it would not retract, ie no decomp, so that one was REALLY hard to kick.

I "fixed" that one by bending the end of the spring and hooking it back over the flywheel weight and its been working properly since.... 6 years so far.

Edited by ronbuell
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turns out the bore has a few marks in it, and very glazed

new piston and rings are going in/.

pulled the valves on the head they are in terrible condition on the inlet side.

glade we pulled it down when we did.

head has been ported and soda blasted new valves on order.


the point of cleaning something is to determine its condition.


water mark stuffed it up pretty bad... should go good with new valves, ported head and high compression piston.

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