Compression testing...

Have a question on compression. I have 2 YZ`s in the garage. Mine is a 04 250 which I get 220psi out of, my sons is a 00 125. I`m able to get 160-165ish out of the 125. Is this good or are we looking at a new top end? I bought the bike used and the previous owner said that nothing had ever been done to the bike. I have no idea how many hours are on it. However it has been ridden enough to wear out the rear shock. It seems to run fine, just a bit low on power in the low rpms, to the point were I thought that maybe the power valve was stuck open. We are about 600' above sea level. Thank you for any input.

Well since you just got it I would go through it to make sure it doesn't tear anything up and leave ya stranded on a trail ride! Sounds like it does need at the least a ring, there's only 1 ring on the piston on that model heck if your in it already then just get a piston and all!!! Don't use Cometic gaskets use Moose their much better. The power valves get sticky with the carbon buildup so take the cover off and spray with brake clean really good, I've seen the arm for the valve come loose and or break so check all that and make sure you run a synthetic oil: Motul,redline,Torco, @ 50.1 or unless your racin 40.1!! Good luck

Thank you. I found a 2 ring Wiseco and thinking of putting that in. My son is running yamalube right now @40:1 with premium pump gas.

Yeah 40.1 is cool and yamalube is ok! if you can get Motul 800 you get 1 qt for the price of 1 pint of Yamalube! 12.00$

a 2 stroke needs 100 psi or 7kg to run.

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