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92 yz250

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Just did a top end on my yz250 but now it has no compression can the O-rings be to thin? they are 2mm thick its the only thing i can come up with..

new cylinder gasket no scars in the cylinder or on the postion, i can start it but it won't idle and when im putting it in gear and driving it won't accelerate.

 

PLEASE HELP!

sorry for bad english.. :banana:

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Pressure check it. Head torqued correctly? Cylinder torqued correctly? Set ring end gap? Measure piston and cylinder size?

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Everything is torqued correctly and ring end gab is like it should bedre but I don't have the equipment to pressure check :(

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I had a 91 YZ250, but last worked on it like 18 years ago.  The only thing I can come up with is that you installed the piston facing the wrong way.  I don't think a bad O-ring (head gasket) is going to cause noticeable amount of compression loss.  So, if no compression, the pressure has to be getting past the piston and the rings.  You say, they are all in specification, so that leaves me to suggest the piston is facing the wrong way (and I'm not even sure that would cause compression loss).

Edited by mch

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I Will buy the tools for the compression test aften that i Will open it up and send My cylinder and piston down to My mechanic to get it mesured, maybe i could have mesured it wrong. :-)

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Why do you think it doesn't have compression?  Does the kick starter move so freely that you can use your hand to on the kick starter to make the piston to go up and down?  Reading your 1st post, it really doesn't sound clear what the problem is, other than it doesn't idle which isn't normally a compression problem.  The piston without rings should slide though the bore smoothly, but no play.  The ring gap can just be checked with a feeler gauge, with the rings (not on the piston) in the bore.

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I accually don't know why it dosen't have compression. I did the top end rebuild and put it back together New gasketkit torqued with the right specs and when i tried the kickstarter i could push it down with my hands. It can start but won't idle and when you put in gear it runs really bad :(

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I can push mine down with my hands too, but its got plenty of compression. What they mean is if you can push it down with very little effort there would be no compression. How are you starting the bike?

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Can i post videos on here? When i get home i could make a video to show it :-) i forgot to tell i bought the bike not so long ago and it needed a top end rebuild so i do not know how the compression fells like on a 250 :-) sorry :(

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when the bike runs it won't idle aswell and it runs bad can bent reeds effect it? and can reeds take off some compression?

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Reeds get worn, cracked, chipped, etc.,  Like the other person said, you do have compression. The amount of effort you exerted looked normal, so I wouldn't go pulling my cylinder just yet.  It could be the reeds / and or carburetor.

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my fresh 04 has a very hard hand pressed compression.  It should take a pretty good effort to roll it over by hand it you can do it at all.  There is a massive leak somewhere.  Not many places for it to go.  See if you can hear air escaping.  I would pressurize the cylinder with air like a leak down gauge set.  I made a tool out of an old spark plug and an air fitting.

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