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Slight whirring/whining noise after top and bottom end

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I just buttoned up a top and bottom end on my '99 KX250. At idle and just off idle I can hear a slight whirring or whining coming from somewhere towards the front of the motor. As soon as I get a bit past idle, the noise goes away. I shoved my head up to the motor and I can't narrow it down any more than that unfortunately. The rebuild went really smooth with no hiccups and I'm not sure what this could be.

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Just curious, when you put the crank in were you absolutely sure the crank was centered and not side loading the main bearings when the cases were sealed? I have seen this before... Please dont ask me where....

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I believe it's all correct. I spun the crank after I pressed the case halves back together and it spun freely.

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Checked the gap and it was tight but within spec. Cylinder was re-plated with a fresh Wiseco piston.

Also the KIPS seemed to be functioning smooth and correct.

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Sounds like the power valve. My KDX200 does the same thing. It seems pretty common on most hapanese bikes. I just added a little bit more gear oil to tone it down a bit. I think the power valve is actuated by some sort of centrifical force and that's where the whinning comes from, I maybe wrong though.

Little Jeff

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I get the rattling from the PV but I don't remember the whistle prior to the rebuild. If that's what it is I'll be thrilled, just want to make sure it's not something else. It does seem that both noises (PV rattle/whistle) go away at the same point above idle.

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Rode a bit this weekend. The noise is still there. No more and no less. I'm leaning away from a bearing noise as I would think it would get louder as the revs come up. Beyond that I'm at a loss. 👍

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I saw your second thread when I was searching. That's the one that started to put my mind at ease with the noise and just let it be. My PV was covered with carbon and was a bit of a pain getting out on dis-assembly. I glass beaded the PV parts and I'm wondering if the added clearance is a big factor with the noise.

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Not sure about that but if it's any further relief the KX125 still makes the noise as does our KX65 and slightly from our 2012 KX85.

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