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Old 05-07-2020, 11:12 AM
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Default 125 rebuild, not holding compression

Let me start off by saying that I've done plenty of reading, and used the search function as well 🤣 I also have quite a few rebuilds under my belt by now, but I'm certainly no pro.

A few years back I bought a 2003 CR125R, and it was low on compression after some riding. Tearing her apart revealed some scratches in the cylinder, so I replaced the cylinder with a new factory one, and rebuilt the rest of the top end while I was at it. After a proper break in cycle, the bike ran great, until the next time I rode it. It sputtered and died, and won't start. It's oddly easy to kick, which is telling me that it's low on compression, although I have not tested it yet. I mean, so easy that I can "kick" it with my hand. What would cause this?

I'd usually expect the piston/ring to be the cause, but they were just replaced, as mentioned.

Is there something that could be wrong with the bottom end that would cause issues up top? Why else would the bike lose compression so easily?
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Old 05-07-2020, 03:24 PM
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Pull your exhaust pipe off and look at the piston and rings. It sounds like it cold seized to me and if so there will be scratches on the piston below the rings.
Have you replaced the crank bearing seals?

Paw Paw
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Old 05-07-2020, 03:29 PM
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I haven't done any bottom end work to it. She still kicks over, in fact way too easily, but doesn't want to start up and run. I've never seen a bike lose compression so quickly
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Old 05-07-2020, 08:27 PM
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I can usually "kick"start a 125 by hand, even after a fresh top end rebuild. Get an actual measurement of the compression by borrowing a compression tester from almost any local car part distributor (Autozone/Oreilly's/etc).
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