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Unread 03-04-2013, 07:27 AM
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Smithers Smithers is offline
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Location: San Luis Obispo, CA
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Default 2005 RMZ250 Top End Rebuild

It's inevitable with any modern 250cc dirtbike - the top end rebuild or at least valve replacement. The valves in the engine I got with my latest used bike purchase had worn down enough to tighten up and cause the engine not to run. This doesn't mean they are worn out at all. The shims just need to be adjusted every now and then. What else can go wrong? Here are a couple things that happen in these bikes that can lead to them running poorly, not running starting at all and needing work:
  • valve shims needing adjustment
  • carbon buildup on valves
  • valve guides worn out
The bike I had was owned by a couple people and was raced a little to begin with. They must have adjusted the valves because I could see the cam cover had been removed and resealed. Judging by the condition of the engine I can believe that all the parts are original but the one thing they didn't do was check for valve movement left and right that would tell them that the guides are worn out. They were really bad so I'm sure they are the originals.



Pop the valve cover off and we have the valve train - cams, cam caps, cam chain etc.



Taking things apart is a matter of having a few basic tools. One good tool to have is the service manual for the RMZ250 or KX250F. This explains the whole process in very good detail. Here is a close view showing the cam journals. It's always to see flawless wear on the cams. If you had an oil shortage in this area due to blockage or an air bubble in the oil system you would surely see some damage here. The surface where the cam journal spins would be galled up or discolored.

On the far side of the smaller cam journal you can see a round shiny surface which is the bucket. This is a "cap" (think cap gun) that sits on top of the shim that is centered in the valve spring retainer. On the closer side of the cam journal you can see the smaller shim (small light colored wafer) in the larger valve spring retainer (dark grey circular) together.


Last edited by Smithers; 03-07-2013 at 12:01 AM.
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