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Repeat Engine Failure


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I was just wondering if anyone else on here has experience repeat engine failures. The bike is a 2000 kx250. The bike is in good shape and I love the way it feels, but I cant bring myself to keep it after I rebuild it again. Here is the story. I bought the bike for $1200 a few years back. It had receipts for a fresh engine build from crank up. I think it lasted 3 months ( I would guess 10 hours or so). The intake side of piston had some scoring, and the exhaust side had a broken ring landing. I replaced the cylinder with a new oem part, and used a wiseco piston. I took it easy for the first half hour and everything was fine. The first time I got it on the pipe, it lost compression. The bike would still crank and run, but I pulled the pipe to look in the cylinder and there was a score on the intake side of the cylinder. I didnt run it anymore and took it to a bike mechanic that a couple of my buddies have been using for years. I finally got it back from him and the motor has 4 hours on it when it failed again. I have not dissassembled it yet but took this picture through the exhaust port. I dunno guys, what ya think????20141129_1349091.jpg

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Without actually seeing the torn down engine and hearing the bike run (when it did) I would make a guess that you have two problems. Sounds like you're sucking dirt into the engine somewhere, hole in filter, leaking boot, etc. or maybe didn't get it all cleaned out the first time? (not sure how far it was torn down) The second problem... is it possible that the power valve is hitting the piston? Not sure about yours, but I've seen power valves installed upside down in other engines that caused very similar looking damage.

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You will have to tear it down.  It appears to have something to do with the KIPS valve or power valve.  Under normal circumstances the movable parts of the valve should never protrude into the cylinder.  This is common when the cylinder is larger than stock and the power valve is the kips valve is not clearanced.  Also make sure the piston is installed correctly and the ring gap is located in the proper location with the pin on the piston.  The same failure multiple times tells me that the valve is probably hitting the piston.  The power valve parts should be cleaned and lubed every time you take the top end apart. 

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Not only should you tear the top end off but you should check for chunks of metal and dirt, etc. in and around the crank bearings and rod. Hopefully everything went out the exhaust port and didn't get hammered into your bearings. Three trashed top ends in a row isn't good for a crank.

Edited by James I
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I would like to see a pic of the center power valve on the cylinder side. Plus, were all the ports chamfered on both cylinders? Lastly, if you're not going to do the work yourself, find a different mechanic as he should have figured it out before giving it back to you. It is fairly simple to figure out if you just approach it methodically.

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