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Need Help; 2000 YZ250 Problems

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I’m having some engine problems and haven’t been able to find the source.

While riding a hare scramble race my bike started running poorly on top end, I can’t tell you if it was sudden or gradual. We hit an open section and I whacked the throttle and the power started breaking up right where the power band should be hitting the hardest. This was about an hour into the race. The only change I had made going into the race was adding a flywheel weight. The bike runs well in the low and mid range but power falls apart when it should be climbing on the pipe. Since the race I have replaced the reads, inspected and cleaned the carburetor, tried new spark plugs, tried taking the flywheel weight off. Compression is good, and the power valve linkage is working properly. Bike starts first kick and the timing plate/disc behind the flywheel hasn’t moved. What could this be?

Thank you

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I always suspect whatever I did last as the source of my problem. Since you added a flywheel that's what I would check, anything to do with the installation of that.

Check to see if the flywheel key is still intact. If the flywheel gets off even a little it affects the timing.

When you installed the flywheel did you use a screwdriver on the flywheel to stop the rotation of the engine while removing the nut. If you did, you may have broken a wire on the stator behind the flywheel. Take the flywheel off and check. You'll need a flywheel remover to check either of these items.

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Try a smaller main jet, it worked for my bike when it had similar symptoms

Try running the bike with no air filter (run it up and down the street) and see if there is not a slight improvement in performance, if there is your problem is carburetor related.

also make sure that your silencer has no blockage... ie. bad packing or high temp silicone if you use it around the front pipe to cylinder connection as I have had that blow back into the silencer causing mass confusion.

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I always suspect whatever I did last as the source of my problem. Since you added a flywheel that's what I would check, anything to do with the installation of that.

Check to see if the flywheel key is still intact. If the flywheel gets off even a little it affects the timing.

When you installed the flywheel did you use a screwdriver on the flywheel to stop the rotation of the engine while removing the nut. If you did, you may have broken a wire on the stator behind the flywheel. Take the flywheel off and check. You'll need a flywheel remover to check either of these items.

agree 100%

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Open the carb bowl and make sure your main jet didn't vibrate out. It sounds weird but this happened to me on my KX250 and resulted in similar issues to yours. Since it starts first kick I doubt you have any damage to worry about.

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I don’t believe it is a carburetor issue. Jetting was suggested before I posted. The bike ran great for the first hour and there was no change in temperature or humidity. However, since the race I checked for blockages or debris in the carburetor and there were none. I also tried the trick where you remove the air filter and it didn’t help. I removed the flywheel weight to see if that was somehow messing up the magneto pick up and it did not fix the problem. To remove the flywheel I put the bike in gear and had a friend hold the brake to keep the engine from rotating, could that have affected timing? Keep the suggestions coming please.

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Please keep this going guys. I only have one chance to prep my bike for this weekend, I need to be able to rule out as much as possible. Thanks again

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For the short amount of time it takes to test it. Unplug the kill switch. A kill switch can short due to vibration of the motor at revs or even randomly. Most think a kill button is either shorted or not shorted. This is not always the case. I would suspect plug then powervalve in this case. You said the linkage is working, but are the powervalves opening quickly and fully? for giggles, try smaller main jet. Was the temperature way high that day? Bad fuel or water in gas can or tank?

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Drain the float bowl and look for water. Blow out all of the jets with air or contact cleaner. Air filter clean? I can't imagine you should change your jetting since it was running good before. You might also have the coil checked out by a dealer or perhaps just check the coil ground and clean it.

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Well it took a while but this one is figured out. And we were all wrong.

The cause: The perforated tube in the silencer had broken in half and fallen down into the silencer clogging up the path for the exhaust to come out. There was still enough room for exhaust to escape with proper velocity at low to mid rpm but the opening was too tight to work at high rpm and volume.

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NICE :busted: I'm amazed you figured that out. That's one of those odd occurances that you'd never guess and would never look for. How in the world did you narrow it down to that?

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I wish I could take credit. I took it to a guy who's very good with these things. He went through the entire motor and found numerous problems (hairline cracks in the cylinder and the piston skirt, broken actuator arm on one of the boost ports and a couple minor things) before he found the culprit.

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