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YZ 250 rebuild - top end problem, where to look

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Greetings - I'm looking for some help on tracking down a problem that developed with my 2003 YZ 250.

This was a complete rebuild - re-plated the cylinder, new piston and rings, cleaned the power valve, cleaned the carb, V-force reeds looked good, so I cleaned them well and reinstalled.

The bike ran great for the first 20 hours or so - lots of power across the board, and plenty of over-rev as you would expect. Now it seems to cough and sputter above say 6 grand-ish - still strong up to there, then the power tapers off and it sputters.

Still starts on the first kick, and no mechanical noise like some thing broke. ???

I'll start with cleaning the carb again - checking the main jet in particular.

I'm also wondering if the reeds could have cracked?

Could something have happened to the power valve?

Can the CDI's go bad?

Thanks for any thoughts.

'Frank

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get a picture of your plug. And the first thing i would do is put in a new spark plug before I started chasing anything else.

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Does the sputtering happen in all gears, and consistently the same way?

Your powervalve linkage may not be hooked up correctly.

This means your power valve is not working, and you wont be making any power up top. Especially when the engine was designed to work WITH the powervalve.

Take the cover off your power valve linkage on the side of the cylinder, and start the bike.

You should see the linkage rapidly moving up and down, if you dont, there's your problem!

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Bad reeds will effect low rpm only,did you paint the frame at anytime?Iv'e seen those things happen with a newly painted frame( poor motor connection at the coil)

I have had a bad cdi,but it was cracked,,smashed by the fork and just happen to effect how well it ran,but still ran. (no power and over heating while fouling plugs too) Ya,,that took a while to figure out..LOL

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Finally getting around to figuring this out.

So with the powervalve cover off here is what I see:

No movement at idle to mid revs - the it seems to slowly open as the revs get higher - then slowly close as the revs come back down. Like a centrifugal clutch opening - no rapid up and down of the rod.

Does this sound right?

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