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YZ250 - seized, then not seized.. help

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hi Chaps, I have a YZ250 that developed a oil leak from the clutch side.. I drained what was left of the oil and it was 500ml, not as much as it should have been but not empty either..

I refilled the oil with 750ml and put it all back together and it ran fine for one lap of the field and then ground to a halt, it was trying to get the drive but the engine was tightening up for sure..

I double checked everything in the clutch side and put it back together again but then it wouldnt start.

I assumed it had seized (or partially seized as it did turn over) so I stripped the top end to check the bottom end and crank etc - they were replaced only about 10 hours ago.

 

anyway, everything is free and silky smooth, bearings, crank, water pump - everything !

 

so im going to put it back together with new gaskets and check it out again

 

does anyone have any ideas what this could be or come across this before ?

Edited by Jaden

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no one ?

 

I have done all the obvious like check the spark but it wont fire up, it kciks ok, good compression etc - it wont bump start either..

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When you say oil leak out the clutch side I'm assuming the clutch cover gasket went?  The first 3 things you check is air, fuel, and spark.  You checked spark, make sure your airflow isn't blocked, and make sure your carb is ok.

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When you say oil leak out the clutch side I'm assuming the clutch cover gasket went?  The first 3 things you check is air, fuel, and spark.  You checked spark, make sure your airflow isn't blocked, and make sure your carb is ok.

 

thanks man, yea I did that, fuel is getting through fine, I removed the reeds and had a look, they were saturated with fuel, I checked the float was working and the jets were clear.  the air filter is cleaned and oiled.. still nothing..  its not even trying to catch..

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I don't have a compression tester, it just feels exactly the same to kick as it did before, you can operate the kick start by hand with some effort and it all feels ok, no grinding etc

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Excellent article, I reckon you're right, I'll get a new piston kit in there..

Any experience with mitaka pistons anyone ? They Are really good value !

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Thinking about it, is there any reason that the existing piston and rings won't run ? I would have thought that they might cough into life just a little ?

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right, got to the bottom of it, im posting as I hope it helps someone out.

 

I was being led on a wild goose chase with the seizing etc, I removed the flywheel weight and the flywheel itself just fell off the shaft, the woodruff key had disintegrated meaning that the timing was out... pure coincidence at the time I had other issues.  the clue was I did get it running with a high speed bump and it popped and banged before backfiring and conking out... had to be timing. 

the guy I bought the bike off helped me out a lot, thanks :)

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