YZ250 engine issues

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5/15/2018 8:35 AM

Bike died at the track few weekends ago. Came out of a corner and just died. It did start up after but sounded awful, so I killed it and put in a new top end. Again, Bike started up, but still sounds bad, almost like a deep echo coming from in the engine. Took the cylinder off and found this oily residue on the new piston. Oil looked kind of milky and didn't really have a distinct smell. Also I’ll lost a picture of the piston that came out after it died, which looks like lots of blow-by. Complete engine rebuild had 16 hours on it. All seals and bearings replaced. Cylinder has not been replated or conditioned for as long as I've had it. Ring gap checked good but that's all I checked.

I’m thinking now possibly warped head and that antifreeze mixed with premix on the piston... or possibly bad right side crank seal and it’s tranny oil.

Any suggestions? Bike is a 05 YZ250.

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Photo Photo

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5/15/2018 9:03 AM

How many hours do you think the cylinder and head have on them?

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

5/15/2018 9:09 AM

mattyhamz2 wrote:

How many hours do you think the cylinder and head have on them?

Honestly man, no idea. They dont appear very beat up, but that could obviously mean nothing. I do have another cylinder at home I got with another bike I could throw on, but I don't think it will solve the oil issue.

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5/15/2018 9:34 AM

mattyhamz2 wrote:

How many hours do you think the cylinder and head have on them?

ChrisB477 wrote:

Honestly man, no idea. They dont appear very beat up, but that could obviously mean nothing. I do have another cylinder at home I got with another bike I could throw on, but I don't think it will solve the oil issue.

No it definitely wont solve the oil issue, but your cylinder could be out of spec

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

5/15/2018 9:41 AM

mattyhamz2 wrote:

How many hours do you think the cylinder and head have on them?

ChrisB477 wrote:

Honestly man, no idea. They dont appear very beat up, but that could obviously mean nothing. I do have another cylinder at home I got with another bike I could throw on, but I don't think it will solve the oil issue.

mattyhamz2 wrote:

No it definitely wont solve the oil issue, but your cylinder could be out of spec

Yeah, what I was thinking for the blow-by issue

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5/15/2018 12:28 PM

Have you checked your piston to wall clearance?

You could also have it checked with a bore gauge to ensure the bore is straight and circle, not oval.

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If you're not mixing gas, you're not haulin ass.

5/15/2018 4:33 PM
Edited Date/Time: 5/15/2018 4:34 PM

Was it smoking real bad? It looks like the right crank seal is sucking oil and that is a lot of blow by on old piston. I think I'd definitely get a new cyl or get that one redone.

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5/17/2018 10:38 AM

Was the coolant level low? If yes, you may have a leak to the motor.
How about up and down play in the crank?
You said the bike was rebuilt, but not the crank?
If the crank gets loose, it will take out the right crank seal. Loose crank can be growly/echo and possibly vibration.
If you inspect the right side crank seal from behind the clutch cover, check that the primary drive gear is still held on tight by the bolt.

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