What do you make of this?

[Korn]

My trusty XR250 has made an attempt to self-destruct, I'm after some advice as to the cause of the damage.

Background: riding along a big A road, half throttle in 6th probably 85% of the way to the redline the bike suddenly loses power and develops a pronounced top end rattle. My first thought was that it had dropped a tappet?? It starts, idles and to some degree still goes so I nurse it home very gently, the noise seems to be proportional to engine load.

Off with the covers and all tappets present and correct, so I start digging. Notice a bit of discolouration around one of the exhaust valves top right: (what happened there??)

https://www.kornel.com/pics/xr/tP1010012.jpg
Big: https://www.kornel.com/pics/xr/P1010012.JPG

The head comes off to reveal a very grotty combusion chamber, notice the same exhaust valve stands out compared to the others (top left):

https://www.kornel.com/pics/xr/tP1010014.jpg
Big: https://www.kornel.com/pics/xr/P1010014.JPG

https://www.kornel.com/pics/xr/tP1010015.jpg
Big: https://www.kornel.com/pics/xr/P1010015.JPG

While doing this I also found the cam chain tensioner had reached its limit. As for the valves/seats I don't have a spring compressor so I haven't taken a better look yet, but removing the barrel also revealed this mess:

https://www.kornel.com/pics/xr/tP1010021.jpg
Big: https://www.kornel.com/pics/xr/P1010021.JPG

https://www.kornel.com/pics/xr/tP1010022.jpg
Big: https://www.kornel.com/pics/xr/P1010022.JPG

https://www.kornel.com/pics/xr/tP1010029.jpg
Big: https://www.kornel.com/pics/xr/P1010029.JPG

Bugger. So apart from lots of TLC I'm looking at a rebore and new piston, valves and oil seals plus a camchain tensioner - if nothing else. But what I really want to know is what caused the damage and do I need to investigate any further?
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3516 Miles, 11 Countries

[stinkwheel]

Not really an answer but it certainly looks like a valve-stem oil seal has let go. Also looks like the head gasket has gone. There is some discolouration round the other exhaust valve too, especially on the gasket surface next to it.

Not a big 4-stroke diagnostics man myself but how is this for a story?

Exhaust valve gap closes up (or possibly camchain jumps a tooth??), burns out valve seat, takes oil seal and adjacent head gasket with it. Runs lean due to head gasket air leak, nips up briefly but the momentum keeps it running and scores the hell out of the piston and barrel.

The one thing that doesn't fit is the piston crown looks fairly tidy in the picture, I would've expected some pitting round the exhaust area.

Or has the head gasket blown through to the oilway and overpressurised the oil circuit?

What did the plug look like? Any oil blowing out/scorching over the cooling vanes next to that valve? Header pipe blued to hell?

EDIT: What about a simple failure of the head gasket next to that exhaust valve?

Looks like a top-end only problem to me anyhow but as I say, I'm no expert.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Guest]

Overheated and/or lack of oil caused it to nip up, at the same time that exhaust valve has stuck slightly proud in the guide?

[Kickstart]

Hi

Does look like the head gasket has had a problem.

Did the oil light come on? Or on at idle but off when moving?

Personal guess is that the head gasket failed and it used a load of oil up causing some pars of the engine to run dry. Any sign of blueing on the cam?

While it could have been a valve clearance tightening up, as the cam is out and the valve is still open this need not be the case. Quite possible that the valve has nipped up in the guide in the same way as the piston has nipped up.

Not sure how the lubrication system works on the XR, but quite possible that it just relies on splash lubrication for the bore, and that would not happen if the oil level dropped substantially.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Robby]

If this is the same head as on the old CB250 RSA - and I'm pretty sure it is - they like to crack the head between valves or between plug and a valve.

So the crack widening up with head would explain less power/more trouble being proportional to load. Also a lot of heat from the combustion chamber discolouring around that valve, messing your mix up and seizing it.

Had it done about 25k by any chance? Seems that that engine does 25k and then dies, it just dies in a variety of different ways chosen entirely at random.

[knockout_bar]

A top end rattle sounds like the top end bearing is gone.
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"egg flied lice" - R6jonny (inspired by McGee)

[Korn]

Cheers for your insight guys. Forgot to mention the head gasket was indeed bollox'd, blowing slightly near the l/h intake valve and some evidence of oil weeping near the suspect valve (r/h exhaust).

[Kickstart]

[stinkwheel]

[Robby]

Mine had 4 valves. Had the head off it and everything.

SOHC, 4 valves, two rockers operating two valves each.

CBX250 was a proper DOHC 4 valve head.

[Kickstart]

Fair enough. Thought they were 2 valve heads.
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing