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Piston chewed up

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(Sorry its so long) Heres the story..when i bought my bike, the cylinder and piston were scored, found out my powervalve valve chipped and scored everything up.

http://i52.tinypic.com/20pcw93.jpg

http://i53.tinypic.com/14lr1mu.jpg

I bought a used cylinder that came with a piston, and the previous guy said the piston had around 4 hours on it, so i used that. So a cylinder and piston later, i did a compression test cause my kickstarter felt easier to kick, and the compression was 125psi. I know this is bad for the bike, but at first when i got it i used these optilube premix packets, and the gas can was 1.25 gallons not 1, so it was running like 62.5:1, so after i noticed that i ran it close to 50 to one. Lean i know, but it was all i had at the time. Now i run 32:1.

Anyway, i took the bike apart today cause it was in need of a new top end. My cylinder wall looks fine, but the piston, not so much.

http://i43.tinypic.com/9902g5.jpg

What could have happened? I dont want to put a new piston in and risk having something come up and mess up my top end.

Any ideas or suggestions?

Thanks

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Possibly detonation? it looks like part of the piston is eroded at 2:00.

But with the scores from the previous piston, could it be some fragments or a bottem end bearing going? The marks only around the edge make me lean towards detonation.

I would check your timing and jetting and see if there is any play or roughness in the crank and rod.

Also the more oil you run the leaner it gets.

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It looks nice and rich...i think your lower end is coming apart.

Both pistons show the same kind of damage. (FOD foreign object damage) from the lower end..It's time to split the cases.

Edited by scalejockey

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Can we get a shot of underneath the piston? I want to see if it got hot enough to burn oil to the underneath of the piston. I'm seeing a lot of detonation across the entire piston. This is most likely caused by either way too advanced timing, engine running way to lean or improper squish band setup. Looks as if is started to seize by the raise edges on the exhaust side of the piston, so likely a chunk of piston broke off causing the major top of piston damage.

If you have taken the stator off, I would recheck timing, otherwise it wouldn't cause the damage shown if untouched from stock.

If it has to do with lean jetting, your flywheel side crank seal might be gone causing an air leak and leaning the mixture way out. The piston is black from being so hot that any oil is burnt on contact with the piston.

If the cylinder was modified to reduce the squish band, you may have to double up base gaskets or something similar to get it set back to an appropriate level. Although this is the least likely, I'm throwing it out as a possibility since it was a used cylinder from unknown origin...

Oh, and by the way, the less oil in the mixture makes the gas/air mixture richer. And vice verse, more oil will displace gas giving a leaner gas/air mixture.

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You've got some trash in the bottom end, either from a previous failure or components which are currently failing.

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I am in the middle of repairing my son's '04 YZ250 which had a ring failure due to detonation. We got the bike a few years back and it ran great so I never checked the jetting. At the time we were using 50:50 race:pump gas. This year I was getting lazy and started using using just pump gas (E10, 91 octane),'

Well my son didn't know the sound of detonation, ping, or knock and just throttled away. His piston looked a lot like yours. When the ring snagged the exhaust port is scratched cylinder. I had to send it to Millenium for re-plating. Even though they didn't have the failed piston they could tell there was excessive detonation.

There was damage in the squish zone at the top of cylinder. Cost me an extra $40 for them to repair and replate, ~$300 total just to refresh the cylinder.

After failure I found the jetting way too lean and have since returned to stock jetting. I'll fine tune the jetting when I get it running again. I will also resume 50:50 race fuel just to reduce the ethynol content and raise the octane.

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