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  1. #1

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    Did lean condition cause this?

    Will Lean condition cause this damage to the top of these pistons. Or is this damage from a needle or other object. The wet spot is from fogging oil I sprayed into the cylinders before I did a compression check. trying to decide on top end only rebuild or complete. The exhaust valves are in place and working properly. Theres also some aluminum built up on the side of the cylinder that had low compression. Sorry I havnt had time to tear down carbs or check oiling system. 2001 800 Yamaha

    4 year old sbt motor.
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  2. #2

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    Looks like a ring hit the exhaust port to me.

  3. #3

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    Didn't think about that. But the worst damaged piston has 120 lbs. if it was the ring breaking I wouldn't think it would have that good of compression. since you said that I noticed there is more damage on the exhaust side which leads to lean condition I think.

  4. #4

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    Can't really see any piston pieces missing but there was a piece of metal banging around in there. I assumed ring pieces.

  5. #5
    john zigler's Avatar
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    There is really very little piston wash, and you do appear to be lean, BUT the marks on your piston are definitely from a foreign object. You need to FULLY disassemble the engine and inspect all bearings, etc and see what is missing. You also have a good amount of aluminum transferred tot he cyl. These are Nickasil plated, and will probably need to be re-done.

    I would go through the carbs, and fuel system completely. Fuel lines, filter, fuel select valve etc.

    Good luck with your project.

  6. #6

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    Thanks John and stcat. I'm going to pull it next week and do it right I guess. A magnet stuck to the cyclinder wall and since it was an sbt motor I assume they already sleeved it with cast. I should be able to use some acid and clean up the bore. I read somewhere that a needle bearing can come loose.

  7. #7
    mudslanger's Avatar
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    Those cylinders are sleeved and acid will not help. the best thing would be to hone them and see if they will clean up. as far as the piston damage on the tops of the pistons it looks like Detonation to me. probably from bad fuel or even water intrusion. If your compression is correct at 120 psi i wouldn't think you would have any crank or rod damage because a missing needle bearing from the crank or rod would produce more damage and lower compression due to more piston to head clearance.


  8. #8

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    I was just going to use the acid to get the built up aluminum off the wall then hone and check bore messurments. I would much rather the damage been from detonation and not debris. Pto has 30 lbs mag 120. Pto has built up aluminum.

  9. #9

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    It also starts and sounds decent at idle.

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