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Burnt Piston!!! But why???

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My son has a restored 82 Cr125 that he burnt the piston on today. I am assuming this is from it running too lean? But why? the air screw was 2 turns out and 92 octance with 34:1 Lucas synthetic pre-mix. Bike was running amazing but died while 3rd gear wide open. No compression, pulled the head and this is what is inside. I also noticed the coil cap on the plug was loose and the electrode end on the plug was loose which I think was giving and intermittent spark, but I think that was a not related to the piston problem?? Is it possible that the bike is just jetted way too lean. It has an aftermarket exhaust. Here are pics of the bike and the piston: Any ideas??

bday055.jpg

cr125piston.jpg

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It's entirely possible the bike was jetted too lean. It's also possible that one of the jets was plugged, causing it to run lean. The pilot screw setting has very little effect on the jetting and will have nothing to do with causing a piston to burn.

How many hours were on the piston and rings?

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The spark plug looked okay but I wasn't looking for a problem when I had it out. Now it looks like the piston with aluminum all over it. I am not sure about the hours as I have not replaced the top end and you never know when the previous owner did. From the condition of the bike, recent restore, I would say it had very few hours. I am pulling the motor down this week to start rebuild. and I will clean and check main jet. I also plan to replace the crank bearings and seals as the piston dust must be in the bearings. Thanks

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lean bikes usually feel like they are running amazing until they die and dying at WOT is a good indication of lean in the main jet or a plugged main. what is stock main jet and what main did you have in it? also check the rubber intake manifold for cracks.

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Have you (or your son) ever run the bike before or was this the first time? If that was not the first time, have any changes been made since the last time it was run? Sure sounds like it was lean. He didn't run it out of gas, did he?

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The top of that piston looks like it was running lean to me. X2 on the suggestion that you could have an air leak from the manifold or somewhere else.

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So, I can't see it that well, but it looks melted near the exhaust port?

That generally means the exhaust was hot and that generally means it was running lean.

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My son has a restored 82 Cr125 that he burnt the piston on today. I am assuming this is from it running too lean? But why? the air screw was 2 turns out and 92 octance with 34:1 Lucas synthetic pre-mix. Bike was running amazing but died while 3rd gear wide open. No compression, pulled the head and this is what is inside. I also noticed the coil cap on the plug was loose and the electrode end on the plug was loose which I think was giving and intermittent spark, but I think that was a not related to the piston problem?? Is it possible that the bike is just jetted way too lean. It has an aftermarket exhaust. Here are pics of the bike and the piston: Any ideas??

bday055.jpg

cr125piston.jpg

btw nice line of evo bikes.

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