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Blown 125, what could have caused this?

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Hello my friend bought a Yz that was supossed to have a fresh top end . He stored it for the winter and flooded the engine and carb with a kind of oil made for storage. Then when he tried it for the first time he said that his bike was lacking of power. He noticed the bike was bogging before powerband and wouldn't idle when warm. Then after another short test the bike cutted out. Result , 45 lbs of compression. What do you think it could be? I think the storage stuff would have clogged jet and make the bike run lean till it blow. He run klotz 32:1. He had coolant. The piston in good shape on the intake side but on the exhaust the ring seized in it and has scracht

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Did he jet the bike for his riding style? looks like lack of lubrication. Piston seems good, if it had more hours seems like it should have carbon build up. Did you check for any piston play and the ring end gap? I would rejett the bike and go from there. Of the cylinder is scored up your friend will have to send that out for repair.

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Did he jet the bike for his riding style? looks like lack of lubrication. Piston seems good, if it had more hours seems like it should have carbon build up. Did you check for any piston play and the ring end gap? I would rejett the bike and go from there. Of the cylinder is scored up your friend will have to send that out for repair.

What do you think about a partly clogged jet causef from the storagle stuff? Because before storage the jetting was fine

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Clogged jet shouldn't affect compression.

Yes I know but don't you think a partly clogged jet would make the bike run heat and affect the temperature/lubrification causing the engine to blow?

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Exhaust side of that piston looks really messed up and the ring is worn down to the pistion. Kind of looks like the ring was getting shredded up on that exhaust port. If its a fresh top end like you said it shouldn't be worn out like that. Make sure your power valves are not contacting the ring or perhaps whoever rebuilt it didn't measure piston end ring gap. That's my best guess. I'm in no way a expert. Your looking at a new top end ether way though that piston and ring are no bueno. Cylinder looks OK .

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Clogged jet should make the bike run poorly, if at all. You would be able to tell if something isn't right from the get go. Piston probably blew awhile ago and just now finally met it's end. I would just dump all fuel from tank and carb before storage. Why try and save a couple gallons for so long?

New piston kit is needed. Depending on how the cylinder is it may be ok? I can't really tell. I would just send it in with new piston and start fresh. Check for any up and down movement from the connecting rod.

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That piston looks like it has suffered a lean mix because of jetting or air leaks. The ring is worn into the ring land and there is alot of scoring in the front exhaust side of that piston All in one place. A clogged jet can cause this. Also a heavy mix of oil can cause this. Ie 25 or 20 - 1. There will be no compression because the ring is no longer making contact with the cylinder walls. That looks to be a sleeved cylinder so that is good news as you can have it honed with out replating. What happens in lean jetting issues or lubrication burning / lack of lube, like these is the exhaust gasses when the come back from the exhaust are so hot that is burns the lubrication from the bore and at just above the exhaust port. That looks to be the case here. Have the bore checked for out of round and out of taper and have it honed with your new oversized piston. It will need to be honed will a proper sunnen hone or the likes. Also I would recommend a 3 thou clearance releif on th exhaust bridge. Check your jetting and needle clip position with the recommended from the manual. Run the correct fuel mix. Check the muffler packing and repack if required and exhaust system for leaks. A leaking main seal on the magneto side will also cause a lean mix. Too much air. Can you post a pic of the inlet side of the piston? Ie has it suffered siezures on this side? 

A few other things to check are correct plug type and heat range, Air filter oiled and clean, cooling system has no leaks and has coolant among others of course. Piston ring gap will need to be checked when you install the new piston.

The other poster above is correct about checking the con rod. If you need more advice on this Pm me. 

 

Kind regards  

Edited by GEOF

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