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1999 CR125r - New Top End - Wont kick over when warm

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Hello guys, so i just got a 1999 cr 125r( it has fmf gold series and silencer) in july and i rode it all summer fine it ran strong intill the end of november. it started to loose power and got harder and harder to kick. then finally i was in 4th gear pretty high reved it bogged down and it wouldnt start again after that. i checked the plug and the electrode melted off or something. so i replaced it . i checked that i was getiing fuel. i had no compression so i took my top end apart turns out the piston ring was melted to the piston. the piston in there was a 53.94 which is stock so i replaced that with a wiesco 53.94 top end kit and cleaned my carb two days ago. i got the bike together and it started up 3rd kick it ran fine intill it warmed up then it didnt want to start. i jump started it ( only with choke on then it can go off) and it runs fine in all gears if i let it start to rev down and try to bring it back up it dies out. (Also previous owner said he did the crank and reeds last spring) any ideas in the rite direction would be helpful. (also took the plug out was alittle black resido notthing that crazy i replaced the ngk with a champion)but i figure it works somewhat since it will run when i jump start it.

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i didnt measure it but it kicked rite over i ran it with the choke on in neutral till warm exahust came out then i turned choke off and ran it low revs through all gears i only broke it in for 20 minutes i just finished today i didnt want to damage anything

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Did you have the bore measured whenever you had everything apart? Seeing as it melted down with the old piston it probably wore the bore as well and replacing the old piston with one of the same size may have been where you made your error. Regardless of what size the piston you took out was you should always measure the bore and order your new piston accordingly. It is very possible your cylinder is worn past it's service limit or you simply needed a slightly larger piston. Was there any damage done to the bore after the original piston melted down? No aluminum from the ring stuck to the cylinder? Was the nikasil flaking at all?

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Were you running it in colder weather? If you were and did not jet for it , you probably leaned it out. You should also check for any intake air leaks.And like others said you most definitely should measure/inspect the cylinder .That black residue could be trans oil being drawn through a failed right crank seal. It would also give you excessive smoke.

Edited by duner96

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The black smells like two stroke oil not transmission oil but I did inspect the sleeve it looked even it dosnt taper and the old piston was tight in the sleeve you couldn't wabble it around its was really tight . I didn't key it for the winter but it did run fine in November. There was no marks of It being damaged. An it really runs through gears fine and rips when I jump start it or rite when I walk up to it cold. Any Ideas on how to check a intake leak I did listen for air and look for leaks but it seems to be fine to me.

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Also the ring was not melted to the sleeve it was melted and stick in to its seat in the piston the out side of the ring were it touched the sleeve was fine there was no metal shavings on the inside of the top end at all. Also I drained my trannision oil and strained it there was nothing it

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The trans oil is completely separate from the crankshaft on a two stroke so the only way you would find shavings or get them out would be to split the cases. Also even a trained human eye would not be able to tell if the bore was out of spec unless it was really bad. The only way to know for sure is to use a micrometer. It could just be something as stupid and simple as bad jetting. If your jetting is out of whack it can very easily cause a hard hot starting condition. Have you done a compression test? First two things I would do is a compression test and then if that checks out open up the carb and see what jets are currently in there. Clean the carb really good inspect and adjust the float height and change jets accordingly. Hopefully it is an easy fix

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