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My 02 kx125 Wont Start!

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My bike wont start.(kx125) I bought the bike a little over a month ago with the motor in peices. Had new hot rod crank installed but he didnt put the top end back together and clutch gears and thats when I got it. I got the motor put together and after got the motor in bike it started like 3rd kick.Ever since then I've had problems so the clutch wouldnt engage. Took it to the shop and got that working. Rode a little bit and after fixing other small things and having it run really good it wouldn't start. I could pull start it but it would cut off if i slow down but wont start kicking it. I dont know whats wrong it has spark,cleaned the carb,and is getting air. I took the head off today today to take a look at the piston it has come coloration but nothing out out the ordinary my head does look a little beat tho. Can Someone help or send me in the right direction! Please!

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pull your piston. I wouldn't be surprised if the piston ring blew. I have only seen the dome getting gouged like that from either the piston itself failing and busting apart, or the ring doing the same....and if its that's what happened I would bet the cylinder wall is damaged as well. which means your going to need to send in your cylinder for repairs. if it felt easier to kick over then that's probably the case. hopefully everything got sent out your exhaust and not into your bottom end if that's what happened.

you could get a compression tester at auto zone or oreillys etc but I don't know if it will fit. also at cycle gear but honestly online may be the best bet (cheaper). aside from that if you already have the top end apart it would be extra security to just get a new piston and ring for it for that warm and fuzzy feeling. not numera....no no no....wiseco or netter. ive seen four 2 strokes blow the top end, and every single one had a numera with easily less than 50 hours.

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My cylinder wall is fine, and the compression doesn't feel any different it backfires so I'm beginning to think its timing I have a flywheel puller on the way.

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it looks like a gouge to me. if your fingernail gets caught on it when you drag it over this then that's bad obviously. it could be just a marking though. hopefully not a gouge. if you bump start it typically the piston starts immediately hauling ass rpm wise, vise kicking it where you can only get it to go so fast while trying to start it. the faster the piston goes the more likely that the air that is getting compressed stays in there long enough for the air/fuel mixture to fire. the slower the compression speed the easier it is to push the air/fuel mix through a small area, like a gouge. that would explain why your faster rpms let your bike run but when you slow down it will start to die.

the reason I'm  leaning towards a compression issue is because of the gouging on your cylinder DOME. unless those were there when you put the engine together, then that's a sure fire sign of metal getting introduced into the cylinder somehow. I hope that isn't the case for you.

it would be weird but not unheard of for timing to retard while in use due to loose bolts though.

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Just by going off what the head looks like you need to pull that motor apart, and at least to a top end and have a good inspection of the bottom.

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Surf_and_brapp lol I noticed that after you said don't get that brand 😂.But I hear you guys do it looks like I may just pull the top end off and look further now before messing with the flywheel. Probably will try a top end and if that doesn't fix it I'll be forced to sell it. I'm running out of time before I go off to school! But thanks guys!! 

Surf_and_brapp and Moose_338 I appreciate it!

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It turns out it is my top end will order a new one tonight!  Bottom end has no debris in it. Hopefully will fix my problem!

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good to see you found the cause of your issue! how is the cylinder wall though? any groves it that then its not going to matter too much if you get a new piston. hopefully everything works out well and you just slap a piston in there and rip!(after you break in the piston though of course):ride:

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Luckily no besides that slight scratch you pointed out, I have a weisco on the way will let you know how things go

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UPDATE SHES ALIVE thanks for all the help. I did the rebuild myself this time and it had way more compression than before. After another break in cruise and adjusting the idle I'll be ripping again.

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