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KX250F Smoking out of exhaust. What needs replacement?

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My four stroke now looks more like my two stroke when i ride it, by spittin out smoke from the rear exhaust pipe.

i am already replacing the top end piston and rings but could that be the only problem or do you think the valves need work too?

if so, usually what do shops do when they adjust the valves and such? (Just for information sake, im not going to try that)

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If you can, i video helps alot in determining the problem. It may be a valve seal but not sure. Please post a vid of it if you can.

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what color smoke is more important to figuring out your problem..

Black smoke = unburnt fuel

white smoke = water/coolant

blue smoke = oil

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Does it only do it on startup or is it constantly smoking as you're riding it? You likely have some blowby happening...

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let's brake it down abit, if it's black smoke when you blip the throttle or at start up under choke, it's unburned fuel which then burned in the exhaust in an oxygen deprived environment which causes the black, sooty smoke, this is normal for 4 stroke fitted with a carby with an accelerator pump (all current high performance 4 stroke that don't run EFI)

if it's blue smoke it's likely one of two issues, if your experiencing smoke under deceleration, it's likely piston/rings, when you let off the throttle there's no load on the rings and this prevents the oil retainer from scraping all the oil off of the cylinder wall, if the ring's are worn badly enough it will happen all the time but worse under decal.

if your experiancing blue smoke when you open the throttle, but it go's away if you hold the throttle at constant RPM, then it's likely intake valve seals (cheap fix), when you let off the throttle it creates a vacuum in the intake, which sucks the oil past the intake valve seals and into combustion chamber, some stay's here, some get's into the exhaust, but because your not on the throttle it's to cold to burn the oil, when you open the throttle and the combustion temperature shoot's through the roof the oil burn's making a ploom of blue smoke.

with badly damaged valve seals they'll leak all the time, so if you leave the bike lidling, then open the throttle it'll try and suffocate you in smoke.

white smoke is coolant, in 99% of bikes the only thing that causes this is a blown head gasket, however in a 4 stroke this will normally result in the combustion gas leaking into the cooling system and over pressurizing it, which will blow the coolant out of the overflow, alot of people who have cooked they're bikes blame the blown head gasket on the heat, when in reality it's the blow head gasket that caused the bike to overhead in the first place.

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mostly blue smoke, mainly when im giving gas or grunting out of a turn.

also probably had a blown head gasket because i had very little coolant left.. got lucky i didnt melt the sucker down yet

already took it apart my question though is should i replace the piston, rings and gaskets, put it all back together and then see if it smokes and if so work on the valves or just get the valves adjusted anyways while its already off?

btw 07 kx250f... previous owner said it stock still

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Do the valve work while you have it apart. Don't forget new stem seals.

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I agree with dgcars...while it is apart....replace or repair everything thats needed.....You would hate to put it back together and so many hours later need to replace the piston or do a valve job.....Do it all at once....come out with a freash engine.......good luck on your project.......

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mxtime.com tell dave Adam From La porte sent you.. $425 for complete head job new seals, new springs, new valves, seat cut, & decarbon head..

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I'm having the same problem, though I don't think mine is as severe. It doesn't smoke like a two stroke, but before it gets warmed up there is a little bit of white(possibly blue?) smoke when I rev it. When it is warm enough to run nicely without the choke, the smoke goes away. Mine is an '06 with alot of hours, previous owner claimed that the top end had just been rebuilt, put in stainless steel valves..

Thanks,

Eric

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