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I recently bought a 2012 kx250f. The guy I bought it from said he never adjusted the valves, but the previous owner bought a new engine in 2014. The bike is burning oil (I haven’t had a chance to do an oil change yet so idk how much) but it burns on startup, and when the bike is revved high quickly. It isn’t hard to start, every time I’ve started it it only takes 1 kick. I’ve heard it could be the valve guides, seats, or the piston rings. Any suggestions on what it most likely is?

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4 hours ago, William1 said:

Do a leak down test. Sounds to me like someone is abusing the engine.

I dont have access to a leak down tester, I was thinking of doing a compression test once and then another with a little oil in the cylinder would that work too

 

The bike also has a lot of compression 

Edited by drewkuze

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Only if you want to check rings in one area of the barrel. Barrels tend to wear unevenly, valves too can be a source of problems. Accept you have a race machine and as such, it requires race machine servicing and repairs.

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9 hours ago, William1 said:

Only if you want to check rings in one area of the barrel. Barrels tend to wear unevenly, valves too can be a source of problems. Accept you have a race machine and as such, it requires race machine servicing and repairs.

This Saturday I’ll hopefully be able to take apartment the valve system and check the cylinder, just hoping it’s nothing too expensive 

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15 hours ago, drewkuze said:

I recently bought a 2012 kx250f. The guy I bought it from said he never adjusted the valves, but the previous owner bought a new engine in 2014. The bike is burning oil (I haven’t had a chance to do an oil change yet so idk how much) but it burns on startup, and when the bike is revved high quickly. It isn’t hard to start, every time I’ve started it it only takes 1 kick. I’ve heard it could be the valve guides, seats, or the piston rings. Any suggestions on what it most likely is?

Starts easy, runs well, except for some smoky tail pipe oil burning; everything else is good and fine?   If yes, then first place to look and first thing to replace are the valve guide stem seals, 90% confidence.  They are ~20$ for a set.  They actually can be done without taking the engine apart.  You do have to take the head cover off and cams out but you do not have to take the head off.  Easier if you do, but not necessary.  And if you have an engine that is otherwise easy to start and running really well, then do not take it apart!  

First thing is to take the valve head cover off and measure the valve clearances.  Write that down and keep someplace safe.  Do some quick math to determine if you need to re-shim them or not.  If so, get the shims on order and get them in hand, if clearance is good or have shims ready carry on.

To change the seals topside involves feeding a rope in through the spark plug hole to fill the cylinder (keep a tag end hanging out the hole), then turn over until the piston is pressing the compacted rope up against the bottom of the valves, which keeps them from falling in.  Leave a wrench/socket with rubber strap or such on the flywheel crank nut or kick starter lever to keep the piston pressed up against the rope and valves while you work on the springs and seals above.  Then go ahead and remove the valve springs and replaced the seals.  Write down which bucket and which shim came from which valve, ensure you put the buckets back in exactly the same spot.  Keep a telescopic magnet close by and DO NOT DROP ANY SPRING KEEPERS!  When it is done, send the piston down a little bit and pull the rope out.  Put the shims back in the right place or the new shims you measured and got earlier.  Put cams back in, set timing, cover back on, etc etc.

Good luck with it.

Screen Shot 2018-10-17 at 8.52.47 PM.png

Edited by FaceDeAce
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On 10/17/2018 at 11:56 PM, FaceDeAce said:

Starts easy, runs well, except for some smoky tail pipe oil burning; everything else is good and fine?   If yes, then first place to look and first thing to replace are the valve guide stem seals, 90% confidence.  They are ~20$ for a set.  They actually can be done without taking the engine apart.  You do have to take the head cover off and cams out but you do not have to take the head off.  Easier if you do, but not necessary.  And if you have an engine that is otherwise easy to start and running really well, then do not take it apart!  

First thing is to take the valve head cover off and measure the valve clearances.  Write that down and keep someplace safe.  Do some quick math to determine if you need to re-shim them or not.  If so, get the shims on order and get them in hand, if clearance is good or have shims ready carry on.

To change the seals topside involves feeding a rope in through the spark plug hole to fill the cylinder (keep a tag end hanging out the hole), then turn over until the piston is pressing the compacted rope up against the bottom of the valves, which keeps them from falling in.  Leave a wrench/socket with rubber strap or such on the flywheel crank nut or kick starter lever to keep the piston pressed up against the rope and valves while you work on the springs and seals above.  Then go ahead and remove the valve springs and replaced the seals.  Write down which bucket and which shim came from which valve, ensure you put the buckets back in exactly the same spot.  Keep a telescopic magnet close by and DO NOT DROP ANY SPRING KEEPERS!  When it is done, send the piston down a little bit and pull the rope out.  Put the shims back in the right place or the new shims you measured and got earlier.  Put cams back in, set timing, cover back on, etc etc.

Good luck with it.

Screen Shot 2018-10-17 at 8.52.47 PM.png

ok, thanks for the help an I'll do that saturday

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ok guys, I decided to do an oil change on my bike. After the change, I rode the bike around and there was no burning oil. I dont know why, but before the bike would smoke more than a 2 stroke on startup and while being revved hard. Now it doesnt and I can't smell any oil burning off either 

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Question, how long had the bike been sitting for before you bought it and ran it?  If it had been sitting for a long time, the smoke may have been stuck rings and dry seals that are now oiled and functioning.

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17 hours ago, drewkuze said:

Now it doesnt and I can't smell any oil burning off either

Maybe do another oil change soon, to be help clean everything and provide better lube and sealing.  A clogged air filter could cause some black smoke due to running rich.

On 10/18/2018 at 2:56 PM, FaceDeAce said:

They actually can be done without taking the engine apart.  You do have to take the head cover off and cams out but you do not have to take the head off.  Easier if you do, but not necessary.  And if you have an engine that is otherwise easy to start and running really well, then do not take it apart!  

What's your reasoning for not removing the head?  I think I'd drop a collet or something in the motor for sure. I reused the gasket and head washers once. But will buy new ones for next time.

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2 hours ago, numroe said:

Maybe do another oil change soon, to be help clean everything and provide better lube and sealing.  A clogged air filter could cause some black smoke due to running rich.

What's your reasoning for not removing the head?  I think I'd drop a collet or something in the motor for sure. I reused the gasket and head washers once. But will buy new ones for next time.

Removing the head is invasive; disturbing gaskets, dumping coolant, removing the "set" of the assembly.  Potential to introduce other problems afterwards that do not have at this time, especially if it has been some time since it was last taken off (old gaskets).  It is not necessary to remove the head if you are on a single track of going after the stem seals.  If on the other hand you generally want to looksee and inspect, go ahead and take it off.  Ideal to have new gaskets available on hand if you take it off.

Of course you are stuffing rags or shop towels in all the openings in the top of the head right?  Nothing is going to fall into the motor if you plug off such things.  If you drop a collet it sits on the rag, and the telescoping magnet you have at the ready promptly retrieves it.

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I thought if we un-torque and re-torque the head bolts gradually and in the right sequence then it's all sweet. I can imagine that loosening or tightening one bolt fully would cause problems.

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In general I agree with Face, but before doing repairs I would advice to change the oil 1-2 times in short term, jut to be sure that everything with oil itself is fine.

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If it is burning Oil (did this engine ingest Dirt by chance) is the air-boot and box inside got signs of dirt if so the top end is in bad shape and the cylinder most likely. Also, the bottom end as well.

Did not see that mentioned - would be a good place to investigate.

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Ok guys, I changed the oil and it stopped burning. Now I have another problem, though. I was riding about 10 mph, dropped the bike on the right side and left it sitting there for about 2-3 minutes, facing downwards on a burm. So the top of the bike was lower than the bottom. Now, it keeps backfirirng horribly (nonstop) and my friends say that its extremely loud backfires too. there's also a noticeable power loss. when it gets into higher rpms, it doesn't backfire as much and while in neutral, not at all. It only backfires when I put the bike into gear. Any suggestions on what this could be?

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9 hours ago, drewkuze said:

dropped the bike on the right side and left it sitting there for about 2-3 minutes, facing downwards on a burm

So you let your motor run for 3 minutes without oil feed to the pump?  Seems like asking for trouble.

Maybe the backfiring happens because your exhaust pipe header is leaking air.

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