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crankcase breathing question???

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I just purchased a 2007 KX250F and noticed that immediatly after starting it cold, a decent amount of oil leaks from the crankcase breather hose. I drained and refilled oil to proper level and no change. also continues to minorly seep some oil when riding it.

I have done much research and noticed this seems to be a common problem with no real remedy and may be caused by the clutch slinging oil around and pushing it out of the breather.

My question is... would it be a reasonable idea to move the crankscase breather from its current location (under the carb) to somewhere ontop of the valve cover. Im pretty sure I could remove the valve cover and find a safe spot to drill and tap a threaded bung for a hose in and I would think it could still breath properly..?

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wouldnt recommend it. It can cause major stalling problems, plus you can not vent it to atmosphere, you would need to run the line to the airbox so the first and every time your crankcase pressure increases it will force oil into your airbox, get sucked into your carb and gum it all up while making your engine smoke like crazy. Plus if you plug the existing crankcase vent the back pressure from the downstroke is going to make you blow lower end gaskets and seals like crazy.

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wouldnt recommend it. It can cause major stalling problems, plus you can not vent it to atmosphere, you would need to run the line to the airbox so the first and every time your crankcase pressure increases it will force oil into your airbox, get sucked into your carb and gum it all up while making your engine smoke like crazy. Plus if you plug the existing crankcase vent the back pressure from the downstroke is going to make you blow lower end gaskets and seals like crazy.

that is not true.. if you run the hose into the air box would work.. What do you think the PCV valve is for in a car? it just route the crank pressure back into the intact track.. I would not do it my self but it would not harm the motor.. you would just get slight oil residue into the airbox that would require cleaning [how ever you should be doing that anyways]

So, honestly I would just not worry about it.. all of them do this it's the nature of the beast.. If you're worried about the intial loss of oil when first starting the bike after an oil change there is a simple solution.. Press the kill button and kick the bike over around 30 times slowly this will allow the oil pump to circulate the oil to equalize between the clutch side of the case [where the crank case breathing port is] to the transmission and crank side of the case.. Generaly when you first fill the the bike up the oil is heavy on the clutch side and with the level higher on that side the clutch will pickup and sling alot more oil then it normaly does and thus pushing it out the breather..

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Its not just after an oil change though, it seems to be all the time, even if oil is not visable in the sightglass. My ktm and other bikes breath from the valve cover area and do not seem to have that problem of pushing oil out. It seems to me that the kawi sprays to much oil at the stock breather location

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Its not just after an oil change though, it seems to be all the time, even if oil is not visable in the sightglass. My ktm and other bikes breath from the valve cover area and do not seem to have that problem of pushing oil out. It seems to me that the kawi sprays to much oil at the stock breather location

I know you're saying its not after just the oil change.. its the fact that it's oil atomization.. See I change my oil after every ride so loosing a smigen of oil each day is not a concern because it will be replaced before the next ride.. how ever if you run the same oil for 5+hrs then I can see you being "worried" how ever if you figure you loose say a cap full of oil every hour or two of run time then just replace a cap full every hour of run time if need be.

I just say don't worry about it.. if you have a yamaha I can tell you it pisses out oil also from it's breather.. I've seem them do it also.

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I still beg to differ. he never said anything about using a pcv valve he said just a threaded bung, but even if he used a pcv valve it would have to be calibrated to that specific engine. pcv valve flow rates vary greatly between models of vehicles and bikes, and the universal ones are shit. Improper pcv valve will cause major stalling and running problems and with the small amount of pinging, backfiring and bouncing off the limiter a full on mx bike does that pcv valve is going to be slamming shut all the time and in turn blowing seals and gaskets all over the place. So like you said, I agree he needs to leave it be. It was designed with the breather on the crankcase and thats where it should stay.

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Wow, you're not grasping the concept... I never said to use a pcv valve.. I said it's the same concept and running the "gases" from the crank case into the airbox will not hurt anything.. at any rate.. I agree he shouldn't do it but if he does it wont hurt anything.

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My ktm and other bikes breath from the valve cover area and do not seem to have that problem of pushing oil out. It seems to me that the kawi sprays to much oil at the stock breather location

This is not a specific Kawasaki problem. My 07 never dumped a drop, neither did my 09.

I think you are going about this the wrong way. The bike did not have this problem from the beginning, and now you plan on fixing it by adding a hole in the valve cover that no other KX needs.

The KX 250f has a sort of a "centrifuge" that slings away oil through small holes in a shaft and leeting the air pressure pass out through the breather tube. This system is passed on from the Suzuki GSX-R line since the original KX 250f engine was designed by Suzuki. It´s a good system.

There are basically 3 things that can cause this problem.

Too much oil, too much pressure in the crank case or a misalignment of the centrifuge holes on the shaft.

You say too much oil is not the problem, so on to the pressure problem. Too high pressure could be from worn out/improperly installed piston rings. Check this before you do anything else.

Pray that you don´t have a problem with the shaft. They say it´s tricky to fix.

There is also a reed valve between the crank case and the transmission. It could be broken, but I´m not sure it would give this problem.

If you drill a hole to releive pressure elsewhere you have not fixed the actual problem, you will only have put a band-aid on a flat tire.

enginelubrication.jpg

engineventing.jpg

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This is not a specific Kawasaki problem. My 07 never dumped a drop, neither did my 09.

I think you are going about this the wrong way. The bike did not have this problem from the beginning, and now you plan on fixing it by adding a hole in the valve cover that no other KX needs.

The KX 250f has a sort of a "centrifuge" that slings away oil through small holes in a shaft and leeting the air pressure pass out through the breather tube. This system is passed on from the Suzuki GSX-R line since the original KX 250f engine was designed by Suzuki. It´s a good system.

There are basically 3 things that can cause this problem.

Too much oil, too much pressure in the crank case or a misalignment of the centrifuge holes on the shaft.

You say too much oil is not the problem, so on to the pressure problem. Too high pressure could be from worn out/improperly installed piston rings. Check this before you do anything else.

Pray that you don´t have a problem with the shaft. They say it´s tricky to fix.

There is also a reed valve between the crank case and the transmission. It could be broken, but I´m not sure it would give this problem.

If you drill a hole to releive pressure elsewhere you have not fixed the actual problem, you will only have put a band-aid on a flat tire.

enginelubrication.jpg

engineventing.jpg

it's most likly due to over filling.. my 07 has dripped from time to time which I equate to inadiquate draining of both sides and filling back with the recommended oil amount.

how ever if you start a bike immediatly after pouring oil it will spit oil out of the breather..

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It is definatly more than a capfull on startup and possibly several capfulls while riding. Im an auto mechanic and have been for a long time and oil coming out of the breather to that extent is not something i can just brush off. I did perform a leakdown test and i have no real leaks and I also tried to pressurize the crankcase and the pressure leaked right out of the breather tube so i dont think its a misaligned shaft. I have been inside just every other manufacter's engines except this kx so im not sure why this breather leaks so much. I need to find the source of the problem somehow....

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I tried routing the hose up along the frame spars (not into my valve cover) and it made it even worse. I found the only way is to stop it is to use the factory loop around the carb boot (cheezy!). I think this acts like a trap in your kitchen sink and stops the oil from getting around (not enough head pressure). If you look at the cases on an 09 or newer KX, the breather pipe exits at a 90 degree angle instead of straight out. Obviously Kawi seen the error of their ways! I have also experienced excess oil out the breather when my rings were worn or incorrectly seated, resulting in blow by which pressurized the bottom end and forced oil out.

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I was sick of my 04 kx250f leaking oil everywhere. Drilled and tapped a hole on the side of my valve cover and routed the line down by the carb vents. Won't cause any issues at all except keeping your oil where it belongs. Works great. There's no way it can affect the way your bike runs either, that happens between the piston and valves, not the piston and crank.

ImageUploadedByThumper Talk1369527015.697915.jpg

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Just a side note: plug the original breather line.

It's a badly engineered system. The CRF's have a good system going from the valve cover to the airbox with a drain line in between.

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Why would a backwards filter cause excess case pressure??? Pretty sure it would pump oil exactly the same either way. Putting it in backwards would just bypass the filter.

Just a bad design. Oil flings off the clutch directly into the kickstart shaft and then the case pressure pushes it to lower pressure, outside the engine.

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Why would a backwards filter cause excess case pressure??? Pretty sure it would pump oil exactly the same either way. Putting it in backwards would just bypass the filter.

Just a bad design. Oil flings off the clutch directly into the kickstart shaft and then the case pressure pushes it to lower pressure, outside the engine.

I would think a backwards filter would cause big problems. That little spring recess in the out-facing part of the filter looks like it would snug up nice to the outlet where the oil is pushed up into the cylinder. That would definitely cause back-pressure, and restrict or block oil to crank, cylinder and head. Even a dirty filter can cause back-pressure and restricted oil flow. It also gives the filter an hourglass shape.

I know there is always room for improvement in engineering, and I am the last one to criticize a novel approach to solving problems. I think venting through the head would be fine, for example, but I also think there is a problem in this particular system, and that problem is likely goo (the technical term) or debris in the screen(s) (like liquid gasket type stuff) or a bad oil pump, or blow-by. This bike did not come off the showroom floor gushing oil from the breather. Something happened to make it do that. Moving the vent is treating the symptom, not the problem.

First, I would route the hose, per manual, up and over the carb. Someone said it's probably like that on purpose, and it seems to be so. Then I would replace the filter, and put it in correctly, if you haven't done so. I would check oil flow at the filter just by looking at it with the filter cover off. Make sure the oil isn't too heavy. 10w30 or 40 should be fine.

You come here for advice, and this could be worth what you pay for it . . . nothing! but it's not the design of the breather that is causing your problem. It is telling you there is one, though. FWIW

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You guys crack me up. This thread is from 2011. :lol:

LMFAO!!!!!! Man, I just HAD to get my two cents in. Hope I helped that guy!!! Nice!

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Ya still people looking at it obviously. When someone googles kx250f breather, this stuff pops up. Good to get different opinions I guess. Also some people just need to have a pissing contest once in a while.

I completely rebuilt my engine, it pissed oil till it hit below the lower level line and I like clean garage floors. Moving the breather worked for me. Pisssssssss

Edited by Troy Cote

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