I am not a techie on engines but i recently had to repair multiple oil leaks due to the side effects of a pinched crank case breather pipe. Now that everything is repaired, there is quite alot of huffing and puffing coming out from the breather pipe. Is that normal? Its almost like putting your hand a few cm behind the exhaust pipe while the bike is idling. I also tried removing the oil cap while the bike is running and once again quite alot of serious huff and puff. i have no idea if this is normal or not and just wondering if anyone here has any links to general crank case pressure theories on the net that i can learn about. Thanks

 

I dont know how mechanically inclined you are... but think about it, oil is at the bottom of the crankcase, then there is a big air void between the oil and the bottom of the piston. When the piston moves up and down it has to suck in air when going up and push air out when going down. Like when your trying to get gas out of a gas can, if you dont have the vent open it doesnt come out too easily, the air has to displace the gas. Same thing in the crankcase, the piston is going up and down sucking air in the combustion chamber so it has the same effect on the bottom side. It will suck and blow alot... Its normal.

 

Of course you'll get a net "blow" due to ring blow by on even new engines. The suck is very minor and likely not noticed. It will feel more like a pulsing blow which is why I am not a fan of putting a filter on the end of the breather hose. Many OEMs just dead end the crank breather into the atmosphere.

 

I put a PCV valve in my crankcase breather...

 

i cant help but feel like i wanna add more holes and pipes after feeling the velocity of air when i opened the oil cap. But ya, i guess the std set up should be fine. Thanks for all the interesting info.

 

I do tend to feel that the existing hole in the airbox is a bit small to provide an adequate venting of the crankcase, but it's probably not as big a deal as it was on my KLR650. Regardless, I drilled out the hole on my KLX250 airbox, as I felt it could do no harm. Some argue that if the bike is lying on its side after a big getoff on the trail, that oil can more easily enter the airbox from the crankcase. While probably true in some extreme cases, I doubt this will be a big problem.

 

Other than losing oil and being 'low' on oil, oil in the Airbox would not be big deal to me. Especially not when you're thinking... i need new hand grips and i'm going to have to shift all the way home by hanging my toe under the shifter and my clutch lever will break if I try to bend it back, etc.

 

Problem with pvc valves is the check ball can not react to open and close fast enough to follow the pulses. Bike engines that use a trap valve like this generally uses small reed valves (ducati), harley uses a mechanical rotating valve to open and close at the right time allowing the pressure and vacuum to trade places under each big piston.

 

I believe there's a Mercedes reed valve device that many of the KLRWorld guys are using on their KLR650's with no issues.