someone explain breather hose princple

[thunderkiller]

Hi friends,

I try to renew a kx 125 engine but I am not so closer to 2-stroke engines. This is my first 2-stroke mx as a beginner learning project.

I opened the crankcase, changed all the bearings and seals with newer ones. The surface of the crankcases were bad, that's why I rectified them 0,25 mm from all sides. I supplied crankcase gaskets 0,50 mm thicker because of the rectified surfaces. And last closed the cases.

Now my question is; when I close the breather hole on the crankcase(at the top of the transmission side) with my thumb and pump air from the oil filling hole using my mouth&lungs to test any air leakage, the air coming out from reed valve port .

Is is normal or not? I am sure the case surfaces are perfect fitting and the gaskets can not leak this little pressure easily like this. Is it normal that the air can pass from transmission side through the crankcase and intake port side, how this breather system works normally. can somebody explain the princple please. Is there any way to test it or any points to check?

Here is the picture for more clear explanation;

 

[Ol'89r]

Now my question is; when I close the breather hole on the crankcase(at the top of the transmission side) with my thumb and pump air from the oil filling hole using my mouth&lungs to test any air leakage, the air coming out from reed valve port .

Is is normal or not? I am sure the case surfaces are perfect fitting and the gaskets can not leak this little pressure easily like this. Is it normal that the air can pass from transmission side through the crankcase and intake port side, ;

No, that is not normal.

You either have a right side crank seal that is not sealing or a leak between the cases. The gearbox should be sealed from the crankcase. Could also be a hole in the case between the gearbox or primary side and the crankcase. (Casting flaw).

Using high air pressure you could possibly blow air past the seal but, if you are only using your lungs to provide the pressure, that won't be enough to blow past a good seal.

Just my $ .02

 

[thunderkiller]

I found the problem by leak-down test. It is leaking from right crankshaft seal.