Motorcycle Repair: blow by, crankshaft seal, rm 250

Question
dont know if you can help, but i have  a 2005 rm 250 f. from the motor breather hose's leaking oil and has blow by there. the motor has  about 6 hrs. on it and the piston is new. i try the piston and rings frist dont no way it is leaking from breather.


               thanks chris

Answer
Chris.... not my brand or type of bike, but I believe that your bike is a 2 stroke.... sealed crankcase for the crankshaft and piston.... If you had blowby, it would be compression/fuel lost past the rings, which would cause it to smoke more and have less power....

I imagine that the breather hose you are talking about is for the transmission, which is sealed from the crankshaft.

Either you (or someone) put too much oil in it or it is frothing for some reason (wrong type of oil, perhaps). A bad crankshaft seal on the transmission side of the crank, will usually suck oil in from the transmission case portion, not pressurize the transmission, in most cases.
That condition will cause more smoking and jetting problems as it upsets the fuel/oil ratio going into the motor.

Check your transmission oil level and keep an eye on it for the next few rides. Drain some out, if the level is too full.

Bill MrHonda Silver