hey all!
I recently rebuilt my 07 crf250r top end, it has some wear on it as the valves and piston where shot. My local mechanic put in all new top end parts and now we have blow by, the bike spits about a 1/4 stick of oil (on the dipstick) per day. After ripping the bike apart again my mechanic came to the conclusion that the cylinder had uneven wear at the top so during the stroke it lets oil by into the valve breather hose. Thanks for any thoughts for those that are knowledge if this is even possible or was there some fault in the mechanic when rebuilding the motor?

It's possible although if he was a good mechanic he should have measured the cylinder for wear before ever putting a new piston in the first time.

No question about measuring the cylinder. You might want him to check the exhaust valve guides. They are prone to wear causing oil to leak past the valves burning oil and contaminating the fuel resulting in loss of power.

Charlie C, on 24 January 2012 - 04:30 AM, said:

How would oil contaminate the fuel mixture if its already burnt and exiting the cylinder at such a high velocity? just a question.

The same way that overlap works on intake and exhaust. Believe it or not, during normal engine operation, there's a period of time where the engine is pulling the exhaust gasses back into the combustion chamber resulting in fuel comtamination which means lower effeciency which equals less power. Anything that enters the cylinder duing combustion contaminates the fuel which leads to less effeciency. Blue smoke coming out of the exhaust is oil mixed with fuel, oil will not burn by itself in these engines. It's contamination of the fuel with oil.

Hi what do you mean by blow by,if its pumping oil into the air box its a ring seal problem or worn cylinder if it is just using oil it could be that the ends of the oil ring expander have been overlapped instead of butted together,its easly done which results in not enough pressure for the oil ring to work properly worth cheaking out.