Motorcycle Repair: heavy oil consumption, oil consumption, black carbon

Question
i have a 4stroke 70cc scooter which earlier demands an oil refill/change after every 1000km but now it is demanding every 250km.

No blue smoke is coming out so oil is not burning.  

I don't have any idea about the cause of this problem because engine sound is as usual.

Answer
See if you can take the air filter housing apart. There should be a tube that comes off the engine and goes into the airbox. You are checking the airbox to see if it has any oil inside. If it does have an excessive amount then you are getting too much blowby out of the engine and it has a compression problem. If you don't see any oil in there, look really close at the motor and make sure it is not leaking oil anywhere. If you dont see anything there, then the only other suggestion I have is to look at the end of the muffler and see if it has excessive carbon around the end. Sometimes things can burn oil in moderation and as long as the exhuast system is in top shape it can  mask this happening. The only visible sign is that the end of the muffler would have a heavy deposit of black carbon around the end of it. Let me know how it goes.