My scooter gives 5 -6 kmpl mileage why??

i don't know why it gives so low mileage and i am able to see blue smoke coming out from the silencer...it suddenly in the middle of the Drive it slows down even though i accelerate fully it slows down to 30 - 35 kmph speed...bt when it slows down the blue smoke doesn't come out... it is a 2 stroke vehicle..i have changed the air filter and the spark plug and also checked the carburetor ..... what do u mean by carburetor tunning ... how to tune tat should i loosen and tighten the screws present somewhere in the middle of the carburetor.