yeah you can increase/decrease it by some changes in fuel mixture setting. for getting best fuel mileage, fuel mixture setting for 70cc bike is 2.5 choori that's what my mechanic told me. fuel mixture bolt is right next to your acceleration bolt in 70cc bike.
if you increase it your ride will drink more fuel but it will give you better accelerate and speed. if you decrease it acceleration and speed will get low.
my suggestion is don't take any panga with fuel mixture setting. first consult with a good mechanic and tell him what you need and then make changes accordingly.