There could be two reasons for bad mileage.
The petrol you are using is of very poor quality. Try Hi-Octane only (Hi-Octane these days is just like a better quality petrol, because the normal petrol is coming with horrible quality).
Your cab's mixture needs some adjustment. You need to 'tune' your bike (tappet adjustment, oil change, carb's mixture adjustment, spark plug check or replace).
The worst you can get with your bike, if you drive it hard, should not be less than 28-30km per liter. Driving normally should achieve upto 38-40km liter. And if you use better quality fuel, then it can go even higher.