Just show your bike to any two wheeler mechanic nearby and ask them to clean the carburetor and tune it well to give mileage. Some of them may tell you to re-bore your bike engine where it is actually not necessary to increase mileage and tune up.
Expert can easily just tune your bike carburetor and increase the mileage, wrong tuning up on carburetor could lead to reduce the mileage. Generally two stroke bikes can give mileage maximum up to 50kms per liter. Yamaha RX100 though well known bike - it may give you approx 35-45kmpl which is average and good, if it is giving less than 30kmpl then there is necessary to clean and tune up carburetor.