Getting good mileage is only possible when your car's engine is tuned at the best. There is not much to do for a Daewoo Racer's engine, being electronic injected, just CO adjustment (~3.0v without air blockage), timing adjustment(~10 degree) and plug gaping (0.7mm). For a CNG driven vehicle intended to be driven on petrol, remove the air blockage from the intake so that the engine can breath properly, and ideally remove the CNG cylinder (you always have one extra passenger [cylinder] in your car all the time). If these things are followed correctly and you are still not getting good mileage on petrol, just check if there is any black smoke emitting from exhaust. If it is, you need to get the ECM scanned to properly diagnose the problem.
Another thing which plays important role in fuel efficiency is the engine compression, the better the compression the better the fuel efficiency. If you feel any blue/white smoke (not steam) from the exhaust, get the compression checked for any problem.
Thirdly, the quality of fuel also plays an important role. I usually get good mileage from Shell than any other company.
Lastly, you have to change your driving habits according to petrol. On CNG, we usually change the gears at bit higher RPMS, to compensate the torque loss on CNG. If you have tachometer in your car, change the gears at 2.5K rpms, if you don't have tachometer, change the gear at every 20km/hr speed (1-2: 20Km/h, 2-3: 40Km/h, 3-4: 60 Km/h, (if exist) 4-5: 80 Km/h).
BTW how much is the petrol average of your car? If it is anywhere above 11 Km, it is considered to be a good average.