There must be some blockage in air intake for adjusting the CNG mixture. For driving on petrol you have to remove that blockage to get better mileage and get the car tuned for petrol ignition timing must be super advanced for CNG get it retarded for petrol (If it is a FF CNG car then there must be a timing control module on board no need to mess with timing then).
Also if there is no thermostat valve in cooling system then with direct radiator fan the engine is running cooler than it should be and that also leads to high fuel consumption. On a colder engine Carburetor dumps more fuel in engine to keep it running.
