I have a Cultus 2004 too and it does 140-150 km in the city. With the A/c it can be anything between 120-130 km in the city.
There are a number of things which come to mind when a car does bad mileage. Clogged Air Filter, Bad spark plugs, incorrect plug gaps, retarded distributor timing at idle, distributor vacuum advance hose not doing its job, bad spark plug cables, CNG kit choked full of oil, mixture too rich or too lean, clutch plate slippage and even an A/C or alternator belt which is way too tight to bog the engine down.
Do you also notice misfiring? Bad pick-up? Inability of the car to go above 100 km/h? To check misfiring let the car idle and go towards the back of the car. Listen to the sound coming from your exhaust pipe. Is it a smooth and rhythmic growl sounding like brrbrrbrrbrrbrrb or is there something like brrrrr pzzt br pzt. I think you will know what I mean when you actually listen to it.
You need to go to an expert CNG mechanic to get it tuned. If your mileage improves after the full tune-up including plug gapping, timing and mixture setting then you are good.
If nothing changes and your engine is still misfiring after the tune-up then you have to start thinking about the hydraulic tappets. One of these could be worn out or pumping up. The cultus is the only car in the pakistani market which is blessed (or cursed) with hydraulic tappets. They can be a real pain in the a**. The Daewoo Racer is the only other car which has them.
If your mechanic really knows his stuff he would do a compression test to check for intake valves not closing properly. To remedy this problem, the timing belt, air filter housing (fry pan) tappet cover and cam shaft have to come off, all six tappets have to be taken out and cleaned with gasoline or other suitable solvent and reassembled. If you are very lucky your engine will become smooth. Otherwise just replace all six of the damn things.
Hope that helps!