you should see about 8 km or 9 km/lit with this engine. Correctly tune it. If I had this car I would do the following.
Step 1 - remove valve cover
step 2 remove upper belt cover
step 3 remove distro cap
step 4 remove spark plugs
ensure timing belt is correctly aligned - the crankshaft TDC mark is seen from the timing hole, its on the bell housing of the gearbox near the engine number.
rotate the engine to cyl 1 TDC seen from the cam pulley and do the valve clearances, you rotate the engine and align the TOP mark of the cam pulley at different positions to do the valves of cyl 1, 3, 2 and 4
step 5 - check the distributor centrifugal advance, if slightly sloppy, remove and overhaul it.
step 6 - ensure the little vacuum pipes are good and correctly positioned.
step 7 - ensure the carburettor secondary throttle opens at the correct time - more than half throttle at high rpm should allow it to open up - If it does not open your engine will suck more fuel and less air as it tries to breathe through the primary throttle - it will cause high fuel intake whether its petrol or CNG.
finally inspect the main jets of the carburettor, if some genius has probed them with a metal needle or similar they are bad.