Brother as for Carburator, you have to visit the specialist one is at Jublie and other is at Tariq Road you have to visit them and check your carburator.
Also check the tappet setting, loose tappet may also cause low milage, change the distributor point as well. it is for Petrol. Are you geeting oil in ur plugs, what is the condition of engine.
As for CNG, you need to service your kit and do proper setting again?