The CNG kit on your car Honda City has not been properly installed and the Lambda Oxygen Sensor has not been correctly accounted for. F
The CNG Kit on your car SHOULD have the following components EXACTLY as stated below:
CNG Kit Landi Renzo TN1B + LC 01
STAP 78 (Spark Timing Advance Processor)
Emulator LR Bosh II (For Honda)
It is important not to mix and match components (such as a Neotek Advancer and landi renzo reducer and all components should come from one manufacturer).
Secondly as stated above as well, the importance of connecting the to all relevent sensors on the engine such as Throttle Position Sensor (TPS), Oxygen Sensor, Crank Sensor etc.
LC01 is basically a small computer operated stepper motor that ensures that the correct amount of gas goes into the engine and it is required for your engine because your engine is of EURO 3 Specifications. LC01 does require a computer and a trained technician to calibrate the computer for your engine the time it is installed.
Be careful of the locally made Kits stamped as Landi Renzo. They are dangerous and hurt your engine.
I strongly suggest that you go to a REPUTABLE WELL KNOWN SHOP (NOT road side conversion Novices) to get your car checked professionally on the basis of the Points mentioned above.
Hope this helps