Is the smell coming in cabin, engine compartment or boot area? First trace the source using sense of smell then check with surf solution. Best way to check CNG Leakage is to make a mixture of surf and water and apply it on all joints you see in the gas pipes. If there is leakage somewhere you will see bubble arising from there. Start checking from boot where the cylinder nozzles are, then engine bay check the whole kit even the CNG pressure guage, mine got leaked and it was the last thing i got checked.....
jerks on 2nd gear (I'm assuming its on both petrol and CNG) can be due to weak plugs, plug wires, ignition coils. Check them one by one...
There is a default noise from FF CNG injectors, but that is on idling only as soon as you press the throttle it disappears. your problem is different i guess. Get it checked properly....