please check the throttle body and plugs and kepp them tuned up and also keep on cleaning th eair filter weekly
the other way you have to install the lambda kit with your cng and it will improves your car acceleration and engines power and you willl feel the difference as when you are on cng it will give you performance as you are on patrol.
this lambda kit costs 10000-16000 but check this that it must be installed from a good mechanics
i have installed this kit in my city 2004 manual and i have touched 175 km/h on cng and its performance is awsome and not bad average as 170 km per 60 kg cylinder on single filling with cng