you sure. Coz the poster didnt even mention that the cars consuming oil or the exhaust tip is having oil deposits or even the coolant is slimy or the OIL cap is creamy???
@Poster, i cant say anything about sound, but try to experience the same rpm problem on petrol. If it is not there, I believe the CNG kit needs a service. Dont just clean the low pressure diaphram, also drain the 1st stage pressure reducer as well (requires more than just simply opening the drain screw at the bottom of the kit.
If it is still there on petrol, then Mr. Ahsan may be right, the choke needs a lookup. I would suggest to either get the carburator sprayed by the petrol jet or use any off the shelf carb cleaner over the choke and other throttle control pulleys. Might be the oil+dirt deposits hindering a free operation.