My car stats are as follows:
Car: Honda VTI Prosmatec 2004
CNG Cylinder: 55 Kg
Kit: Non-Electronic
I think it also has an advancer and an emulator.
City: Islamabad
City Mileage: ~100 with A/C.
Highway Mileage: 150 - 170 with A/C.
VTIs are very special cars. Most of our mechanics do not know them. Yes, that right.
VTIs come with a state of the art technology Honda invented and installed in her cars/bikes long time back. That is called VTEC Engines.
These engines are no ordinary EFI engines. I know my VTEC has a 3 profile CAM which means that the engine has three different profiles for valves opening and closing (in other cars there is only one setting). THis has been done to get the engine moving in the most optimum speed range ALWAYS.
Personally I guess that since no one REALLY understands the engine, changing the engine in anyway is not recommended in a VTI.
However anyone who is getting less than 140 - 150 km on a 55Kg cylinder with A/C in his honda on long route (motorway), should keep trying on finding the problem.
Let me also tell you (from my experience) that Honda is a sophisticated car and probably wants cleaner air too. Be very careful on cleaning of the air filter as that can remarkably improve your car performance on CNG.
Also before putting any blame on CNG, run your car on Petrol and find its average etc. Then switch to CNG and do the same.
As a rule of thumb, if your car behaves nice (mileage wise) on petrol it should be good on CNG.