i have same expereinces as dharjava
I go to Murre on my spectra factory fitted GV
Till Murree it is fine, i reach there in an hour. ON way to Nathia Gali there is a 2 KM stretch where CNG dont work and petrol is needed, rest is fine.
For an EFI car on CNG, it is not adviseable to go downhill continuously on CNG. u mught end up with lot of backfiring and in extreme situations a blown up or cracked air cleaner houisng.
Ths happens because throttle is not very much open which makes spark weaker whereas engine (Pistons) are sucking full amount of gas because of downhill force on the wheels. this is a perfect recipe for a backfire on CNG.
In short engine revs fast due to downhill forces whereas u dont keep your foot on the throttle so ECU retards spark timing considering that Throttle is not fully open. It works fine on petrol but on CNG ECU is made fool by the emulator. So be carefull.