if its cold and you face starting problems with CNG in winter then you should know that CNG engines are supposed to start on petrol when cold then switched to CNG. The 8 hour period is cooling down the engine to dead cold morning levels.
Make sure that your engine has a thermostat valve in the cooling system, if its missing install it - the paagal ke bacche mechanics will tell you that its gunaah, kufr etc etc. to do that, but its there for a reason - to warm up the engine quickly and keep it there, secondly make sure that your engine cooling system is also connected to the CNG regulator - it prevents icing and sticking of the regulator.