for a carby car - when dead cold - remove the air cleaner housing, and fill the carb float chamber from its breather, do not switch the CNG switch to petrol (keep it in the middle) and have a freind start the car, the cleaner will clean out the jets, pump the throttle once or twice and keep on refilling the bowl from the breather port.
Switch to petrol, once petrol starts to flow in, clean out the induction throats.