cng burns lean. it burns at much higher temperature than petrol.
your engine is not designed for this.
continuous use will lead to leaking valves and rings. engine will start smoking.
cng has lower thermal value / energy than petrol, you will need to rev the engine higher to get some acceleration out of a cng engine. this continuous high reving will lead to reduced engine life.
i have kept cuore on cng driven to 140,000 km, engine started slight smoking after 110,000 km
i have civic, driven 218,000km always on petrol, no sign of smoke or oil consumption.
point is simple, petrol engines are not designed to be used on cng.
dedicated cng engines have different design to petrol engines