Bro your fuel is leaking. Check your petrol tank cap and pipe connections.
Once after a full refill, the peteol station guy did not properly put the cap back on and I lost almost 1/3 of my tank while I was driving at 120 on motorway on CNG I stopped by and closed the cap and everything was good again.
P.S. if you have carby engine. Petrol & CNG burn together when you are converting from CNG to Petrol during the neutral phase. But you will feel clear jerks when petrol & CNG are burning together in the engine.
Sent from my Nexus S using Tapatalk 2