Must be careful, I had this problem my Fuel (Petrol) pump was leaking and if petrol is not leaking then get your CNG/Petrol switch valve checked bcoz it's not stopping petrol. Leaking can cause burn engine... do get it checked...
What is the case that these suzuki cars have mechanical petrol fuel pump which runs continuously and brings fuel from engine whether it's on petrol or cng... then the kit switch valve decides whether to send this petrol to carburetor or back to fuel tank.
I got electronic fuel pump for 750, this is straight forward now and it turns on when I switch CNG button to petrol so valve by-passed.