For a carby car, there is another thing to look into. To avoid any future problem
With appearnace of this problem indicating that external fuel cut solenoid might be stuck or leaking, the fact is also highlighted that even on CNG, the fuel pump is working and filling up the float.
Have faced a dead fuel pump in the past. Turns out, Suzuki never thought to disengage the pump while disengaging the fuel supply on a FF CNG kit. And since this fuel cut was on fuel supply line before carb, and the return circuit comes from within the carb, there's no way to use the return line which could have saved the pump under high pressure conditions (caused by the CNG kit fuel solenoid).
So to conclude, after getting a new solenoid, You also have install a relay in line with the power wire going to the pump. This relay is activated by CNG switch on the pin that activates the subject solenoid. This turns the fuel pump on only during Petrol use. and prolongs its life.
Any desi electrician can do this job. Not a big hassle.
And for an EFI car, if you dont want to repair the emulator and like doing DIYs or like explaing to your electrican about what to do, find the positive wire that is common to all injectors. A multimeter can be used for this and tested by turning the ignition ON (not vehicle start). Connect an external relay in line with it that is activated by CNG switch on petrol. But this is an improper cost saving solution.