I have the same problem of backfiring on CNG, just got my car tunned in Karachi. Initially it was doing good but now it gives a jerk and backfire when i shift it to 2nd gear and this is occassionl. This is the most problematic car i have ever bought... I have bought it for 415k and spent almost 70k uptill now.
I am stuck with this car and i guess this will give my a hefty bump when im about to sell this car.
Can any1 please help me fixing this suzuki ****.