Actually I am running my santro on cng so it occasionally back fires on cng and air filter box cover opens up by back fire pressure, the back fire issue was so annoying,
So I being an engineer, had already changed spark plugs to denso iridium which I imported & installed myself in the car,
the other reasons for back fire suspected to be ignition cables or ignition coils , which upon checking by a mechanic came out to be ignition coil
causing missing/ weak spark in a cylinder so replaces the coil.
It costed 4500 including labor, its not new though but kabli coil, mechanic gave guarantee of 1 month also that if back fire arises again then he will be responsible