Dear axim,
Main problem arises due to two reasons.
Spark plug wires (get them changed)
Coil.
Basically the fault is of the coil. I had that changed and problem resolved. Also had the spark plug wires changed. Changing them increases the pick of the car quite a bit.
I have santro since 2006, when new it had a very strong engine and visited murree etc and simply no problem, engine power simply un matched by any other car in same category.
But after just 2 years of use, back fire problem started.
There are too many sensors in this car and they usually go bad very quickly
By the way sometimes a Jinn goes in to the car and then he will not come out easily. This is the main problem of this car