I have 2007 santro, and had misfire problem.
My mechanic was very straight in telling that most mechanic including me cannot pin point on the reason directly.
There is a hit and try required, and each need to changed in sequence.
1) Plugs (may be not functioning proper due to gap) - they mainly ignite.
2) Plug Wire (Sometimes connections are not proper in flowing the current to plugs)
3) Coil (sometimes it don't generate enough current due to malfunctioning)
First time the culprit were plugs gap and second time it was the coil. Best would be to take a good santro along with bad, and exchange the items one by one and when issue is confirmed, purchase the required thing.
Most people waste lot of money in buying items that are later tested as good and not need to be changed.
Another item could be the head, that may have leaking grip, but it rarely happens.
If someone don't find a good santro to test things, the plugs and plugwires are usually same in small cars and can be verified easily. For coil, go to Gulf Autos (near aisha manzil), they usually have repaired coil that they allow to be checked in case of backfire.
Hope my experience help someone.
Thanks to Mechanic (Mustaqeem Sb - Karachi U.P., Next to U.B.L bank) who told me clear facts and didn't make money buy playing with this process. For someone in karachi searching for trusted mechanic who don't play with customers, go there.