98 civic problem

What causes car to backfire?

Improper engine timing could be one cause.

Yes, and a slipped or improperly installed timing belt (or worse, a broken one) can do major engine damage.

I have a 1999 Civic and have changed the T belt twice - it’s now close to 190,000 miles and running well. IIRC the recommendation in 1999 was every 90,000 miles and 7 years.

All that has been done before it started was one spark plug was changed because it looked like it was burnt

Take the spark plug that was changed out and see if it’s burned again.

If not, take the other spark plugs out and see what kind of condition they’re in.

Check their gaps, while you’re at it.

If it’s burnt again what all should I do?

This car has a distributor. Its cap and rotor might have corroded electrodes, or one or more wires to the spark plugs could be bad. A spark plug rarely goes bad on its own, so the thing is to look "upstream’ of the plug. Be sure to use only the plug recommended by Honda for this engine. I prefer Honda original caps and rotors, too, but have only replaced them once or twice in 19 years.

Does the check engine light come on? A poorly-working ignition system should show up quickly that way. You might also have a bit of an exhaust leak - the unburned fuel can ignite in the exhaust system if its gets air.

Is this the same car that has been severely overheated and boiled dry, mechanic thinks the block or head are cracked, and the SRS light is on ?

If this is the same abused vehicle in your other threads cut your losses and take what you can get and buy a better vehicle.

Engine light is on. It’s code is cylinders 1&2 misfire and a random misfire.

Yes same one. The SRS light was a fuse and has gone off after changing. I know it’s a lot but it’s all I have for a few more months. I have to do whatever I can to baby it and try to get things done.