Improper engine timing usually
Could also be leaking / damaged intake valve(s)
or on an older engine with a carburetor if the accelerator pump is bad it will spit back when throttled up quickly
Unburned fuel in the exhaust system causes the backfire from the exhaust pipe.
I think you are hearing pressure pops. Not a backfire.
a backfire is caused by incorrect timing or not enough back pressure in the exhaust pipe
It's lean.
It's lean.
Backfire is normally caused by engine out of time, or spark plug wires installed incorrectly.
bad gas
Your car is running too rich.
Out of time.
Too much PSI.
It's lean. You have to figure out why.
As with any engine, timing, and also carb problems.