It could be a few things. possibly the timing is off or the carbeurator isn't adjusted properly
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.
It backfires because if you are going high speed and then suddenly let off the gas the extra gas that is flowing in has to come out some time another way it backfires is that something on the exhaust or carbeurator is loose or is missing something.
Your car is running too rich.
bad gas
Too much PSI.
Out of time.