Too much PSI.
Out of time.
Backfire is normally caused by engine out of time, or spark plug wires installed incorrectly.
As with any engine, timing, and also carb problems.
It's lean. You have to figure out why.
Timing is off. You may need a new timing belt.
When downshifting, the transmission shifts to lower gears which increases engine speed and provides more power for acceleration or braking. It can help control speed during descents or prepare for a faster exit from a corner in driving. However, improper downshifting can cause engine wear or damage if done at high speeds.
Backfiring is usually an indication of a lean condition. When it does it during cranking, it's probably timing related.
Several issues can cause a back fire:OverheatingLean mixtureFaulty ignition
engine backfire during cranking.
That usually means the engine is lean. Could be a vacuum leak.
Incorrect Timing Can Cause Backfire.