I, too, would like to know the answer! I have a 1993 Polaris Trail Boss ATV with the 250 cc engine... it tends to backfire when letting off the throttle.
Your car is running too rich.
running lean can cause your 400ex to backfire ..if it backfires on deceleration its lean ...if it pops on acceleration its running rich.====carb issues
It's broke
Unburned fuel in the exhaust system causes the backfire from the exhaust pipe.
It is timed incorrectly or plug wires are installed incorrectly.
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.
There are many causes to this problem, in a 1999 Ford Ranger 3.0L. It is usually caused by a spark plug firing when its not suppose to, or late/delayed timing.
It's lean.
Backfire is normally caused by engine out of time, or spark plug wires installed incorrectly.
If you are running at high rpms and turn the key off the unburned fuel will be pumped from the cylinders in the engine into the muffler. The muffler is still hot enough to ignite the unburned fuel.