If your engine backfires when trying to start and the valve timing is correct, then it could be electronic ignition unit or the pick up that causes the spark to occur with wrong timing.
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.
Out of time.
Too much PSI.
Backfire is normally caused by engine out of time, or spark plug wires installed incorrectly.
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. You have to figure out why.
As with any engine, timing, and also carb problems.
Timing is off. You may need a new timing belt.
Several issues can cause a back fire:OverheatingLean mixtureFaulty ignition
That usually means the engine is lean. Could be a vacuum leak.
Incorrect Timing Can Cause Backfire.
Timing , pure and simple. If the timing is out then your fuel / mixture will detonate when the piston is in the wrong position in the cylinder. This can lead to a backfire