Out of time.
Too much PSI.
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.
Backfiring is usually an indication of a lean condition. When it does it during cranking, it's probably timing related.
Incorrect timing or plug wires connected incorrectly.
Several issues can cause a back fire:OverheatingLean mixtureFaulty ignition
A backfire is caused by many different things. Using the wrong grade fuel will cause a backfire, as the engine's compression ratio is usually too high and will detonate the Air/Fuel mixture, causing a backfire. It can also be caused by advanced or retarded timing. Advanced timing will cause a backfire through the intake, as the spark occurs too early and ignites the Air/Fuel mixture before the intake valve is closed. Retarded timing causes a backfire through the exhaust as the spark occurs after the exhaust valve has opened. Your problem seems to be advanced timing. A backfire is caused by many different things. Using the wrong grade fuel will cause a backfire, as the engine's compression ratio is usually too high and will detonate the Air/Fuel mixture, causing a backfire. It can also be caused by advanced or retarded timing. Advanced timing will cause a backfire through the intake, as the spark occurs too early and ignites the Air/Fuel mixture before the intake valve is closed. Retarded timing causes a backfire through the exhaust as the spark occurs after the exhaust valve has opened. Your problem seems to be advanced timing.
That usually means the engine is lean. Could be a vacuum leak.
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.
The timing of the engine is off or the spark plug wires have been hooked up incorrectly.