A friend of mine has a 96 Polaris 425 Magnum 4x4 with low miles. There is a backfiring sound when you decelerate rapidly after getting on the gas. Any idea what causes this.

Plugged muffler? Spark plug?

Thanks

 

It could be something as simple as the throttle cable stretching and getting loose. Newer Polaris machines with the wires going into the throttle control have what they call an ETC control. What it does is activate a rev limiter (the newer ones may cut the spark entirely) if for some reason the throttle sticks partially open. Sometimes if the throttle cable stretches enough, it will activate the ETC even if the throttle isn't stuck. Set the cable free play and see if this helps.
Backfiring can also be caused by too rich fuel mix, as well as several other possibilities, but the most common cause for backfiring in a Polaris I found was excess throttle cable slack causing the ETC to activate.

 

I've heard a fuel-lean mixture causes backfiring.

Momentary backfiring on deceleration is no serious problem in my view.

Diogenes

 

From my experience when backfiring occurs through the exhaust it's because the motor is running too rich.

 

From my experience, turning the idle screw to fuel-richen the mixture has cured backfiring on deceleration.

Theory; fuel-lean mixtures burn more slowly than fuel-rich mixtures; a lean mixture may be burning at the bottom of the power stroke; when the exhaust valve opens, some burning mixture exits, expanding and causing the "pop" of backfiring.

I'm no authority, sharing my experience and repeating the words of others. Contrary experience/explanation may be just as good.

Diogenes

 

Good points. Anything is possible. But I would still say it’s the unburned fuel entering the exhaust and popping in the muffler. It really shouldn't hurt anything except the pocket book for fuel charges and maybe a fouled plug or two.

 

Whatever. Just one of the references from the search engine, (Gasoline FAQ's):

"If the mixture is weakened ["leaned"], the flame speed is reduced, consequently less heat is converted to mechanical energy, leaving heat in the cylinder walls and head, potentially inducing knock. It is possible to weaken the mixture sufficiently that the flame is still present when the inlet valve opens again, resulting in backfiring."

Diogenes

 

I've found that beans is the biggest cause of back firing, lots and lots of beans!

 

Hiofcer, you make some good points too. Search engineing further, I found some sources suggesting: 1) backfire in intake manifold, too lean; 2) backfire in exhaust system, too rich.

Or, as stated, whatever.

Thanks for an interesting and civil discussion.

Diogenes

 

I was wondering what would cause an engine to backfire through the carb. Thanks