Outdoor Power Equipment and Small Engines - Lawnmower Backfires When Shut Down

What are some common causes for a 17hp lawnmower engine backfiring when shut down? And how can I fix the problem? Thanks in advance! WolfpackRon.

I have an 18 hp troy-bilt. The manufacturer says that the backfire upon shutdown is annoying but normal. You might check with the manufacturer and see what its position is.

Hope this helps.

When you shut the engine off you kill the spark. That doesn't mean that the engine stops turning immediately. There will be many piston strokes before the engine stops turning. On each stroke air and gasoline are sucked into the combustion chamber, but is blowed out because there is no spark to ignite the mixture. On the last couple of stokes as the piston gets real slow the air/gas mixture is slowly moving through the hot muffler and exhaust pipe. That's when you will get ignition. Fuel is actually ignited by the exhaust system and pops when ignited. About the only way to stop that is to redesign the exhaust system and/or shut the fuel off with the spark. Small engines mostly aren't designed with features like that in mind.

Thanks Jughead and CHFITE. It is annoying and sometimes loud. Being an engineer, i appreciated the detailed answer. Thanks, WolfpackRon.

Ron,

What kind of engine? Many engines have anti-backfire solenoids, and some engines can be fitted with one. Also, slowing the engine speed to idle will normally stop the backfiring on shutdown.