Walking behind a lawnmower while it's backfiring could make you feel like you're a character in a comedy movie if it weren't so annoying. According to Briggs & Stratton, a leading manufacturer of lawnmower engines, muffler backfires won't harm your lawnmower, but some backfires originate inside the engine and aren't as benign. Neither type will win you any points with the neighbors. Understanding how lawnmowers work can help you to understand how backfiring happens.
Combustion Chamber Backfires
Most lawnmowers have a four-stroke engine. In such engines, a mixture of gas and air enter the combustion chamber on the intake stroke and are ignited by the spark plug on the compression stroke. The explosion drives the piston on the power stroke, and the engine expels exhaust on the exhaust stroke. Backfiring can happen if combustion occurs during any other than the compression stroke. The cause may be sticking valves or the wrong gasoline mixture, and it may affect the performance of the engine. It isn't as common as muffler backfires, which happen for a different reason.
Pops and Crackles
If you hear a loud explosion or popping sounds when you lower the speed of the engine or just after you shut it off, the muffler is backfiring. It's happening because fuel is making its way into the muffler -- which is hot -- and igniting. It usually happens because, as the engine is slowing down, some unburned fuel exits through the exhaust port. Muffle backfires can be particularly disconcerting when they occur after you shut off the engine. The cause is often a combination of engine and muffler overheating.
Watch the Gas
Because muffler backfires don't harm lawnmowers, many people just put up with them. If you prefer the purr of a well-tuned engine to the crackling sound of a backfiring one, however, examine the fuel you're using. Gasoline that has a high concentration of ethanol burns hotter than pure gasoline and is one of the main causes of muffler backfires. It's also advisable to throttle-down more gently. If your mower backfires after you shut it off, it's because the engine and muffler are too hot. Letting the engine idle for 15 to 30 seconds before shutting it off gives it a chance to cool down.
Combustion chamber backfires are generally more noticeable than muffler backfires, and diagnosing and correcting them takes a bit more effort. They can happen because the spark plug is misfiring. If cleaning or replacing the plug doesn't correct the problem, the carburetor mixture may be too lean -- too much air in the mixture makes it more volatile. That may be because you inadvertently left the choke on, but if it isn't, try adjusting the carburetor mixture, following instructions in your owner's manual. If nothing works, the best course of action is to take the lawnmower in for service. The valve seals could be worn or the carburetor in need of replacement.