With almost anything engine related, there can be several causes for a riding lawn mower sputtering and backfiring. Most of them relate to the fuel system. Read on to find out more.
Perhaps a more accurate way to say that is that most commonly, it’s related to the fuel system. Generally, the things that cause a backfire are: a timing issue, electrical issue, or fuel issue.
What Causes a Backfire
In simple terms, a backfire is when fuel vapors ignite in the exhaust system and/or carburetor. This can include a starling bang and even balls of flame.
Tools Needed (Commissions Earned)
Diagnosis of a Backfire
First, try to identify when the machine backfires – starting, running, or shutting off? This can help point you in the right direction.
Riding Lawn Mower Sputtering and Backfiring During Startup
If it’s backfiring when you attempt to start it, start with the timing. A flywheel is what “times” the engine so the valves open and close in the correct order and at the right time. That time coordinates with the spark generated by the plug to create ignition.
So if the vapors are igniting and producing a backfire, that means that the ignition is occurring, just not in the combustion chamber, which would mean that the timing is probably off.
Fortunately, this is a simple enough fix. The timing of the flywheel is governed by the flywheel key. This small rectangular piece of metal in the shaft holds the flywheel in the correct sequence.
If you hit a rock with the riding mower, for instance, the flywheel key can become sheared. This allows the flywheel to become dislodged and offset the timing.
Riding Lawn Mower Sputtering and Backfiring While Running
If the engine is backfiring while running, the most likely cause is the carburetor. This device proportions the air and fuel into the appropriate mixture for the machine to run efficiently.
That being said, if it is “too rich” or “too lean” the machine is getting either too much or not enough fuel. Either case can produce backfiring.
You can simply adjust the air/fuel mix screw on the carburetor, but I would recommend doing a complete overhaul of the carburetor.
If the mixture has gotten to this point that you need to start adjusting, then that can be a symptom of a bigger problem. Instead of just putting a band-aid on the issue, let’s go ahead and fix the problem.
Typically what happens is that the carburetor jets get dirty and fouled from old fuel and/or dirt. This causes the machine to run too lean, so you adjust the mix up to add more fuel.
That will work for awhile, but over time it will continue to get worse and worse. A better option is to simply take the carburetor apart and clean it thoroughly with an ultrasonic cleaner.
Riding Lawn Mower Sputtering and Backfiring While Shutting Off
This is simply caused by shutting the machine off too abruptly. When cutting the grass, it’s important to run the throttle wide open; that’s the way the engine is designed.
However, if you abruptly shut the machine off with the throttle wide open, it’s still sending fuel to the combustion chamber as it is not anticipating the shut off.
Since the machine is now shut off, however, there is no accompanying spark to ignite the vapors. Instead, they travel through the exhaust and are ignited by the heat of the muffler. This ignition in the muffler produces the backfire.
A simple fix to this issue is to simply throttle down the machine before shutting it off. This will reduce fuel into the combustion chamber and prevent backfiring from occurring.
What Else to Check
Anytime there’s an issue with an engine, it’s customary to check the “Big 3.” That is, compression, spark, and air/fuel mix. So I’d be remiss to not mention spark in this article.
If the spark plug is dirty or damaged, the spark can become weak. This can effectively prevent the spark from firing the vapors in the combustion chamber.
So essentially, it might be firing every other cycle, for instance. These “wasted” vapors then exit the exhaust where the heat from the muffler can ignite them.
Furthermore, fuel type can have an effect on backfiring as well. I highly recommend using ethanol free gasoline for power equipment, but in certain parts of the country I understand that it’s not readily available.
One of the problems with ethanol is that it is more easily ignited. This can produce backfiring as the timing of the machine is thrown off.
Recap
So there you have it; some of the common causes for a riding lawn mower sputtering and backfiring. Any extra tips? Let me know below!
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