If your lawn mower is making a sputtering sound and running rough, then this indicates that it’s time to service your lawn mower’s engine. Not only will this make mowing much easier and enjoyable, but your lawn mower will be thankful. So, let’s find out why your lawn mower’s engine is struggling and look at common causes of a sputtering lawn mower engine.
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Lawn Mower Sputtering (14 Potential Causes)
Lawn mower engines rely on a long list of components to achieve combustion. If even one of these components fails to perform correctly, your lawn mower will likely make a sputtering sound. Therefore you’ll need to diagnose each component to ensure that they are working correctly and are not the cause of your sputtering lawn mower engine.
- Fuel Type
- Fuel Age
- Gas Cap
- Fuel Tank
- Fuel Lines
- Fuel Filter
- Fuel Pump
- Air Filter
- Carburetor
- Choke
- Ignition Coil
- Spark Plug
- Spark Arrestor
- Oil
Causes of a Sputtering Lawn Mower (A Closer Look)
As you can see, there is a pretty long list of potential reasons that can make your lawn mower sputter. So, let’s dig in and closely examine each item. I’m betting your sputtering problems are caused by one of these.
Check Lawn Mower Fuel Type
First, you must ensure that you use the right type of fuel for your lawn mower. Most mowers typically use regular unleaded 87 octane 10% ethanol gasoline. Using the wrong gas can be enough to cause our lawn mower’s engine to have performance issues, including sputtering. Lawn mowers are designed to create specific engine pressure within the cylinder head to match the octane rating of one particular gasoline. So, buying the wrong gasoline for your mower will definitely cause a few issues.
Check if the Gas is Bad
Does gas go bad? Well, gasoline only lasts about 30 days before it starts to go bad and loses its combustibility. So if you have unprotected gasoline sitting around for a while, your lawn mower will certainly have performance issues. Basically, the older gasoline gets, the harder it is to ignite.
Also, if you leave old gas in your lawn mower for a while, a varnish or fuel gum will build up on various components. If fuel gum accumulates on the fuel lines and carburetor jets, it will affect their performance. So, even if you get rid of the bad fuel, you’ll still need to repair all the components that the fuel gum came into contact with. So, inspect the gasoline visually to see if the fuel is discolored and old.
Faulty Gas Cap
A gas cap does more than just keep the fuel from spilling out of the tank. It also regulates the fuel pressure in the fuel tank. For example, as your lawn mower engine consumes fuel, the volume of gas in the gas tank reduces and is replaced with air. The air enters the gas tank through vents, usually found on the gas cap.
If the gas cap vent is clogged or damaged and air can’t enter the tank, it will create back pressure and reduce fuel flow to the engine until the flow completely stops.
So, if you think you have pressure building in the tank, just remove the gas cap. You should hear the air rush into the gas tank if there is back pressure. Also, by removing the gas cap, the sputtering should stop. So, inspect the gas cap and check if it needs to be cleaned or replaced.
Clogged Fuel Tank
Typically, a gas tank is nothing more than a container to store a volume of gasoline for an engine. There really isn’t much that can go wrong with a gas tank that could cause an engine to sputter. Well, other than the gas tank becoming blocked with debris. If fuel can’t get to the carburetor, your lawn mower will sputter as it is starved of fuel. So you’ll need to check if anything, such as grass clippings, has made its way into your mower’s gas tank that could be causing a blockage.
Check for Clogged Fuel Lines
The fuel line is next on the list of potential reasons why your lawn mower is sputtering. If your mower has been running on bad gas or debris has passed through the gas tank, your fuel lines will likely be restricted or blocked. Either way, your lawn mower won’t receive enough fuel, and the engine will sputter. So, remove the fuel lines and check them for blockages, then clean the fuel lines if necessary.
Inspect the Fuel Filter
Fuel filters need to be replaced from time to time to ensure that the fuel is filtered and to maintain adequate fuel flow. So if your fuel filter is doing its job properly, you need to replace it at least once a year. If you forget, the dirty fuel filter will restrict the fuel flow, and your lawn mower will sputter.
If you have a transparent filter, you can inspect the filter material to see if it’s clogged or discolored. Either way, it will need to be replaced. If your filter is not transparent, try to remember the last time you changed it. If it’s been more than a year, then you’ll need to go ahead and replace your lawn mower’s fuel filter.
Test the Fuel Pump
Bigger mowers like riding and tractor lawnmowers typically use separate pulse fuel pumps. These types of pumps operate on the pressure the overhead valve (OHV) creates. For a pulse fuel pump to work efficiently, it needs a clear path for the fuel to flow, a working diaphragm, and sufficient pressure from the OHV. If there is a problem with any of these processes, your lawn mower will likely make a sputtering sound.
One way to test a lawn mower fuel pump is to remove the outflow pipe and check how the fuel flows. If the flow isn’t strong and constant but sputters instead, you’ll need to clean the lawn mower fuel pump, repair the fuel pump or replace the fuel pump. Unfortunately, most new fuel pumps are sealed units; therefore, you have no choice but to replace the pump.
Check the Air Filter
The next serviceable part of your lawn mower is the air filter. The air filter is designed to allow a certain amount of airflow through the carburetor. This specific amount of airflow enables the carburetor to create a precisely regulated air-to-fuel mixture. If the air filter is clogged, then the fuel mix will be rich, which will not only cause your lawn mower to blow out black smoke, but the engine will likely also sputter.
If the air filter is damaged, say it has a hole, then too much air will enter the carburetor and cause the engine to run lean. This would cause issues such as the mower overheating, the engine making white smoke, and, you guessed it, a mower making a sputtering sound. So, pop out the filter and check if you need to clean your lawn mower’s air filter or if you need to replace it.
Check The Carburetor
Lawn mower carburetors are pretty complex devices that can go wrong in many ways. First, locate the carburetor on your lawn mower. The three main issues I think are worth checking out are the carburetor jet, whether the carburetor is secure, and the air-to-fuel mixture screw.
To start, let’s look at the carburetor jet. The jet is the tiny device the fuel is forced through to create fuel vapor. If the jet is blocked, no fuel vapor will be produced, and there will be nothing for the spark plug to ignite. Therefore you’ll need to remove the jet from the carburetor to ensure it isn’t obstructed or blocked. Typically, cleaning the carburetor will solve any blockage.
Next, a loose carburetor isn’t uncommon, especially with small walk-behind mowers. Basically, the vibrations from the mower cause the carburetor mounting bolt to back off, resulting in the carburetor becoming loose. Once the carburetor is loose, additional air can make its way between the air filter and carburetor and the engine and the carburetor. Consequently, the excess air throws off the air-to-fuel mixture, and the engine will run lean and make your mower sputter. Therefore, pull the carburetor a bit to see if it is loose.
Finally, the carburetor adjustment screw adjusts the fuel-to-air ratio. For example, if your lawn mower runs lean, you can adjust the screw to make the engine run richer. Alternatively, you can adjust the carburetor to run leaner if your lawn mower runs rich. As carburetors and mowers age, they start to need some fine-tuning. Like the carburetor, the carburetor adjustment screws can turn with the engine’s vibration. So if your mower is running either rich or lean, then you’ll want to adjust the carburetor screws.
Choke Settings
The purpose of a choke is to enrich the fuel during cold starts. To do this, the carburetor uses a butterfly valve to limit the opening from the air intake to increase the fuel-to-air proportion in the mix. As complicated as this might sound, the main thing to remember is that the choke is only intended to be used during cold starting. So, if you leave the choke on once your lawn mower has warmed up, your mower will not only sound rough but also continuously sputter. If you have a manual choke, remember to turn it off once your lawn mower has heated up, and if you have an auto choke, make sure it switches off when it should.
Ignition Coil Not Making Charge
A lawn mower ignition coil is a small device that sits against the flywheel and creates the charge to spark the spark plug. If the coil has a short or the gap isn’t correctly set, then the coil won’t consistently produce the charge needed to fire up your lawn mower’s engine. Problems with the coil will make the engine misfire, stall, and sputter during operation.
First, check that your ignition coil gap matches the manufacturer’s specifications. Secondly, check that the resistance through the coil is within range of the manufacturer’s specifications. If either of these is off or out of range, then this is likely the reason your lawn mower is sputtering.
Spark Plug Not Sparking
Connected to the coil is the spark plug. The spark plug grounds the coil to generate a spark that ignites the fuel in the cylinder head. A few things need to be working properly for the spark plug to ground effectively.
First, the gap between the center and ground electrodes must be a specific distance. Secondly, both electrodes must be in working order and not burned out. Finally, the spark plug must be clean and free of carbon deposits.
To check, remove your lawn mower spark plug, check its cleanliness and the gap, then finally, test the spark plug. If your spark plug fails any of these tests, you’ll need to install a replacement spark plug.
Spark Arrestor
So what is a spark arrestor? It’s a small device inside the muffler that catches sparks or flames exiting the engine. The spark arrestor is important because it prevents unintended fires. If the mower’s spark arrestor is clogged, it can make your lawn mower run rough and sputter because it prevents air from exiting. As important as it is for air to enter the engine, it is also essential that air can exit.
For example, as fuel combusts in the top of the cylinder head and pushes down the cylinder, the exhaust valve opens and allows the exhaust gases to escape. This is called the engine’s exhaust stroke and comes after the power stroke. For the engine to continue running, it has to be able to complete the intake and compression strokes.
However, if burned oils and carbon deposits clog the arrestor, the exhaust gases won’t escape efficiently, disrupting the engine’s stroke. The engine will basically suffocate. To check the arrestor, remove it from the muffler and check its condition. In some cases, you may be able to clean it with a blow torch, but installing a replacement spark arrestor is super easy.
Oil Level, Type & Condition
Engine oil plays a big part in keeping your lawn mower lubricated and preventing it from overheating. Not only will your lawn mower overheat with low old oil, but it will also struggle and sputter. An oil change is normally enough to fix the issue, but only if done correctly. If you add too much oil, you’ll likely have too much oil pressure in the sump, which will damage the head gasket. This would force oil into the top of the engine and quickly burn away any oil in the sump. If this happens, you’ll be back to low oil with the added issue of your mower blowing white smoke and the mower overheating again.
Ok, so this might sound pretty dramatic. But not having the right amount of good clean oil is definitely an issue for any lawn mower.
So, first of all, check your lawn mower oil level. If the oil level is low, you’ll need to top it off. If the oil appears dark and dirty or has been in your mower for more than 50 hours or a season, you’ll want to do an oil change, including changing the oil filter. Also, if you add too much oil, remember to drain the excess oil from your mower to avoid a blown head gasket.
Causes of a Lawn Mower Sputtering (Checklist)
Whew–that is a lot of information to take in! To make things easier to remember, I have created a lawn mower sputtering checklist that includes each potential cause, what to check, and the action I recommend you take. Let’s check it out.
Lawn Mower Sputtering Checklist
| ITEM | DIAGNOSIS | ACTION |
|---|---|---|
| Fuel Type | Confirm fuel is the correct type | Remove incorrect fuel |
| Fuel Condition | Do a visual inspection | Remove bad fuel |
| Gas Cap | Inspect the gas cap vent | Repair/replace the gas cap |
| Gas Tank | Check for blockages | Clean out the gas tank |
| Fuel Line | Check for blockages | Clean out the fuel lines |
| Fuel Filter | Check for cleanliness/age | Replace the filter as required |
| Fuel Pump | Test the flow from the outflow pipe | Clean the fuel pump Repair the fuel pump Replace the fuel pump Confirm pressure from OHV |
| Air Filter | Check for damages | Replace the air filter |
| Air Filter | Check for dirt | Clean the air filter |
| Carburetor | Check if the carburetor mounting is loose | Secure the carburetor |
| Carburetor | Check if the fuel jet is clogged | Clean the jet |
| Carburetor | Check the mixture adjustment screws | Adjust the fuel-to-air mixture |
| Choke | Check the manual choke position | Switch choke off |
| Choke | Check the auto choke position | Repair auto choke |
| Ignition Coil | Check coil gap | Reset the gap if incorrect |
| Ignition Coil | Test the coil resistance | Replace the coil if resistance is out of range |
| Spark Plug | Check the gap | Gap the spark plug |
| Spark Plug | Test 2x electrodes | Replace if electrodes are burned off |
| Spark Plug | Check cleanliness | Clean sparkplug and the gap |
| Spark Arrestor | Check if clogged | Clean or replace |
| Oil | Check Condition | Replace if old |
| Oil | Check level | Top off as required |
| Oil | Oil filter | Replace if changing oil |
