When troubleshooting why your lawn mower’s engine begins to sputter or run rough, begin by looking at items that can keep the engine from getting sufficient air, fuel, and spark. Then look for items that can put extra strain on the engine.
12 Reasons John Deere lawn mowers are running rough:
- Plugged air filter
- Stuck choke
- Old gas
- Clogged fuel line
- Plugged fuel filter
- Dirty carburetor
- Bad gas cap
- Dirty spark plug
- Fast ground speed
- Low engine speed
- Plugged mower deck
- Dull mower blades
Before working on your lawn mower, wait for the engine to cool and for all moving parts to stop moving. Remove the spark plug boot(s) prior to performing repairs and follow the safety precautions listed in the John Deere operator’s manual.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
This is Why Your John Deere Lawn Mower Runs Rough & Sputters
1. Plugged Air Filter
The air filter can become plugged with grass clipping, dirt, and other debris when it isn’t regularly cleaned to keep it in good condition.
It offers protection to the engine by only allowing clean air to pass through it and into the engine. Without an air filter, dirt and other contaminants can cause engine damage.
I recommend replacing the air filter once a year and cleaning it several times throughout the mowing season.
You may have to clean or replace the filter more often when you are running the John Deere in very dusty conditions or you use your mower more than the average homeowner.
SOLUTION: Check the condition of the air filter and clean it if it’s a little dirty. When the filter is extremely dirty, damaged, or no longer creates a good seal over the air intake, replace it with a new air filter. Follow these instructions for cleaning your type of air filter.
Clean a John Deere lawn mower paper air filter:
- Remove the air filter from the housing.
- Wipe out any dirt remaining in the housing. Be careful to not let any dirt fall into the air intake.
- Tap your filter against a solid surface. What you are trying to do is knock as much dirt out of the filter that will come loose and fall out.
- Hold your air filter up to a light source and make sure you can still see light shine through the paper element. If you can, go ahead and reuse your air filter as long as it isn’t damaged. If you can’t, it’s time to buy a new one.
- Reinstall the air filter and attach your air filter housing cover.
- If your air filter also uses a foam pre-filter, wash the filter with mild detergent and water. Rinse and allow to air dry. DO NOT APPLY OIL to the pre-filter.
2. Stuck Choke or Wrong Choke Setting
The choke is used to restrict the amount of air that is mixed with gas to form an explosion to start a cold engine. This allows the fuel mixture to run rich with more fuel and less air.
Once the engine starts and warms, the choke lever must be adjusted to the off position to allow additional air required for the engine to continue to run.
When the John Deere choke is in the wrong position or is stuck in the open or closed position, the engine may begin to run rough and bog down.
SOLUTION: Verify the choke lever is in the closed position to start a cold engine and open position to start and run a warm engine.
If the choke lever is in the correct position and you continue to have airflow problems, check the choke plate to ensure it is opening and closing when adjusting the lever.
Use a carburetor cleaner to free up a stuck choke and replace a choke cable that is worn and no longer functioning correctly.
3. Old Gas
Gas only stays good for about 30 days before it begins to break down. The ethanol used in most gasoline today has negative effects on the fuel system and engine.
Ethanol is an alternative fuel added to make gas more environmentally friendly because it’s made of a renewable resource: high-starch plants.
This fuel naturally attracts water from the air to the fuel. Water doesn’t mix with gas. The ethanol and water mixture is corrosive and will separate from fuel leaving behind varnish that causes fuel restrictions and component failure.
The fuel restrictions created will keep the engine from getting the fuel it requires for good engine performance.
SOLUTION: Replace old fuel in the fuel tank. Add fresh gas and a fuel system cleaner and stabilizer like Sea Foam Motor Treatment to help reduce residue and moisture left behind by old gas.
Use a gas that has a minimum 87-octane rating and maximum 10% ethanol content. An ethanol-free gas is best. For more information on choosing the right gas, check out This is the Type of Gas John Deere Lawn Mowers Use.
4. Clogged Fuel Line
Sticky deposits that develop in the fuel system can get lodged in the fuel line restricting the fuel flow. In order to find a clogged fuel line, use the fuel shut-off valve or hose crimp pliers to stop and start fuel flow as you check the flow coming out of a line.
Use a container to collect fuel as you check the fuel flow. This container must be placed lower than the fuel tank. Fuel cannot run uphill without the use of a fuel pump.
SOLUTION: Shut off the fuel supply and remove the fuel line that is clogged. Try to loosen the clog by spraying carburetor cleaner into the line.
Next, blow compressed air into the line to attempt to dislodge the clog until it is removed. Repeat using the carburetor cleaner and compressed air until there is no longer a clog.
Replace a fuel line with one of the same length and diameter when you are unable to remove the fuel restriction or you find the fuel line is developing cracks.
5. Plugged Fuel Filter
An inline fuel filter is used on the mower to protect the fuel system and engine from dirt and other contaminants. Like the air filter, the fuel filter is a maintenance part that must be replaced to be kept clean.
The fuel filter can become plugged with dirt so a good flow of fuel isn’t able to pass through it. The carburetor won’t get the fuel it requires and may cause the engine to bog down and run rough.
It’s best to change the fuel filter annually and replace it more frequently if it becomes plugged or is damaged.
SOLUTION: Remove an old dirty filter and install a new inline filter. Look for an arrow on the new filter and make sure the filter is installed with this arrow pointed in the direction of the fuel flow.
6. Dirty Carburetor
The carburetor is responsible for mixing gas and air to form combustion in the engine. When the carburetor isn’t functioning as designed, the engine may run rough because it isn’t getting the correct gas and air mixture.
When isolating a fuel problem to a faulty John Deere carburetor, first make sure you are getting fuel to the carburetor. If you are not, check for a clogged fuel line, fuel filter, or bad fuel pump.
Next, remove the air filter from the air filter housing. Spray carburetor cleaner into the air intake and allow the mower to run.
If the engine runs well and then begins to run sluggish again, chances are you need to clean the carburetor.
SOLUTION: Clean the carburetor using these instructions in for cleaning a John Deere lawn mower carburetor.
7. Bad Gas Cap
A John Deere gas cap is designed to allow air to pass through the cap to allow the fuel tank to vent. When the gas cap is bad and no longer vents, a vacuum will form as fuel is consumed and air isn’t able to get into the tank.
This vacuum will keep sufficient flow from flowing out of the fuel tank. If you experience a rough running mower, use a pressure gauge to test the vacuum in the tank or follow the instructions below.
Tips for identifying a plugged fuel tank vent in the gas cap:
- When the engine is running rough, loosen the fuel cap to allow air into the fuel tank. I
- f the engine immediately begins to run better, you may have a bad gas cap that is no longer venting properly.
- To confirm the problem, retighten the gas cap and continue to allow the mower to run.
- When it begins to run sluggish, loosen the cap and listen to the engine. Again, if it no longer runs rough after you loosen the cap, the gas cap is likely the problem.
SOLUTION: Purchase and install a new John Deere gas cap.
8. Dirty Spark Plug
A fouled spark plug may fire intermittently causing the engine to run rough and sputter. Other items that can affect spark is the electrode gap on the plug and loose spark plug wires.
SOLUTION: Remove and inspect the spark plug. If you find it is very dark in color, has a burnt electrode, or is damaged, replace it with a new one. If it’s in good condition and only a little dirty, remove the buildup with a wire brush.
Verify the spark plug is gapped to the engine manufacturer’s specification. Securely attach the spark plug wire once all repairs have been made.
9. Ground Speed is Too Fast
The mowing conditions determine the speed you should be mowing at. A lawn that has thick, tall, or wet grass must be mowed at a slower speed than a lawn that is dry and doesn’t have tall or thick grass.
Mowing at too fast of a ground speed for the mowing conditions will put extra strain on the engine and can cause it to bog down. It will also leave you with a poor cut quality with your John Deere.
SOLUTION: Assess your mowing conditions and adjust your ground speed. Slow down when you hear your engine bogging down and not running strong.
When mowing tall grass, it’s best to make multiple cuts. It will take a lot longer, but the cutting results will be better and you won’t overwork the engine.
Do this by setting your mower’s cutting height at its highest setting for the first cut and then lowering the cutting height for subsequent cuts.
10. Engine Speed is Too Low
It takes a lot of engine power to run a John Deere mower and mower deck. The engine speed should be run at its highest speed when engaging the blades or the mower will begin to run rough.
SOLUTION: Adjust the throttle lever to the highest engine speed. On many mowers, this is signified with a rabbit icon.
11. Plugged Mower Deck
A mower deck that is plugged full of grass clippings and debris will cause the engine to work harder in order to turn the blades through the plugged deck.
Having to turn the blades through this additional material will strain the engine and cause it to run sluggishly.
SOLUTION: Scrape the mower deck with a deck scraper or wire brush to keep it clean. Avoid cutting wet or damp grass. This grass is more prone to clumping and sticking to the deck.
You can attempt to use a deck spray to keep debris from sticking. While it does help, it is not a miracle solution that is going to keep all grass from sticking to the deck.
12. Dull Mower Blades
Sharp mower blades help achieve a nice cut. Dull mower blades may tear the grass leaving it with brown tips a couple of days after mowing due to bruising.
In addition to a poor-cut appearance, dull mower blades can also magnify the problem of a plugged lawn mower deck. This will further contribute to the engine bogging down because it takes more engine power to turn dull blades through a deck full of grass.
SOLUTION: Remove the mower blades from the deck. Sharpen and balance the blade following the instructions in Change & Sharpen Your John Deere Mower Blades.
Still Having Problems with Your John Deere Lawn Mower?
As a John Deere owner, you will encounter a variety of problems over the life of the equipment. These can include problems with starting, dying while mowing, vibrating, cutting unevenly, and not moving.
To help you identify the reasons your mower is having problems, I put together a handy guide to help you troubleshoot your mower. Check out Common John Deere Lawn Mower Problems and Solutions.