Whether you have a riding mower or a zero-turn mower, there may be times when you experience the engine beginning to sputter and no longer running strong.
A Kubota lawn mower begins running rough when the engine isn’t getting sufficient air, fuel, and spark due to a plugged air filter, stuck choke, clogged fuel line, plugged fuel filter, dirty carburetor, bad gas cap, or dirty spark plug.
It may also be the result of overworking the engine due to a fast ground speed, low engine speed, plugged mower deck, or 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 wire prior to performing repairs and follow the safety precautions listed in the Kubota 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 Kubota Lawn Mower Runs Rough & Bogs Down
1. Plugged Air Filter
The air filter is used to keep dirt from entering the air intake and causing engine wear. It must be kept clean and in good condition.
If you neglect cleaning or replacing the filter at regular intervals, it can become plugged with so much dirt and debris that a sufficient amount of air isn’t able to pass through the filter and to the engine.
This can cause the engine to run sluggishly and possibly overheat the engine.
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 Kubota 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 Kubota 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 combustion in the engine. Placing the choke lever in the choke position will reduce the airflow needed to start a cold engine.
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 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 begins to break down as soon as a month after purchase. The ethanol found in most gasoline sold today includes ethanol. This is an alternative fuel that is developed from plants.
Ethanol and the water it attracts separate from gas over time and sinks to the bottom of the fuel tank. This mixture not only leaves behind sticky deposits but is highly corrosive and can damage fuel components.
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 octane rating of 87 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 Kubota 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 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 the instructions in How to Clean Your Kubota Lawn Mower Carburetor: Step-By-Step.
7. Bad Gas Cap
The fuel tank must be able to vent allowing air to pass in and out of the fuel tank. On a Kubota lawn mower, the fuel tank vent is built into the gas cap.
If the gas cap vent becomes plugged and the air isn’t able to pass into the tank, a vacuum will form keeping fuel from leaving the tank.
When the engine is running rough, loosen the fuel cap to allow air into the fuel tank. If the engine 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 and bogs down after you loosen the cap, the gas cap is likely the problem.
SOLUTION: Purchase and install a new Kubota gas cap.
8. Dirty Spark Plug
The Kubota mower may experience intermittent spark when the spark plug is dirty. This can result in the engine running rough. An electrode gap that isn’t set correctly or a loose spark plug wire can also cause intermittent spark issues.
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.
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 give you a poor cut.
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 Kubota 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 Kubota Mower Blades.
Still Experiencing Problems with Your Kubota Lawn Mower?
Own a lawn mower long enough, you’ll start running into problems with it starting, not continuing to run, smoking, leaking gas, giving a bad cut, vibrating, or another issue.
To help you save time and money, I have put together a guide to help you troubleshoot the next problem that develops on your Kubota mower.
You can find this guide at Common Kubota Lawn Mower Problems & Solutions.
If you are unsure how to perform diagnostics and repairs on your Kubota lawn mower safely, it’s best to have a professional complete the repairs.
This will help you avoid personal injury or additional damage to the mower. Your local Kubota lawn mower dealership or lawn mower repair shop will be able to help you solve your problem.