A Cub Cadet 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. Remove the spark plug wire prior to performing repairs and follow the safety precautions listed in the Cub Cadet 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.
12 Reasons a Cub Cadet Lawn Mower Runs Rough & Bogs Down
Plugged Air Filter
If you don’t regularly clean or replace the air filter to keep it in good condition, it can become so plugged with dirt that sufficient air isn’t able to pass through it.
A clogged air filter will not only cause an engine to run rough, but it can also overheat the engine.
The air filter’s function is to keep dirt from being pulled into the air intake resulting in engine wear. Always run your mower with an air filter and make checking the filter part of your mowing pre-check routine.
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 mower in very dusty conditions.
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 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. 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.
Clean a foam air filter:
- Remove the air filter from the housing.
- Wipe out any dirt that is in the filter housing. Don’t allow any dirt to fall into the air intake.
- Inspect your filter. If you find any dark spots, or tears or your filter is dry and brittle, you must replace your filter with a new one. If it appears to be in good condition proceed with cleaning it.
- Wash your foam filter with water and mild detergent. Rinse to remove the soap from the filter.
- Lay flat to dry. Placing your filter in the sun will help speed up the drying process.
- Once the filter is completely dry, coat it with foam air filter oil. You want it completely covered with oil, but you don’t want it to be dripping with oil. If you get too much oil on the filter, ring out the extra oil or use a paper towel to absorb the excess oil. (NOTE: Apply oil to a primary foam filter. Never apply oil to a foam pre-filter that is used in conjunction with a paper air filter).
- Reinstall the air filter and attach your air filter housing cover.
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.
Old gas is often the root cause for a Cub Cadet running rough and sluggishly. This is because gas begins to break down as soon as 30 days after purchase becoming less effective.
Most gas contains ethanol. This is an alternative plant-based fuel that attracts moisture to the fuel system. The water and ethanol mixture leaves behind varnish and sticky deposits when it evaporates.
These sticky substances can cause fuel restrictions and component failures.
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 Cub Cadet Lawn Mowers Use.
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.
Plugged Fuel Filter
Most Cub Cadet mowers use an inline fuel filter placed in between the fuel lines to strain fuel as it comes out of the fuel tank to keep dirt and debris from passing into the fuel system.
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.
The fuel filter is a maintenance part that should be replaced annually to reduce the likelihood of a clogged filter.
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.
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 the Carburetor, first make sure you are getting fuel to the carburetor. If you are not, check for a clogged fuel line or fuel filter.
Make sure the fuel pump is pushing fuel to the carburetor if your Cub Cadet requires a 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 Cub Cadet Lawn Mower Carburetor: Step-By-Step.
Bad Gas Cap
The fuel tank must be able to vent allowing air to pass in and out of the fuel tank. On a Cub Cadet lawn mower, the fuel tank vent is built into the gas cap.
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 gas cap.
Dirty Spark Plug
The Cub Cadet 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.
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.
Engine Speed is Too Low
It takes a lot of engine power to run a Cub Cadet 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.
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.
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 Cub Cadet Mower Blades.
Still Having Problems with Your Cub Cadet Mower?
If these tips haven’t solved your problem or if you are experiencing a different problem with your mower, check out my guide showing the most common issues and solutions: Common Cub Cadet Problems.
Here you’ll find the causes of issues like the mower not starting, a bad cut, a vibration, or a smoking problem. I include solutions along with links to more in-depth information.