You know your mower best especially when you’re using it weekly throughout the busy mowing season. You notice the engine isn’t running at its best. It begins running rough, sluggish, and bogging down. There are many causes for an engine to begin running rough in addition to engine damage.
A lawn mower will run rough when the air filter is plugged; water is in the fuel tank; old gas causes fuel restrictions in the fuel filter, fuel lines, or carburetor; the spark plug is bad; the gas cap is bad; the mower deck is plugged, the choke is set incorrectly or the ground speed is too fast.
Follow the safety guidelines provided in your operator’s manual. This includes waiting for all moving parts to stop and removing the spark plug wires so your mower doesn’t start while working on it.
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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.
10 Reasons a Lawn Mower is Running Rough
Old Gas or Water in the Gas Tank
Gas that sits in your lawn mower or in a fuel can begins to break down and lose its combustible properties as quickly as 30 days after purchase. Most gasolines contain ethanol, an alternative fuel added to gasoline to make it more environmentally friendly.
Ethanol naturally attracts moisture from the air. It will cause a gummy solution to form causing restrictions in the fuel system. The loss of combustible properties and fuel restrictions can cause your lawn mower to run rough.
In addition to old fuel affecting your mower, water in the fuel system can also result in poor running performance. This can happen when the mower is left outside in a hard rain.
I’ve even seen a little boy trying to help his grandpa by placing a garden hose into the fuel tank. (This little boy didn’t understand there’s a difference between a fuel pump hose and a garden hose).
When water and gas enter the engine, the engine will sputter because water isn’t combustible and the engine has a hard time compressing it.
Solution: Drain your fuel tank using a siphon. Refill it with fuel for your engine type. If you are unable to consume the fuel within 30 days, stabilize your fuel with a product like Sea Foam Motor Treatment. Read more about Sea Foam and its effects on the fuel system here.
Plugged Air Filter
Mowing isn’t the cleanest task no matter what type of mower you are using. Mowers stir up dirt and debris. The dusty conditions can plug the air filter causing the lawn mower to run rough and sluggish when the engine can’t get sufficient air.
Solution: Remove your air filter and check its condition. Clean a dirty air filter and replace it when it is extremely dirty, covered in oil, or not in good condition.
Follow the instructions below to clean a paper air filter. If you have a different type of air filter, refer to “Guide to Lawn Mower Air Filter: Differences & How to Clean Them“.
Clean a lawn mower paper air filter:
- Remove the air filter cover from the air filter housing. The cover is held on by snaps, wing nuts, knobs, or clips.
- Carefully remove the air filter. Don’t allow dirt to fall into the air intake.
- Wipe out dirt and debris remaining in the air filter housing. Again, don’t allow any of it to fall into the air intake.
- Tap your paper air filter against a solid surface. Use the plastic or rubber edge to hit against the surface so you don’t damage the paper. What you are trying to do is loosen the dirt so it falls out of the filter. (Do not use compressed air as this will damage a paper air filter).
- Hold your air filter up to a light source, if you are unable to see light shine through the paper element or it is covered in oil, do not reuse it. If it appears in good condition and you can see light, reuse the filter.
- Install the air filter into the housing.
- Reattach the air filter cover.
Bad or Dirty Spark Plug
A lawn mower may begin running rough and have intermittent running problems when the spark plug is dirty and covered with carbon buildup or oil.
A spark plug must be replaced when the tip is very dark in color, the porcelain is cracked or the electrode is burnt. Make sure the spark plug gap is correct and the spark plug wire is attached securely to avoid problems with your mower not starting or running.
Solution: Remove the spark plug with a socket wrench and replace it with a new spark plug. Because spark is a necessary component to your lawn mower running well, I prefer to use a new spark plug instead of cleaning it.
If you choose to clean a dirty plug, use a wire brush to remove the buildup.
Plugged Fuel Filter
The fuel filter strains the fuel as it comes out of the tank to prevent dirt and other contaminants from entering the fuel system. When the fuel filter isn’t changed out regularly or the fuel is very dirty, the filter can prevent a flow of fuel from passing through the filter.
A lack of fuel required for the engine to run can cause it to run sluggishly when a higher concentration of air than the fuel needed is burned in the combustion chamber.
Solution: Replace a filter that is dirty and doesn’t allow fuel to flow through it. When installing the fuel filter, the arrow on the filter must be placed facing the direction of the fuel flow. The arrow should be pointed toward the carburetor and away from the fuel tank.
Fuel Line Restrictions
The gummy deposits left behind by old fuel can keep a good flow of fuel from moving through the fuel lines. This can cause your mower to run sluggishly or not run at all.
Solution: Find the clog in the fuel line and remove the restriction. For instructions on locating the clog and removing the blockage in the line, check out “Your Lawn Mower Isn’t Getting Fuel: Fix It NOW!“.
The carburetor is used to regulate the ratio of fuel and air mixed to form combustion in the engine. You will find the carburetor mounted to the top or side of the engine block. It is usually below or behind your air filter.
When the carburetor is dirty from deposits left behind by old fuel, the fuel passageways can become clogged, and the small components of the engine can stick and fail to work as designed. This can prevent the right mix of fuel and air to get to the engine causing the engine to run rough.
If you have confirmed the bad spark plug and air filter are in good condition and you are getting fuel to the carburetor, there’s a good chance the carburetor is causing your sluggish running problem.
FIX: Clean the carburetor. This is something most homeowners can do if they are a little mechanical and don’t mind working with small parts.
If that’s not you, your local lawn mower repair shop can do this for you. For instructions on cleaning the lawn mower carburetor on your lawn mower, read this article.
Fuel Gas Cap
A gas cap must be vented to allow air to pass through it. A gas cap that is broken or has a clogged vent will cause the fuel tank to form a vacuum preventing fuel from leaving the tank. This lack of fuel flow will cause the mower to run sluggishly and eventually shut down.
If your mower is running sluggish, remove the fuel cap and allow it to run to see if it continues to run sluggish. (Be careful not to allow dirt or debris into the fuel tank).
If the engine runs fine but starts to run sluggish and eventually dies a little while after you reinstall the cap, you may have a fuel cap venting problem.
Solution: Replace with a new fuel cap.
Plugged Mower Deck
A mower deck that is plugged with grass clippings and debris can put the engine under load where it begins to bog down and run rough.
The engine has to work much harder when it has to turn the blades through a packed deck. Turning dull mower blades through this debris makes the problem worse.
Solution: Scrape the mower deck using a deck scraper or wire brush until the deck is clean. Avoid mowing wet or damp grass as it is more likely to form clumps and stick to the mower deck.
Choke Set in the Wrong Position
If your mower has a choke lever, it might be set in the wrong position. The choke restricts airflow to allow a higher concentration of fuel into the combustion chamber when starting a cold engine.
Once the engine is warm, the choke must be adjusted to allow air to mix with fuel to continue to run.
When the choke isn’t adjusted correctly after the mower warms up, the mower will run rough or stop running because it isn’t getting sufficient airflow.
Solution: Check that the choke is set in the correct position while running the mower. If you continue to have choke problems, you may need to clean the choke and linkage to get it to open and close.
Not Adjusting for the Mowing Conditions
The engine has to work harder when cutting thick lawns or tall grass. You must adjust the mowing speed for your mowing conditions. Your ground speed must be slower when cutting thick or tall grass than cutting thinly covered lawns so you don’t overwork your engine.
When the grass is tall, it’s best to cut at the tallest cutting height for the first cut and then lower the cutting height for your next cut. When the grass is very tall, a brush cutter or rotary mower is a better choice.
Performing routine maintenance on your lawn mower and checking your engine oil before you mow can prevent engine problems that cause your mower to run rough and sluggish.
If none of the items above solve your rough running problem, take your mower to a local small engine mechanic to be diagnosed.
There are internal engine problems that are hard for the average homeowner to diagnose without the proper engine tools to accurately perform tests on the engine.
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.