12 Reasons a Push Mower Runs Rough and Sputters: FIXED!

You notice the engine is sputtering and no longer running strong. This will affect your push mower’s performance.

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.

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 push mower 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 Push Mower Runs Rough & Sputters

1. Clogged Air Filter

While the air filter is used to make sure the engine receives clean air that is free of dirt and debris, it can also be the reason the engine doesn’t get enough air.

The air filter can become plugged with grass clipping, dirt, and other debris when it isn’t regularly cleaned or replaced to keep it in good condition.

This may cause the engine to begin to run rough or possibly overheat and cause the engine to smoke.

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 your push mower in very dusty conditions or you use your mower more than the average homeowner.

NEVER attempt to run your push mower without an air filter. Dirt and debris that reach the engine can cause permanent engine damage.

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 push 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.

Clean a push mower foam 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.
• Wash the filter with a mild detergent and water to remove dirt.
• Rinse until the soap is removed. Squeeze the excess water out of the filter and lay it flat to dry.
• Once the filter is dry, use filter oil or engine oil to lubricate the filter. Get it wet enough to be lightly saturated with oil, but not too much where it is dripping oil. Squeeze the filter to remove excess oil.
• If the foam filter is excessively dirty and has dark spots or tears, you need to replace it. 
• Reinstall the air 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 push mower 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.

Note: Some push mowers utilize an automatic choke. If you have problems with these types, contact your local lawn mower dealer for assistance.

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.

This alternative fuel naturally attracts water from the air to the fuel system. Water doesn’t mix with gas. It leaves behind varnish and is highly corrosive causing fuel restrictions and component failures.

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 fuel and a fuel system cleaner and stabilizer like Sea Foam Motor Treatment to help reduce residue and moisture left behind by old gas.

Push mower with 2-cycle engines and 4-cycle engines have different fuel requirements.

• 2-cycle engines: Use a gas and oil mixture. Mix unleaded gas that has a minimum 87-octane rating and maximum 10% ethanol content with an air-cooled 2-cycle engine oil.
  
  Refer to this article for the fuel mix rates for the most common push mowers.
  
• 4-cycle engines: Use straight gas with a minimum 87-octane rating and maximum 10% ethanol content. These types of engines will have separate fill ports for gas and engine oil. Do not mix.

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.

Remove the line from the carburetor. 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.

Start the fuel flow and watch the flow of fuel coming out of the fuel line into the container.

SOLUTION: If you are not getting good flow, shut off the fuel supply and remove the fuel line from the mower. 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 push mower carburetor, first make sure you are getting fuel to the carburetor. If you are not, check for a clogged fuel line or fuel filter.

Next, remove the air filter from the air filter housing. Spray carburetor cleaner into the air intake. Start the engine.

If the engine runs well and then begins to run sluggish again after the carburetor cleaner burned in the engine, chances are you need to clean the carburetor.

SOLUTION: Clean the carburetor using these instructions for cleaning a push mower lawn mower carburetor.

7. Bad Gas Cap

The 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.
• If 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 push mower 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 push mower.

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 push 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.

You may find a throttle lever or speed settings on your push mower. Make sure the throttle lever is at the highest setting and the speed setting is right for your mowing conditions.

Note: Some push mowers utilize an automatic throttle and you are not required to make any adjustments. If you have problems with these types, contact your local lawn mower dealer for assistance.

SOLUTION: Adjust the throttle lever to the highest engine speed.

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 blade through the plugged deck.

Having to turn the blade 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

A sharp mower blade will help achieve a nice cut. A dull mower blade 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 blade from the deck. Sharpen and balance the blade.