A string trimmer engine that bogs down will not run at full RPMs and give you the power you need to trim the grass and weeds in your yard. You will end up with a subpar trim job where all of the grass isn’t cleanly cut. It may not even have the power to cut the thick weeds.

A string trimmer runs rough when old or dirty fuel causes fuel restrictions in the fuel filter, fuel lines, or carburetor. It also runs rough when the string trimmer isn’t getting sufficient airflow or spark due to a plugged air filter, dirty air plug, or plugged spark arrestor.

Be careful when working on your string trimmer to avoid injury. Follow all safety guidelines provided in the operator’s manual. This includes removing the spark plug wire and waiting for all moving parts to stop.

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Follow all safety instructions provided in your equipment operator’s manual prior to 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.

Reasons Your String Trimmer Runs Rough & Bogs Down

Old or Bad Fuel Causes a String Trimmer to Run Rough

Gasoline can have negative effects on your string trimmer. It’s important to use the correct fuel and properly care for it. Old fuel attracts moisture that can leave behind varnish and gummy deposits that clog the fuel system and cause components to fail.

Because gasoline breaks down as quickly as 30 days after purchase, it’s important to use fresh gas and consume it within this time frame. If you are unable to use a tank of fuel this quickly, add a fuel additive like Sea Foam to stabilize it and reduce the negative effects of fuel.

Most string trimmers used today have a 2-cycle engine that requires gasoline and oil to be mixed before filling the fuel tank. You will also find some 4-cycle string trimmers. Oil and gas are not mixed for these types of engines.

Adding the wrong type of fuel to your string trimmer can damage the unit beyond repair. Read more about this in, “This is the Type of Gas to Use in a String Trimmer“.

Dirty Spark Plug Causes a String Trimmer to Run Rough

A dirty or damaged spark plug can cause it to misfire causing the string trimmer to run rough. A loose spark plug wire or incorrect gap can also be responsible for a poorly running string trimmer that bogs down.

Remove the spark plug using a socket wrench. Check its condition. Replace the spark plug if you find the tip is very dark in appearance, the porcelain is cracked or the electrode is burnt.

If you find your spark plug is just dirty, clean it with a wire brush or replace it with a new spark plug.

Check the spark plug gap using a feeler gauge. Install the new or clean spark plug and make sure the spark plug wire is securely attached.

Plugged Fuel Filter Causes a String Trimmer to Run Rough

The fuel filter attaches to the end of the fuel line and sits in the fuel tank. Fuel is filtered to prevent dirt and debris from entering the fuel system.

When the filter becomes clogged so a good steady flow of fuel isn’t able to pass through the filter, the trimmer will begin to run rough because it isn’t getting sufficient fuel.

A plugged fuel filter must be replaced. Set the trimmer on a flat surface, and remove the fuel tank cap. Pull the fuel filter out of the tank. You may want to use a clean bent wire to “fish” the filter out of the tank. Holding the fuel line and ring clip in one hand, securely grab the filter and pull it out of the fuel line.

Do not lose the ring clip and keep it on the fuel line. Install a new fuel filter at the end of the fuel line. Slide the ring clip so it sits over the fuel line and fuel filter end to secure the filter to the line.

Clogged Fuel Lines Cause a String Trimmer to Run Rough

The fuel lines can become clogged if dirt gets into the fuel system. Gummy deposits left behind from running old fuel can also cause a blockage. Look for a clog or a kink in the fuel line that can cause the string trimmer to run roughly from a lack of fuel.

Replace any fuel lines when you find a restriction, fuel leak, or dry cracked lines.

Plugged Fuel Tank Vent Causes a String Trimmer to Run Rough

The fuel tank vent can become plugged or damaged preventing air from passing through the vent to equalize the air pressure in the fuel tank. A plugged vent can cause a vacuum to form in the fuel tank to keep fuel from flowing out of the tank.

If fuel isn’t running through the fuel lines and you don’t have a clogged line or a plugged fuel filter, there is a good chance the fuel tank vent is plugged.

You can test this by loosening or removing the fuel cap to allow air into the tank. Start the trimmer, and allow it to run.

Be careful and keep your string trimmer level so you don’t spill any gas from the tank. If the trimmer runs strong and doesn’t bog down and run rough, tighten the fuel cap and see if you can replicate the issue to confirm the fuel tank vent is the problem.

Replace the fuel tank vent if your trimmer starts to act up and run rough again after installing the fuel cap.

Plugged Air Filter Causes a String Trimmer to Run Rough

The engine requires air to run. Without enough air, the string trimmer will run sluggish and rough. One of the items that can restrict airflow is a plugged air filter. The air filter can become plugged from dirt and debris when not regularly cleaned and changed.

If you find your air filter is plugged, I recommend replacing the filter. The small air filter is usually not very expensive. It is an important component when it comes to protecting the engine.

A foam, fabric or felt-style filter can be cleaned using mild dish soap and water. Wash the filter to remove as much dirt as possible. Rinse until clear.

A paper filter can be cleaned by knocking it against a solid surface to remove as much dirt as possible. Hold it up to a light source to see if you can see light through the element.

Reuse the filter if you can still see light. Replace the filter if you cannot see light, your filter is very dirty or it is covered in oil.

Because there are so many different types of filters used which varies by manufacturer and model of the trimmer, refer to your operator’s manual for steps to clean the style of air filter in your string trimmer.

Dirty Carburetor Causes a String Trimmer to Run Rough

The carburetor mixes the correct proportion of air and fuel needed for your string trimmer to run. The passageways can become clogged and the small components can fail to function correctly. This can cause an insufficient amount of fuel mixed with air resulting in a rough running string trimmer.

You may be able to clean your carburetor to get it working again. You may end up having to rebuild (if rebuild kits are available for your carburetor) or replace the carburetor if cleaning doesn’t help.

Plugged Spark Arrestor Causes a String Trimmer to Run Rough

There is a small metal screen that keeps hot exhaust material from shooting out of the string trimmer and causing injury or starting a fire. This small screen will become plugged with a carbon buildup that will affect how the engine runs.

Solution: Disconnect the spark plug wire. Remove the engine cover and the engine exhaust cover. Carefully remove the spark arrestor screen. Clean the spark arrestor screen using a wire brush and reinstall it.

If the screen isn’t able to be sufficiently cleaned or you find it is damaged or has a hole in it, replace it with a new spark arrestor screen.

Service Your String Trimmer to Keep It Running at Its Best

Keep your string trimmer running at its best by performing an annual service. This includes replacing the maintenance items: air filter, fuel filter, and spark plug.

This will also include cleaning the string trimmer and removing dirt and debris from the engine covers and around the engine. This is to keep air passages clear to keep the engine cool.

Check out this guide for steps to maintain your string trimmer: “How to Service a String Trimmer“.