Skip to Content

This is Why Your Ryobi String Trimmer is Running Rough

A string trimmer that is running rough and sluggish will not be able to perform at its best. When you experience this problem, your trimming job may take longer since the trimmer is unable to quickly and efficiently cut through the grass and weeds.

A Ryobi string trimmer runs rough when the engine does not get the air, fuel or spark it requires to run efficiently. This can be due to a plugged air filter, clogged fuel line, clogged fuel filter, dirty carburetor, bad spark plug, plugged fuel tank vent of plugged spark arrestor.

Before working on your string trimmer to avoid injury, wait for the engine to cool down and for all moving parts to stop. Follow the safety precautions listed in your operators manual.

Ryobi String Trimmer Runs Rough

This post may include affiliate links. Purchases made through these links may provide a commission for us, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.

Follow all safety instructions provided in your equipment operators manual prior to diagnosing, repairing or operating. Consult a professional if you don’t have the skills, knowledge or are not in the condition to perform the repair safely.

8 Reasons Your Ryobi String Trimmer Runs Rough & Bogs Down

Old or Bad Fuel Causes a Ryobi String Trimmer to Run Rough

The type of gas you use to fill your fuel tank is one thing you don’t want to get wrong. Gasoline can have negative effects on your string trimmer when you don’t use the right type of gas or you let the gas sit in your trimmer and get old.

Always use a fresh unleaded gas with a minimum octane rating of 89 and a maximum ethanol content of 10%. Old gasoline and ethanol can cause your Ryobi to run rough due to these reasons:

  • Ethanol attracts moisture to the fuel system.
  • Moisture in the system causes corrosion.
  • Old fuel leaves behind a varnish and gummy deposits that restrict the fuel supply by clogging and preventing fuel components from functioning correctly.

Because gasoline breaks down as quickly as 30 days after purchase, it’s important to consume gas within this time. 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 so it last a little longer.

Ryobi offers string trimmers with a 2-cycle engine or 4-cycle engine. You need to know what type of engine is used on your trimmer to avoid damaging the engine when adding fuel. Read more about the right type of fuel and how to care for it in, “This is the Type of Gas and Oil Ryobi String Trimmers Use“.

  • 2-cycle Ryobi string trimmer: Requires a gas and oil mixture.
  • 4-cycle Ryobi string trimmer: Requires a straight gas (no oil mixed in).

Dirty Spark Plug Causes a Ryobi String Trimmer to Run Rough

A fouled spark plug on your Ryobi can cause it to misfire and run sluggish. A damaged spark plug, an incorrect spark plug gap and a loose spark plug wire can also result in running problems with your string trimmer.

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 in good condition and 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 to make sure it matches the engine manufacturers required specification. Install the new or cleaned spark plug. Make sure the spark plug wire is securely attached.

Plugged Fuel Filter Causes a Ryobi String Trimmer to Run Rough

A Ryobi has a fuel filter attached to the fuel line to keep any dirt or debris in the fuel from entering the fuel system. This filter can become plugged when it isn’t changed out regularly. A plugged filter will prevent a steady flow of fuel through the lines because fuel isn’t sufficiently able to pass through it.

A plugged Ryobi fuel filter must be replaced. Set the trimmer on a flat surface, wipe around the fuel cap and then remove it. 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 into the end of the fuel line. Make sure the ring clip is securely holding the fuel line to the male end of the fuel filter.

Clogged Fuel Lines Causes a Ryobi String Trimmer to Run Rough

Inspect the fuel lines and look for a clog or a kink that restricts the amount of fuel that is able to flow through the lines. Clogs can develop from dirty fuel or gummy deposits left behind by running old fuel.

Replace a line with a new fuel line when you find a clog, kink, puncture or leak in the line.

Plugged Fuel Tank Vent Causes a Ryobi String Trimmer to Run Rough

The fuel tank cap has a check valve and filter that can become plugged and no longer allow air to pass through the cap into the fuel tank. When this happens, the fuel tank will form a vacuum keep fuel from flowing out of the tank.

An engine that is no longer able to get enough fuel will run rough and possibly shut down. If you are not getting fuel and you don’t have a restriction due to a clogged fuel line, plugged fuel filter or dirty carburetor, your problem may be a bad fuel cap.

You can test for a plugged fuel vent (check valve) 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 your Ryobi 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 cap if your trimmer starts to act up and run rough again after reinstalling the fuel cap. The cap contains a non-serviceable filter and check valve that must be replaced when the fuel tank is unable to vent.

Plugged Air Filter Causes a Ryobi String Trimmer to Run Rough

A Ryobi 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. You may also choose to clean the filter if it is not damaged.

A foam style filter on your Ryobi sting trimmer can be cleaned using a mild dish soap and water. Wash the filter to remove as much dirt as possible. Rinse until clear and allow to dry.

Add clean engine oil to the filter or a foam filter oil so the filter is lightly saturated. Squeeze out any extra oil. Reinstall the filter.

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

Dirty Carburetor Causes a Ryobi String Trimmer to Run Rough

The carburetor mixes the correct proportion of air and fuel needed for your Ryobi 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 getting it working again. However, you may end up having to rebuild (if rebuild kits are available for your carburetor) or replace the carburetor if cleaning doesn’t help.

Damaged carburetors due to old fuel running through them is a common issue. One that can be minimized by using fresh fuel and properly caring for that fuel.

Plugged Spark Arrestor Causes a Ryobi 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.

Disconnect the spark plug wire. Remove the engine cover and the engine exhaust cover. Carefully remove the spark arrestor screen. You can either clean it with a metal brush or a torch.

If you choose to use a torch, protect your hands and eyes. Hold the screen away from your body using pliers. Torch the screen to burn off soot buildup. Wait for the screen to cool and reinstall it. Reattach the engine exhaust cover and engine cover. Reattach the spark plug wire.

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 Ryobi String Trimmer to Keep It Running at Its Best

Keep your Ryobi string trimmer running at its best by performing an annual tune up. This includes replacing the maintenance items (air filter, fuel filter and spark plug); cleaning the trimmer; changing the engine oil on a 4-cycle model; and replacing missing or damaged parts.

Read more about completing this annual service in “How to Service a Ryobi String Trimmer: Step-By-Step“.