It’s important to keep up on maintaining your string trimmer in order to keep it running at its best and to extend its useful life. However, this doesn’t mean your string trimmer won’t be problem-free.
I’ve put together this checklist to help you the next time you run into an issue with your string trimmer. You’ll find troubleshooting tables for the most common string trimmer problems along with links to articles for information in more detail.
Always follow the safety precautions found in your operator’s manual. This includes removing the spark plug wire and waiting for all parts to stop moving and cool down.
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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.
Common String Trimmer Problems & Solutions
1. Starting Problem
If your string trimmer has a starting problem, look for items that may restrict the air, fuel, and spark required to start the engine.
Most string trimmers you find being used today are powered by 2-cycle engines. You will see some 4-cycle engines now that several manufacturers have released these trimmer models.
You must know the type of fuel to use for your type of engine and how to care for that fuel to minimize the effects old fuel has on the engine that may prevent starting.
- String trimmers with 2-cycle engines require a gas-to-oil mixture.
- Use unleaded gasoline with a minimum 89-octane rating and a maximum ethanol content of 10%.
- Add a premium 2-cycle engine oil like Briggs & Stratton oil.
- Mix gas and oil before adding it to the fuel tank.
- String trimmers with 4-cycle engines have separate fill ports for gas and oil.
- One fill port for unleaded gasoline with a minimum 89-octane rating and maximum ethanol content of 10%.
- One fill port for an SAE 30 engine oil.
- Do not mix oil for a 4-cycle engine. The exception to this is with STIHL string trimmer 4-MIX engines which use an oil and fuel mix.
Mix a fuel additive with the fuel to help clean the fuel system and reduce moisture.
Detailed information can be found at:
Reasons Your String Trimmer Won’t Start
| Cause | Reason for the Problem | Solution |
| Bad or old fuel | Fuel breaks down over time making the fuel less efficient and prone to clogging the fuel system. | Drain the fuel tank and fill it with fresh fuel. Use a fuel additive like Sea Foam to stabilize the fuel and assist with cleaning the fuel system. Use the right fuel. |
| Incorrect 2-cycle oil mix for a 2-cycle engine model | An incorrect mix can fail to provide the lubrication the engine requires. | Mix gas and 2-cycle oil. DO NOT mix gas and oil for a 4-cycle model. |
| Bad spark plug | A dirty or damaged spark plug; loose spark plug wire, or incorrect gap can prevent the engine from starting. | Replace with a new spark plug, secure connections, and ensure it is gapped to manufacturer specifications |
| Plugged air filter | Dirt can plug the air filter preventing airflow | Remove the filter and clean it. Replace the filter if it is in bad condition. |
| Plugged fuel filter | A dirty fuel filter will restrict fuel flow. | Replace the fuel filter. |
| Blocked fuel lines | Deposits left behind by old fuel and dirt can clog the fuel lines preventing the engine from getting fuel. | Use a carburetor cleaner and compressed air to clear the clog out of the fuel line. Replace the line if necessary. |
| Plugged fuel tank vent | A plugged fuel tank vent will no longer allow the tank to vent. A vacuum will form keeping fuel to flow out of the tank. | Replace a plugged fuel tank vent. |
| Plugged spark arrestor | Hot air isn’t able to exit the string trimmer when the spark arrestor is plugged with carbon deposits. | Clean the spark arrestor screen. |
| Dirty carburetor | The carburetor can become dirty and clogged due to bad fuel and ethanol deposits preventing fuel flow. | Clean the components making up the carburetor. Replace it if needed. |
| Bad primer bulb | A cracked primer bulb will fail to move fuel to the carburetor. | Replace the primer bulb. |
| Flooded engine | The fuel-to-air mixture is running too rich that it can’t ignite. This can be caused by pressing the primer bulb too many times or pulling the starter rope too many times while the choke is closed. | Turn the switch to the run position. Move the choke lever to the run position. Press the throttle trigger while pulling the starter rope over and over. |
| Broken starter recoil | A damaged or broken recoil prevents the recoil from starting your engine. | Check the recoil to see if it can be restrung. Broken parts must be replaced. Replace the recoil assembly if needed. |
2. Starts Then Dies
Many of the items that prevent a string trimmer from starting are also the same items that can cause it to stall and die after it’s been running.
If the trimmer starts and then shuts down right away, make sure you are following the right starting procedures. The choke lever must be placed in the on position to restrict airflow to start a cold engine.. It must then be adjusted to off once the engine warms.
Detailed information can be found at:
Your String Trimmer Starts, Stalls, and Dies
| Cause | Reason for the Problem | Solution |
| Wrong choke setting | The engine will die when the choke lever isn’t placed in the open position after the engine warms. | Close the choke to restrict air to start a cold engine. Open it once the engine warms. |
| Bad fuel | Fuel breaks down over time making your fuel less efficient and prone to clogging the fuel system and engine damage. | Drain the fuel tank and fill it with fresh fuel. Use a fuel additive like Sea Foam to stabilize the fuel and assist with cleaning the fuel system. |
| Plugged air filter | Dirt can plug the air filter preventing airflow. | Remove the filter and clean it. Replace the filter if it is in bad condition. |
| Plugged fuel filter | A plugged fuel filter won’t allow sufficient gas to pass through it. | Replace the fuel filter |
| Clogged fuel lines | Bad fuel deposits and dirt can build up and clog the fuel lines preventing the engine from getting the required fuel. | Use a carb cleaner and compressed air to clear the clog out of the fuel line |
| Dirty carburetor | The carburetor can become dirty and clogged due to bad fuel and ethanol deposits preventing fuel flow | Clean all the components making up the carburetor. Rebuild or replace if needed. |
| Plugged fuel tank vent | A plugged fuel tank vent will no longer allow the tank to vent. A vacuum will form keeping fuel to flow out of the tank. | Replace a plugged fuel tank vent. |
| Plugged spark arrestor | The spark arrestor can plug with carbon so hot air can no longer exhaust. | Remove the spark arrestor screen and clean it with a metal brush. Replace a damaged screen. |
3. Runs Rough and Sputters
A string trimmer runs rough and sputters when the engine isn’t getting sufficient fuel, the spark plug is dirty, or hot exhaust air is not able to exit the trimmer.
Detailed information can be found at:
Your String Trimmer is Running Rough and Sputters: Fix it!
| Cause | Reason for the Problem | Solution |
| Old fuel | Old fuel leaves behind gummy deposits that can restrict fuel and cause component failure. | Remove old fuel and replace it with fresh fuel. Add a fuel additive like Sea Foam Motor Treatment to stabilize gas and clean the fuel system. |
| Dirty spark plug | The intermittent spark from a dirty spark plug can cause it to run sluggishly. | Replace the spark plug. |
| Plugged fuel filter | A good flow of fuel is not able to pass through a plugged fuel filter. | Replace the fuel filter. |
| Clog fuel lines | Fuel is restricted when the fuel line develops a clog due to old fuel or dirt. | Clean or replace the fuel line. |
| Plugged fuel tank vent | When the fuel tank is unable to vent, it forms a vacuum that keeps fuel from leaving the tank. | Replace the fuel tank vent. |
| Plugged air filter | The air filter can become plugged with so much dirt that sufficient air isn’t able to pass through it. | Replace the air filter. |
| Dirty carburetor | Fuel passageways in the carburetor become plugged with old gas deposits keeping sufficient fuel from getting to the engine. | Remove the carburetor and clean it. Rebuild or replace it if needed. |
| Plugged spark arrestor | Hot exhaust air isn’t able to exit the trimmer when the spark arrestor is plugged with carbon. | Remove and clean the spark arrestor screen. |
4. Runs With the Choke On
When your string trimmer only runs with the choke on, you need to look for items that restrict fuel flow or allow too much air to get to the engine. The trimmer may need the choke engaged to correct the fuel-to-air ratio so the mixture is ignited.
You will also need to look for items that are allowing too much air to get into the system. Pulling in too much air will cause you to have to restrict air using the choke.
Detailed information can be found at:
Your String Trimmer Only Runs with the Choke On
| Cause | Reason for the Problem | Solution |
|---|---|---|
| Old fuel | Old fuel can leave behind varnish and deposits that cause fuel restrictions and fuel component failures. | Drain the fuel tank. Refill with fresh fuel. Add a fuel stabilizer and fuel system cleaner. Start and allow the fuel to work its way through the fuel system. |
| Fuel line puncture | Air can be sucked into the line increasing the amount of air getting to the engine. | Replace the fuel line. Check for fuel line restrictions. |
| Plugged fuel filter | A dirty plugged air filter restricts airflow. | Replace the air filter. |
| Bad carburetor gasket | The gasket may no longer seal allowing excess air into the carburetor. | Remove the carburetor and replace the gasket. |
| Dirty carburetor | Fuel passages can clog and no longer allow sufficient gas for the combustion process. | Remove and clean the carburetor. Rebuild or replace it if needed. |
| Plugged fuel tank vent | A plugged fuel tank vent will cause a vacuum to form in the fuel tank. This will keep fuel from flowing out of the tank. | Replace the fuel tank vent. |
5. Loses Power and Bogs Down
When you feel your string trimmer isn’t giving you good power or is bogging down, the cause is often a dirty carburetor and other items that are restricting fuel.
Other items to check are those that keep hot air from exiting the string trimmer, air from getting to the engine, or spark from igniting the fuel and air mixture.
Detailed information can be found at:
Reasons String Trimmer Loses Power & Bogs Down
| Cause | Reason for the Problem | Solution |
|---|---|---|
| Old fuel | Old fuel can leave behind varnish and deposits that cause fuel restrictions and fuel component failures. | Drain the fuel tank. Refill with fresh fuel. Add a fuel stabilizer and fuel system cleaner. Start and allow the fuel to work its way through the fuel system. |
| Plugged air filter | Good airflow cannot pass through a plugged air filter. | Replace the air filter. |
| Dirty spark plug | A dirty spark plug can cause an intermittent spark. | Clean or replace the spark plug. |
| Plugged fuel filter | Sufficient fuel is unable to pass through a plugged filter. | Replace the fuel filter. |
| Clogged fuel line | Clogged fuel lines can keep fuel from getting to the carburetor. | Try to clean the fuel line or replace it. |
| Plugged fuel tank vent | A plugged fuel tank vent will cause a vacuum to form in the fuel tank. This will keep fuel from flowing out of the tank. | Replace the fuel tank. |
| Plugged spark arrestor | The spark arrestor can plug with soot so hot air can no longer exhaust. | Remove the spark arrestor screen and clean it with a wire brush. |
| Carbon buildup on the exhaust port | Carbon that builds up in the exhaust port and hot air isn’t able to exit through the muffler. | Clean the exhaust port. |
| Dirty carburetor | Fuel passages can clog and no longer allow sufficient gas for the combustion process. | Clean the carburetor to remove varnish left behind by old fuel. Rebuild or replace it if necessary. |
Don’t Skip Maintenance on Your String Trimmer
While you will experience a problem with your string trimmer at some time, you can minimize significant problems by performing regular routine service.
I recommend completing a full maintenance service on your string trimmer once a year to keep it in good condition.
The annual service includes replacing the maintenance parts: air filter, fuel filter, and spark plug. You should also check the fuel system and cooling system, and clean the exterior.
Tighten any loose hardware and replace any damaged or missing parts. In addition to this, you must inspect the string trimmer before use to make sure it is in good condition before operating.