Here’s What to do if Your String Trimmer Won’t Stay Running

A string trimmer comes in handy to get rid of weeds and grasses in your backyard. However, it can be frustrating when the tool malfunctions and won’t stay running. Understanding the issues behind a string trimmer’s mechanics is vital to keeping it running. 

What to do if your string trimmer won’t stay running? Identify why the string trimmer is not running and address that issue. Some common causes are:

  • Clogged carburetor
  • Improper gas vent
  • Clogged fuel filter
  • Dirty air filter 
  • Clogged spark arrestor etc.

Make sure to clean the clogged part and replace the faulty components if necessary. 

Oftentimes some basic troubleshooting fixes the problem with the tool. However, it is important to know why such problems occur to prevent them from happening in the first place.

Also, some tips and tricks can be really useful to maintain your string trimmer. So let’s dig deeper and have a better understanding of the topic!

Why a string trimmer won’t stay running and how to fix it

Most string trimmers are operated by a two-stroke internal combustion engine. Hence, they need a constant fuel supply, air circulation, and spark. If any of these components is off-balanced, the engine won’t function properly.

If the major parts and attachments, such as carburetor, spark plug, fuel circulation system, air filter, and exhaust system are not well-maintained, the string trimmer may not stay running. 

I am going to break down the causes for you and also provide a solution for each.

Clogged Carburetor

The carburetor supplies the internal combustion, i.e., spark-ignition engine proper mixture of fuel and air. A clogged carburetor results in reduced engine performance, overheating, and even black smoke. 

The clogging happens if you leave fuel in the trimmer while storing for a long time. That fuel will eventually go stale and partially evaporate, leaving behind a thick, sticky sludge. There you have your culprit! This sludge can easily clog the carburetor that will cause the engine to stall.

What to do: 

1. Disassemble the carburetor and try cleaning the clogs with a carburetor cleaner. This can be a bit complicated if you have never done it before, so check out this video if you aren’t sure what you are doing:

2. Get a carburetor repair kit to replace damaged components. You can usually buy these online.

3. Replace with a new carburetor if it’s completely broken.

Poorly Adjusted Carburetor

Sometimes an improperly adjusted carburetor can also result in sputtering and stalls. Usually, there are two small screws on the carburetor labeled ‘L’ (meaning low) and ‘H’ (meaning high). These screws adjust the trimmer’s fuel mixture usage while running at idle speed and the highest speed respectively. 

If the screws are not adjusted correctly, the fuel mixture would not be correct and excess air or fuel will lead to an engine cut out.

What to do: 

  • To readjust the ‘L’ screw, start the trimmer and turn the screw anticlockwise until the engine begins to slow down. Note the position and do the same again for clockwise direction. 
  • For final adjustment, position the screw in such a way that it is somewhere in between the two positions you just noted.
  • Repeat the same procedure for adjusting the ‘H’ screw.

Improper Venting in Gas Cap

As the volume of gasoline expands and shrinks with combustion and cooling respectively, venting is necessary to keep the pressure steady and prevent vacuum formation. In most string trimmers, the vent is basically an inlet hole in the gas tank cap that allows air to enter into the fuel tank. This inlet hole can get blocked by dirt. 

What to do: 

  • Start by slightly loosening the cap.
  • Run the trimmer to check if it improves the operation.
  • If it does, replace the gas cap.

Clogged Fuel Filter

The fuel filter is usually at the end of the intake fuel line. It could get dirty because of residue from old fuel stored in the tank or impurities from low-quality fuel. 

What to do:

  • Make sure to drain the old fuel from the fuel tank.
  • Replace the fuel filter on a regular basis; the process is simple, and inexpensive, too.

Dirty Air Filter

Air circulation is essential for combustion to happen. If the air filter gets dirty, there will be partial  combustion, and the engine will breakdown. 

What to do:

  • Clean or replace the air filter regularly, which is pretty cheap and easy to do.

Clogged Spark Arrestor

Spark arrestor is a small screen component at the exhaust port that prevents the emission of flammable debris from the combustion engine. Over time, it can get clogged with these debris and soot, which obstructs the engine from operating.

What to do:

  • Remove the arrestor and clean it with a wire brush.
  • Replace it with a new one if necessary.

Faulty Choke

The choke is responsible for supplying fuel to the engine that helps to cold-start the trimmer. There should be a full choke while starting, i.e., choke should be at the highest position. It should be moved to half choke position when the engine attempts to start and should be completely disengaged when the engine starts.

Having a faulty choke that moves freely, or gets stuck at the full position may cause the trimmer to stall while running.

What to do:

  • Check the choke and make sure that it is in the correct position.
  • Replace if the choke is faulty. 

Dirty or Cracked Spark Plug

A spark plug is crucial to generate spark and ignite the fuel-air mixture for combustion. The spark plug can get dirty from carbon buildup or other residues. If this happens, the plug won’t spark or may even misfire.

Also, there is ceramic insulation in the spark plug, which, if cracked, will cause the spark to take a shortcut through the insulation instead of igniting the fuel mix. As a result, the engine will die.

  What to do:

  • Using a wire brush, clean the buildup or residue on the spark plug.
  • Replace the spark plug if it is cracked or deformed.

Tips and Tricks to Maintain a String Trimmer

Now it is time to share some preventive measures to avoid clogging in the components, and a few tips for maintaining your string trimmer.

Use Fresh Fuel with the Right Fuel Mix

You may have already realized that most of the problems associated with clogging involve keeping the old fuel stored in the fuel tank. Gasoline stored for more than 30 days should be discarded as it begins to turn into varnish and gum.


  • Always use fresh fuel if possible.
  • You can put some fuel stabilizer into the gas tank while storing the trimmer.
  • Make sure to use the right fuel mix, according to the instructions in your trimmer’s manual. Also, don’t mix more than a month’s supply at a time.
  • It is recommended to use an ethanol-free premixed fuel and oil product. When there is alcohol in the fuel, it is called an ‘oxygenated fuel’. This oxygen is capable of breaking down the organic compounds in the fuel. In such instances, a fuel stabilizer doesn’t help much.

Store the String Trimmer Properly

Once you get rid of the grass, naturally you will store the string trimmer for a while, especially during winter. If you take care of a few things before storing the trimmer, you can avoid a lot of hassle later on. After all, prevention is better than cure!


  • Drain all the fuel out of the trimmer before storing it for a long period of time.
  • Thoroughly clean the fuel tank to get rid of any leftover or deposits.
  • It is always a good idea to have a quick cleaning of the components such as air vent, filters, etc. You can use a metal brush or a rag.
  • Store in a dry place to prevent corrosion.

If you follow the easy tips mentioned above, your string trimmer should keep working without stalling. Nonetheless, regular and proper maintenance is always ideal for getting the best performance from your string trimmer.

Go for it and enjoy the trimming game!

Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!

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