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STIHL String Trimmer Runs Rough & Bogs Down (FIXED!)

Your string trimmer won’t give you that power to get the job done. I’ve put together a list of items to check so you can get the performance you need out of your trimmer.

A STIHL strimmer runs rough and bogs down when the engine isn’t receiving sufficient fuel, air, and spark.

This can be due to old fuel; a plugged fuel filter; a clog or puncture in the fuel line; a dirty carburetor; a bad fuel tank vent; a plugged air filter; a clogged spark arrestor screen, or a dirty spark plug.

Consult the STIHL operator’s manual for safety precautions. Remove the spark plug wire and wait for the engine to cool prior to performing repairs.

String trimmer runs rough and bogs 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.

8 Reasons Your STIHL String Trimmer Runs Rough & Bogs Down

Old or Bad Fuel Causes a STIHL String Trimmer

When you start experiencing your trimmer beginning to run rough and act like it’s bogging down, you should check the quality of fuel running through it.

Old fuel can begin breaking down as soon as 30 days after purchase. The ethanol found in most gasoline attracts moisture to the fuel system.

This moisture and ethanol mixture will leave behind varnish and sticky deposits when the gas is old. This can cause fuel restrictions and carburetor failure.

The type of fuel and how quickly it is consumed are important to obtain good performance from your trimmer. Getting the fuel selection right and caring for it properly will reduce the problems that develop from running old gas.

STIHL string trimmers require a gas and 2-cycle engine oil mixed at a ratio of 50:1. Keep these items in mind when selecting fuel for your STIHL string trimmer:

  • Purchase fresh unleaded gasoline with a minimum 89-octane rating and maximum 10% ethanol content.
  • Mix gas and 2-cycle engine oil at a ratio of 50:1 before adding to the fuel tank. Read more about fuel in STIHL String Trimmers Use This Type of Gas and Oil Fuel Mix.
  • Consume fuel within 30 days.
  • Add a fuel stabilizer if you are unable to consume fuel within 30 days so it lasts a little longer.
  • Store fuel indoors in a dry location away from combustible items.

SOLUTION: Empty old fuel from the fuel tank. Mix 50 parts unleaded gasoline with 1 part 2-cycle engine oil in a gas container. Add a fuel additive like Sea Foam Motor Treatment to stabilize the fuel and clean the fuel system.

Add the fuel mixture to the fuel tank. Start the string trimmer and allow it to run so the new fuel works its way through the system.

Your string trimmer may immediately begin to run better with the change to fresh fuel. If it doesn’t, proceed with checking the remaining items in the list until you find one that works.

Plugged Fuel Filter Causes

Once you have confirmed you have fresh gasoline mixed with 2-cycle engine oil at a rate of 50:1, continue looking for other items that can restrict fuel like the fuel filter.

The fuel filter is a small cylinder-shaped part located inside the fuel tank. This filter is a routine maintenance item I recommend replacing annually.

When the filter isn’t changed out regularly, it can become plugged with the dirt it is keeping out of the fuel system. A plugged filter will keep a sufficient amount of fuel from getting to the carburetor.

SOLUTION: Replace a plugged fuel filter.

  • Set the trimmer on a flat surface.
  • Wipe around the fuel cap, and then remove it.
  • Pull the fuel filter out of the tank. Using a clean bent wire to hook the fuel line works well. You may be able to reach it with a pair of long needle nose pliers.
  • Securely holding the fuel line, securely grab the filter and pull it out of the fuel line.
  • Install a new fuel filter at the end of the fuel line.
  • Place the filter back inside the fuel tank so it sits on the bottom and back side of the tank.

Clogged Fuel Line

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

In addition to a clog, look for a puncture in the line that may cause air to be sucked into the fuel line causing the trimmer to blog down.

SOLUTION: Replace a STIHL fuel line with a new fuel line when you find a clog, kink, puncture, or leak in the line. You can purchase OEM parts from a STIHL dealer.

Dirty Carburetor

The carburetor is the part that regulates the amount of gas that is mixed with air that is needed to form combustion so the STIHL starts and runs.

When this proportion of gas to air is not correct, the string trimmer may begin to run rough. The passageways in the carburetor can become clogged and the small components that make up the carburetor can stick.

This can cause the STIHL carburetor to fail to function properly and get enough fuel to the engine.

SOLUTION: You may be able to clean your carburetor to get it working again. However, you may end up having to rebuild it (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 are a common issue. Minimize carburetor damage by using fresh fuel.

Avoid the negative effects ethanol has on the carburetor by using a non-ethanol fuel like pre-mixed MotoMix fuel.

Dirty Spark Plug

A dirty spark plug can cause the trimmer to misfire and run sluggishly. Not only can a dirty spark plug cause it to run rough, but also an incorrect electrode gap and loose wire cause the same problem.

SOLUTION: 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, you may choose to clean it with a wire brush or replace it with a new spark plug.

I prefer to replace it with a new one since it’s an inexpensive part that greatly affects the string trimmer’s performance.

Plugged Fuel Tank Vent

Another item to look at is the fuel tank vent. This is a small part added to a STIHL string trimmer to allow the fuel tank to vent.

It allows air to pass in and out of the fuel tank to keep the air pressure inside the tank equal to the pressure outside of the tank.

When the fuel tank vent doesn’t work or becomes plugged, the fuel tank will form a vacuum restricting fuel flow to the carburetor. This lack of sufficient fuel can cause the trimmer to run rough.

You can test for a plugged fuel tank vent by loosening the fuel cap to allow air into the tank. Start the string 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 STIHL trimmer engine runs strong and doesn’t bog down or run rough, tighten the fuel cap and see if you can replicate the issue to confirm the fuel tank vent is the problem.

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

Plugged Air Filter

The air filter is used to protect the engine from dirt and debris. The filter keeps dirt from entering the carburetor throat and ensures the engine receives clean air.

When the air filter isn’t checked, cleaned, and replaced to ensure the air filter is in good condition, it can become plugged with so much dirt that sufficient air isn’t able to flow through the filter.

Without enough air, the engine will begin to run rough. It’s important to keep the air clean to protect the engine and not cause it to overheat due to a lack of air.

SOLUTION: Inspect the air filter and clean it if it is in good condition using the steps below. If the filter is very dirty, fuel-covered, or damaged, it must be replaced with a new one.

Steps to clean a string trimmer air filter:

  • Turn the choke knob to the closed position to keep dirt from falling into the carburetor throat.
  • Remove the air filter cover.
  • Remove dirt from around the air filter.
  • Remove the air filter.
  • Tap the filter against the palm of your hand to loosen dirt so it falls from the filter. Use an air compressor if needed. If the filter is very dirty or damaged, replace it with a new one.
  • Install the air filter.
  • Reattach the filter housing.

Plugged Spark Arrestor

The spark arrestor screen is installed on a STIHL string trimmer as a safety precaution. It keeps hot exhaust material from shooting out of the muffler that may cause injury or start a fire.

This screen must be checked and cleaned. If it becomes plugged with soot, the airflow is restricted making the engine run sluggishly.

SOLUTION: Before you begin cleaning the spark arrestor, wait for the muffler to cool. Remove the spark plug wire.

Remove the spark arrestor screen and use a small wire brush to remove the exhaust material that has collected on the screen. You must replace a screen that has a hole in it or is damaged.