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Troy-Bilt String Trimmer Runs Rough & Bogs Down (9 Reasons)

It’s difficult to get the job done when your string trimmer continues to run rough and sputter. I put together this guide to help you find the cause of your problem so you can get the task done.

A Troy-Bilt string trimmer runs rough and bogs down due to old fuel, a dirty carburetor, a clogged fuel line, a plugged fuel filter, a dirty spark plug, a plugged fuel tank vent, a clogged air filter, or a plugged spark arrestor screen.

Remove the spark plug boot before beginning any repairs. Follow all safety precautions listed in your Troy-Bilt operator’s manual.

Troy-Bilt String Trimmer primer bulb

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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.

Your Troy-Bilt String Trimmer Runs Rough & Bogs Down

1. Old or Bad Fuel

A primary reason a Troy-Bilt string trimmer starts having running problems is the quality and age of the fuel. The first thing to check when your trimmer starts to run rough is the fuel.

Gas can begin to oxidize and break down over time. This can have negative effects on a Troy-Bilt engine.

Most gasoline includes ethanol, an alternative fuel that naturally attracts moisture to the fuel system. The ethanol and water mixture will not only separate from gas, but it will also leave behind varnish that may affect the amount of fuel getting to the engine.

Using the right type of fuel and consuming it while it is still fresh is important for good engine performance.

Keep these items in mind when selecting fuel for a Troy-Bilt string trimmer:

  • Purchase fresh unleaded gasoline with a minimum 89-octane rating and a maximum 10% ethanol content.
    • 2-cycle engines require a 2-cycle oil mix at a ratio of 40:1. Mix gas and 2-cycle engine oil before adding it to the fuel tank.
    • 4-cycle engines require a straight gas.
  • 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.

Read more about choosing and storing fuel in This is the Gas and Oil Fuel Mix Troy-Bilt String Trimmers Use.

SOLUTION: Empty old fuel from the fuel tank. Mix fuel for your string trimmer with a fuel stabilizer like Sea Foam Motor Treatment or STA-BIL to keep fuel stable, help clean the fuel system, and reduce moisture.

Add the fuel mixture to the fuel tank and allow it to run for 5 minutes so the new fuel works its way through the system. The Troy-Bilt string trimmer may begin to run better.

If it doesn’t, proceed with checking the remaining items in the list until you find what is causing your running issue.

2. Plugged Fuel Filter

The fuel filter is a small cylinder-shaped part attached to the fuel line and located inside the fuel tank. Its function is to strain fuel to keep dirt from entering the fuel system and wearing on the engine.

When the filter isn’t changed out regularly, it can become plugged with dirt keeping a good flow of fuel from getting to the carburetor. This will cause the trimmer to run rough and bog down.

SOLUTION: A plugged Troy-Bilt fuel filter must be replaced using the following procedure:

  • Set the trimmer on a flat surface.
  • Wipe around the fuel cap to remove dirt, and then remove it.
  • Take note of where the fuel filter is placed inside the tank so you can place the new filter in the same location.
  • Pull the fuel filter out of the tank using a clean bent wire to hook the fuel line.
  • 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.

3. Clogged Fuel Line

Gummy deposits left behind by old fuel can stick to the fuel line narrowing the passageway for fuel to flow. This will keep fuel from getting to the carburetor.

SOLUTION: Replace a Troy-Bilt line with a new fuel line when you find a clog, kink, puncture, or leak in the line.

4. Dirty Carburetor

The carburetor regulates the amount of gas that is mixed with air for combustion so the Troy-Bilt starts and runs.

When this proportion of fuel-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 Troy-Bilt 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 (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.

5. Dirty Spark Plug

A fouled spark plug on your Troy-Bilt can cause it to misfire and run sluggishly.

A worn or damaged spark plug, an incorrect spark plug gap, and a loose spark plug wire can also result in running problems with your string trimmer.

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, 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 manufacturer’s required specifications. Install the new or cleaned spark plug. Make sure the spark plug wire is securely attached.

6. Plugged Fuel Tank Vent

The fuel tank must be able to vent to allow air to enter the tank while fuel is consumed. If the vent is plugged, the tank will form a vacuum keeping a good amount of fuel from getting to the carburetor.

This will cause the engine to run rough and bog down. A fuel tank vent on most Troy-Bilt string trimmers is built into the fuel cap.

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 plugged fuel tank vent.

You can test for a plugged fuel tank vent with a pressure gauge to identify a vacuum. If you don’t have a gauge, you can also determine if the tank vent is plugged by performing a running test with and without the fuel cap tightened.

Start by loosening 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 Troy-Bilt trimmer 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 cap if your trimmer starts to act up and run rough again after reinstalling the fuel cap.

7. Plugged Air Filter

An air filter is essential to run on a Troy-Bilt string trimmer. It provides protection to the engine by keeping dirt out of the air intake.

When the air filter isn’t checked, cleaned, and replaced to keep a clean filter on your trimmer, 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.

Never run your string trimmer without an air filter. This not only will void your warranty but may also cause permanent damage that can ruin the engine.

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.

Clean a Troy-Bilt FOAM string trimmer air filter:

  • Remove the air filter cover and air filter.
  • Clean the foam air filter using water and mild detergent.
  • Rinse the filter until the water runs clear and allow it to air dry.
  • Once dry, lightly saturate the filter with clean SAE 30 engine oil. Squeeze excess oil from the filter.
  • Reinstall the filter.
  • Reattach the air filter cover.

Because there are so several different types of air filters used on Troy-Bilt products that can vary by model, refer to the Troy-Bilt operator’s manual for steps to clean a different style of air filter.

8. Plugged Spark Arrestor

There is a small metal screen that keeps hot exhaust material from shooting out of a Troy-Bilt 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 with a metal brush.

Install the clean spark arrestor screen. Reattach the engine exhaust cover and engine cover. Reattach the spark plug wire.

If the screen cannot be sufficiently cleaned or you find it is damaged or has a hole in it, it must be replaced with a new spark arrestor screen.

9. Carburetor Adjustment

The carburetor may need to be adjusted to change the RPMs at idle speed and at full throttle. It may be causing the engine to run lean or rich causing the trimmer to run rough.

There are adjustment screws on the carburetor to make these adjustments, however, you most likely need a special tool to make these adjustments.

If none of the items listed above have solved your problem, I recommend having a mechanic at your local Troy-Bilt service center troubleshoot the trimmer and make any necessary carburetor adjustments. Over-adjusting the carburetor can permanently damage the engine.