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9 Reasons a DeWalt String Trimmer Runs Rough & Bogs Down

You notice your string trimmer engine is performing as it should. It begins to run sluggishly and bog down when you press the throttle trigger.

A DeWalt 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.

Follow the safety precautions found in your operator’s manual to avoid injury while performing repairs on your string trimmer. This includes removing the spark plug wire and waiting for all parts to stop moving.

string trimmer

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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 DeWalt String Trimmer Runs Rough & Bogs Down

1. Old or Bad Fuel

Gas is often the reason you encounter performance problems with your DeWalt string trimmer. This is because gas oxidizes and breaks down over time.

Ethanol is added to most gasoline being sold today. This alternative fuel attracts moisture from the air to the fuel system. This can have negative effects on a DeWalt engine including running rough and bogging down.

This water and ethanol mixture not only leaves behind varnish that results in fuel restrictions, but it is also highly corrosive and damaging to fuel components and the engine.

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

Keep these items in mind when selecting fuel for a DeWalt 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 50:1. Mix gas and 2-cycle engine oil before adding it to the fuel tank.
  • 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 DeWalt 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 treated 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 DeWalt string trimmer may begin to run better.

If it doesn’t, proceed with checking the remaining items in the list until you find one that works.

2. Plugged Fuel Filter

Continue checking other items that may be causing a fuel restriction. Inspect the fuel filter. This is the small cylinder-shaped part that can be found inside the fuel tank attached to the fuel line.

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 DeWalt 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 the fuel filter placement so you can place the new filter in the correct position.
  • 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

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.

SOLUTION: Replace a DeWalt 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 DeWalt 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 DeWalt carburetor to fail to get enough fuel to the engine resulting in a rough running 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 DeWalt trimmer can cause it to misfire and run sluggishly.

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.

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 a DeWalt string trimmer is a small part attached to the end of a line coming out of the fuel tank or it may be 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 DeWalt 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 vent 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 DeWalt 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.

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

If a different style filter is used, consult your operator’s manual for cleaning instructions.

Clean a DeWalt 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 SAE 30 engine oil. Squeeze excess oil from the filter.
  • Reinstall the filter.
  • Reattach the air filter cover.

8. Plugged Spark Arrestor

There is a small metal screen that keeps hot exhaust material from shooting out of a DeWalt string trimmer and causing injury or starting a fire. This small screen will become plugged with a carbon buildup over time 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 DeWalt service center troubleshoot the trimmer and make any necessary carburetor adjustments. Over-adjusting the carburetor can permanently damage the engine.