A chainsaw that is bogging down and losing power will not run at full RPMs. When this happens, you won’t get a good cutting performance and your task will take a lot longer to complete.

A Stihl chainsaw may lose power due to a dirty spark plug, plugged air filter, clogged spark arrestor, plugged fuel filter, bad fuel, clogged fuel lines, or a dirty carburetor. The air and fuel mixture may need to be adjusted on the carburetor to prevent a loss of power.

Keep reading and I’ll explain more about why your Stihl chainsaw may be losing power. Follow all safety precautions before performing repairs including removing the spark plug wire and waiting for all moving parts to stop.

Things that cause your Stihl chainsaw to lose power:

• Plugged air filter
• Clogged spark arrestor
• Clogged fuel filter
• Plugged or punctured fuel line
• Old fuel
• Dirty carburetor
• Dirty spark plug
• Carburetor requires adjustment
• Chain brake is engaged

Stop the chainsaw and allow it to cool before working on it. Remove the spark plug and follow your operator’s manual for all safety precautions. Contact your local Stihl dealership for additional help or if you don’t feel comfortable performing repairs.

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Follow all safety instructions provided in your equipment operator’s manual prior to 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.

Lack of Air Causes a Stihl Chainsaw to Lose Power

Air is one of the essential items your chainsaw requires to start and run. When looking for a reason your Stihl chainsaw loses power, check for items that will restrict airflow like a plugged air filter or clogged spark arrestor.

Plugged Air Filter in a Stihl Chainsaw

A clean air filter is a necessary component to have installed in your Stihl chainsaw. This filter keeps dirt and debris from entering the carburetor throat to prevent damage and wearing of the engine.

When using your chainsaw, you are creating some very dirty conditions. Airflow can become restricted when the air filter becomes plugged with dirt and debris because it isn’t being cleaned or replaced regularly.

A plugged air filter will cause a loss of power. When you experience this, check the filter. Clean it or replace it if necessary. Never run your chainsaw without an air filter. Doing this can result in significant engine damage.

How to Clean a Stihl chainsaw FLEECE air filter:

• Remove the cover.
• Clean around the air filter to remove any dirt and sawdust.
• Remove the air filter and continue to clean the air filter housing.
• Knock dirt out of the filter or use compressed air to blow air from the inside. DO NOT use a brush.
• If the filter is very dirty, wash it in a mild dish detergent and water solution, and rinse from the inside outward until the water runs clear.
• Allow the filter to completely air dry. DO NOT use heat on the filter as this can damage it. DO NOT apply oil to the filter.
• Reinstall the clean filter.
• Purchase and install a new air filter if the old filter is extremely dirty or damaged
• Reattach the cover.

How to Clean a Stihl chainsaw HD2 air filter:

• Remove the cover.
• Clean around the air filter to remove any dirt and sawdust.
• Remove the air filter.
• Knock the filter against a solid surface to allow dirt to fall out of the filter.
• Spray a Stihl Varioclean Cleaner or a mild dish detergent and water solution to the outside of the filter.
• Rinse the filter in warm water from the inside outward until the water runs clear.
• Allow the filter to completely air dry. DO NOT use heat on the filter as this can damage it. DO NOT apply oil to the filter.
• Reinstall the clean filter.
• Purchase and install a new air filter if the old filter is extremely dirty or damaged
• Reattach the cover.

Clogged Spark Arrestor on a Stihl Chainsaw

You will find a spark arrestor on your Stihl chainsaw. This is a small mesh screen installed to prevent hot exhaust material from leaving the muffler.

When the screen becomes plugged with carbon buildup, you may experience a loss of power where the Stihl chainsaw won’t run at full RPMs. This is because the engine isn’t able to exhaust sufficiently.

This screen should be checked often and cleaned to prevent buildup. To do this, first, disconnect the spark plug wire and wait for the saw to cool down.

Carefully remove the spark arrestor screen. Clean it using a metal brush. If you find any damage or holes in the screen, replace it with a new spark arrestor screen.

Once you have installed a new or clean spark arrestor, reattach the spark plug wire if you are done performing all repairs and maintenance.

Lack of Fuel Causes a Stihl Chainsaw to Lose Power

Of course, your Stihl saw will lose power if it isn’t getting enough fuel so you’ll need to look for items that restrict fuel flow. Bad or old fuel often contributes to fuel restrictions and a loss of power.

If you find the fuel in your chainsaw is old, begin by draining the fuel tank and adding fresh fuel. Read more about the right fuel for your Stihl chainsaw here. Then continue with checking for a clogged fuel filter; plugged or punctured fuel line; or dirty carburetor.

Clogged Fuel Filter on a Stihl Chainsaw

The fuel filter can be found inside the fuel tank. It is attached to the end of the fuel line to strain the fuel in the tank. Its purpose is to keep dirt and debris from entering the fuel system.

When the fuel filter is not changed out regularly or you are using very dirty fuel, it can become plugged keeping a sufficient amount of fuel from flowing through the filter. This lack of fuel flow will cause a Stihl chainsaw to lose power.

Plugged or Punctured Fuel Line on a Stihl Chainsaw

Old fuel sitting in the chainsaw is likely to leave behind varnish and sticky deposits that clog the fuel line. This can restrict fuel flow.

When you find there is a clog, remove the fuel line and clean it to remove the clog using carburetor cleaner. Spray the carburetor cleaner into the line to loosen the clog. Then blow compressed air through it to remove the clog.

Repeat spraying the cleaner and blowing compressed air until the line is free of the blockage. Replace with a new line when you find it is dry or cracked.

Another thing to look for is a punctured fuel line. A punctured line can introduce air into the fuel system. Additional air to the cylinder can make your Stihl chainsaw lose power when there is no longer a correct mixture of fuel and air.

Dirty Carburetor on a Stihl Chainsaw

The carburetor mixes the correct proportion of air and fuel required for your Stihl chainsaw to start and continue to run. The passageways can become clogged and the small components can fail to function correctly which can be the reason the chainsaw stops running.

Old fuel is a big reason why a Stihl carburetor will stop working. You may be able to clean or rebuild your carburetor to get it working again. You will have to replace the carburetor if this doesn’t work.

Carburetor Needs Adjustment on a Stihl Chainsaw

The carburetor may need to be adjusted to change the RPMs at idle speed and at full throttle. There are adjustment screws on the carburetor to make these adjustments.

The screws are labeled “L” for low-speed and “H” for high-speed. Begin the adjustment by making sure the screws are at their standard settings.

To do this, turn the screws clockwise until they stop. Then turn each screw counter-clockwise the number of turns indicated above the screw.

For example, in the photo below, the H screw is turned counter-clockwise 3/4 of a turn. The L screw is turned counter-clockwise one full turn. The number of turns needed may be different for your model Stihl saw. Refer to the number above the screw or your owner’s manual for the standard setting.

Next, let the chainsaw idle and adjust the low-speed screw clockwise and counter-clockwise until you find the “sweet spot” where it runs smoothly and not sluggishly.

Turn the idle screw (LA) counter-clockwise if your chain begins to move at idle. You need to turn it until the chain stops moving and then continue another 1/4 further.

Then, adjust the H screw to get a good smooth RPM at full throttle. Don’t over-adjust and allow the RPMs to increase too much or you will damage the engine.

Stihl does have some limits to the adjustments you can perform to the carburetor. If you are having problems with the carburetor, bring your chainsaw to your local Stihl dealer. A special tool may be required so only your dealer can make the necessary adjustments.

Lack of Spark Causes a Stihl Chainsaw to Lose Power

Spark is required to ignite the fuel and air mixture to form a combustion in the cylinder so the chainsaw will continue to run. When the Stihl chainsaw isn’t getting consistent spark, it will bog down and lose power.

Dirty Spark Plug in a Stihl Chainsaw

The spark plug is a wear item. It must be replaced regularly. I recommend replacing it once a year for casual usage. However, if you are using your chainsaw often, you will need to change it more frequently.

The spark plug will become dirty over time causing an intermittent spark and a loss of power. Remove the spark plug and inspect the tip. If it is dirty, but not very dark in color, you can try to clean it using a wire brush so you can reuse it.

If you find the spark plug is very dark in color, and has a broken porcelain, or a burnt electrode, you must replace it with a new one.

Other Item That Causes a Stihl Chainsaw to Lose Power

Chain Brake is Engage on a Stihl Chainsaw

The chain brake is a plastic handle that sits in front of the regular handle you use to hold the chainsaw. This is a safety feature installed to protect the user when the saw kicks back.

When the chain brake is engaged, the Stihl chainsaw will bog down losing power. Check the chain brake and make sure it is in the off position.