It’s hard to get the job done when your string trimmer isn’t giving you sufficient power.
A STIHL string trimmer bogs down and loses power due to a fuel restriction caused by old gas, a plugged fuel filter, a clogged fuel line, a dirty carburetor, or plugged fuel tank vent.
It may also be due to an airflow restriction caused by a plugged air filter or a plugged spark arrestor screen; or a lack of spark caused by a dirty spark plug.
Stay safe and follow all safety procedures listed in the STIHL operator’s manual. Consult a professional mechanic if you are unsure how to complete repair procedures.
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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.
STIHL String Trimmer Bogs Down & Loses
Check the fuel quality when your trimmer begins to bog down and you experience a lack of power. Old gas is often a reason this happens.
Over time, the ethanol used in gasoline along with the moisture it attracts will evaporate and leave behind varnish and sticky deposits. This will cause fuel restrictions that result in a sluggish engine.
Because ethanol is harmful to the small engine on a STIHL string trimmer and its performance, use fresh gasoline with a minimum octane rating of 89 and a maximum ethanol content of 10%.
Mix with 2-cycle engine oil at a ratio of 50:1. (50 parts gas to 1 part oil). Never use gas with ethanol levels higher than 10% like E15 and E85 fuel. These contain up to 15% and 85% ethanol respectively.
Follow these tips when selecting and storing fuel for your STIHL string trimmer:
- Purchase fresh fuel: Purchase fuel you can likely consume within 30 days.
- Use a fuel stabilizer: Add a fuel stabilizer to make your fuel last a little longer. Many 2-cycle oils include fuel stabilizers. Depending on the manufacturer of the oil, it may keep fuel stable from 30 days to 2 years.
- Use the right gas: Use a gas that has a minimum octane rating of 87 and maximum ethanol content of 10%.
- Use a 50:1 gas-to-oil mixture: 50 parts gasoline must be mixed with 1 part premium 2-cycle engine oil.
- Store fuel in a dry location away from combustible products.
For more information on choosing the right fuel for your trimmer check out STIHL String Trimmers Use This Type of Gas and Oil Fuel Mix.
SOLUTION: Drain the old fuel remaining in your string trimmer and fill it with a fresh 50:1 gas-to-oil mixture.
Add a fuel stabilizer like Sea Foam Motor Treatment that also helps reduce moisture and clean the fuel system.
Start the string trimmer and allow it to run. You must allow the fresh fuel and Sea Foam mixture to move through the system.
You may find your STIHL trimmer is starting to run stronger and no longer losing power after replacing it with fresh fuel. If not, continue to troubleshoot the problem by proceeding through this list.
Plugged Air Filter
The air filter is essential to use on a trimmer. It keeps dirt and debris from passing through the air intake and causing engine damage.
While the air filter offers protection for a trimmer, it can also cause problems when it isn’t regularly cleaned and kept in good condition.
When the air filter is neglected, a buildup of dirt can plug the filter so the engine doesn’t get the air it requires to perform at its best. A lack of air may cause the engine to lose power.
I recommend replacing the STIHL air filter annually and then checking and cleaning it regularly throughout the year. You may need to replace it more often when working in very dusty conditions.
SOLUTION: Check the air filter and clean it if it is a little dirty. If it is very dirty or damaged, it must be replaced.
Clean a STIHL 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.
Dirty Spark Plug
A dirty spark plug may cause an intermittent spark. Your string trimmer may lose power as a result.
SOLUTION: Remove the spark plug and check its condition. If the spark plug tip is very dark in color, worn, or damaged, it must be replaced.
If it is in good condition and only a little dirty, you can clean the spark plug tip using a small wire brush.
Next, check the electrode gap. This must be gapped to the manufacturer’s specification. The spark plug wire must be securely attached as well. A loose spark plug wire or an incorrect gap can cause a loss of power.
Plugged Fuel Filter
You will find the fuel filter located inside the fuel tank. This cylinder-shaped part is attached to the fuel line to keep dirt out of the fuel system. STIHL also refers to the fuel filter as the pickup body.
The fuel filter should be changed annually to keep it in good condition. When it isn’t replaced regularly, the filter can become plugged with dirt.
This can keep the engine from getting sufficient fuel and can cause it to run sluggishly and lose power.
SOLUTION: Replace a fuel filter that isn’t allowing fuel to pass through to the fuel line. When placing the filter back inside the fuel tank, make sure it is placed at the bottom of the fuel tank.
Check the fuel tank to make sure the fuel is of good quality and it’s not contaminated with dirt and debris. If the fuel is very dirty or old, replace it with fresh clean fuel.
Clogged Fuel Line Causes
Another place to check for a fuel restriction is the fuel line. Old fuel may leave behind gummy deposits that can restrict fuel flow through the fuel line. You may have also developed a puncture that is sucking air into the fuel system.
A fuel restriction or introducing too much air through the fuel system can cause your STIHL to bog down and run rough.
SOLUTION: Inspect the fuel line looking for any clogs preventing fuel flow. Replace a fuel line that is clogged, kinked, or has developed cracks.
A carburetor is required on a gas STIHL string trimmer to regulate the amount of gas mixed with air to create combustion. Old fuel can gum up and cause the small components in your carburetor to stick so it no longer functions right.
SOLUTION: If you are a little mechanical you should be able to handle cleaning your carburetor. Clean the carburetor by taking it apart and using carburetor cleaner to clean it.
Use your phone to take photos of the carburetor as you disassemble it so you can refer to them when reassembling later.
If the carburetor does not function after being cleaned, you may need to rebuild it (if a rebuild kit is available) or replace it with a new carburetor.
Plugged Fuel Tank Vent
The fuel tank must be able to vent allowing air to pass in and out of the fuel tank. You will find a fuel tank vent that allows this to happen.
When the fuel tank vent becomes plugged, it won’t allow air to pass through it to equalize the air pressure inside the fuel tank with the air outside of it.
This will cause the STIHL string trimmer to bog down and lose power because the fuel tank forms a vacuum when it isn’t able to vent properly. This vacuum restricts fuel to the carburetor.
To help determine if you have a plugged fuel tank vent, loosen the fuel cap to allow air in the tank. If after doing so your string trimmer begins to run better, there’s a good chance the fuel tank vent is plugged.
You can further confirm the problem by trying to replicate the issue by allowing the trimmer to run with the cap tightened to see if you experience the loss of power and then loosening the cap to experience a better-performing trimmer.
Be careful and keep the trimmer level so you don’t spill gas from the fuel tank. If you are able to replicate the issue, you will have to check and replace the fuel tank vent.
SOLUTION: Replace a plugged fuel tank vent.
Plugged Spark Arrestor
The spark arrestor is a small metal screen that prevents hot exhaust material from exiting the muffler and starting a fire.
When this small screen becomes plugged with soot you may experience a loss of power where your STIHL string trimmer won’t run at full RPMs.
SOLUTION: Place the trimmer in the off position. Disconnect the spark plug wire. Wait for the muffler to cool. Carefully remove the spark arrestor screen. Clean it with a metal brush.
If the screen isn’t able to be sufficiently cleaned or you find it is damaged or has a hole in it, replace it with a new spark arrestor screen.
When to Have a Mechanic Repair a STIHL String Trimmer?
If you’ve gone through the list above and it didn’t solve your loss of power issue or you don’t feel comfortable performing any of the repairs, it is time to consider consulting an experienced small engine mechanic.
You may have a good small engine repair shop near you that has been recommended by a neighbor or friend. You can also find one by visiting STIHL’s dealer locator page.
I prefer to go to have repairs completed at a STIHL dealership when I run into a significant issue. STIHL dealerships are required to have factory-trained mechanics. Because of this, you won’t find STIHL in big box stores.
STIHL dealers have the required specialty tools to perform testing. I can also be assured that any warranty work is completed and that OEM parts are used in the repair.
When visiting a repair shop, keep in mind the labor rate for the mechanic to diagnose your problem. There is typically a flat rate charge to diagnose the problem and then labor and parts fees in addition to this fee to complete the repairs.
Paying a diagnostic fee, parts, and a labor bill may not be sensible if you are running an old inexpensive STIHL trimmer that’s on its last leg.
You have the weigh the reliability, quality, and age of your current string trimmer along against the cost of the repair. You may be better off investing in a new STIHL string trimmer.