Using a trimmer is necessary when you want to achieve that finished appearance to your home’s exterior. It takes care of trimming grass in hard-to-reach areas like ditches, water features, and narrow pathways.
While a string trimmer that doesn’t work won’t put you in a dire situation, it does keep you from having a well-manicured lawn.
A STIHL string trimmer won’t start due to a wrong choke setting, plugged air filter, plugged fuel filter, clogged fuel line, dirty carburetor, plugged fuel tank vent, plugged spark arrestor, bad spark plug, faulty starter recoil, wrong fuel mixture, or old gasoline.
The engine may also be flooded and won’t start. Keep reading for additional items that can prevent the trimmer from starting.
Follow all safety precautions in your operator’s manual. This includes removing the spark plug wire before performing repairs.
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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.
Reasons a STIHL String Trimmer Will Not Start
Choke Knob is in the Wrong Position
If your string trimmer isn’t starting, check to make sure the choke knob is in the correct position. The choke is used to restrict airflow when starting a cold engine.
SOLUTION: Set the choke knob in the closed position when the engine is cold. If the engine is already warm, the choke knob must be in the open position.
Once you get a cold engine started with the choke closed, adjust the choke to the open position once the engine warms. The engine will die if you don’t make this adjustment.
Clogged Air Filter
The air filter keeps dirt from getting to the engine through the air intake. Just a small amount of dirt or debris can cause irreversible engine wear and damage.
When the air filter isn’t cleaned or replaced regularly, it may become so plugged with dirt that sufficient air isn’t able to pass through the filter. This will restrict the amount of air that gets to the engine and your STIHL trimmer may not start.
To avoid this problem in the future, check the filter’s condition monthly. Remove the filter and clean it using the steps listed below. Always replaced a very dirty or damaged filter.
NEVER run your trimmer with a very dirty air filter or without an air filter. It is a sort of insurance policy protecting the engine from engine wearing due to dirt.
SOLUTION: Remove the air filter cover and clean or replace the air filter. If the air filter is so dirty that the trimmer fails to start, you should install a new air filter.
How 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.
Bad Spark Plug
The spark plug is another maintenance item that should be replaced annually to keep your string trimmer running at its best. When the spark plug is fouled, it may not provide the spark needed to start.
SOLUTION: Before removing the spark plug, wipe around the plug to remove all dirt and debris. Remove the plug and check its condition. Replace a spark plug that is very dark in color, has signs of wear, or is damaged.
You can attempt to clean a mildly dirty spark plug with a wire brush and reuse it. I prefer to replace it. It is an inexpensive part and one of the primary items responsible for keeping your string trimmer running.
Check the electrode gap and make sure it is set to STIHL’s specification. Install the spark plug and securely attach the boot. (If you are continuing to work on the trimmer, detach the spark plug wire.
Incorrect Fuel Mix
Using a fuel mix without sufficient lubrication may cause the engine to seize and fail to start. Never add straight gas to a STIHL string trimmer.
It is a sure way to damage an engine and possibly result in having to spend money to purchase a new trimmer. Find out more information on choosing the right gas and mixing it with oil in STIHL String Trimmers Use This Type of Gas and Oil Fuel Mix.
2-Cycle Gas to Oil Mix
All STIHL string trimmers require a gas-to-oil mix ratio of 50:1. This is the same requirement for STIHL 2-cycle engines and STIHL 4-MIX engines.
| Mixture | 1 Gallon Gas | 2 Gallon Gas | 2.5 Gallon Gas |
|---|---|---|---|
| 50:1 | 2.6 oz Oil | 5.2 oz Oil | 6.4 oz Oil |
When creating the oil and gas mixture, use unleaded gasoline with a minimum octane rating of 89 (mid-grade) and maximum ethanol content of 10%. Add a 2-cycle premium oil that is ISO-L-EGD and JASO M345 FD certified.
Mix it in an approved gas can before adding it to your string trimmer. STIHL recommends using a 2-cycle oil like their High Performance or HP Ultra oils.
2-Cycle Premixed Fuel
A great option to reduce fuel problems and extend engine life is using an ethanol-free fuel mix. This is an ethanol-free blend of oil and fuel that is ready to pour into your string trimmer’s fuel tank.
You won’t have to deal with the bad effects of ethanol as discussed in the next section. Also, it’s convenient to have fuel available on your shelf when you need it. STIHL MotoMix pre-mixed fuel is a good 50:1 premixed fuel option.
SOLUTION: Drain the fuel tank and fill it with the correct gas-to-oil mix. If you continue to have problems, have a small engine mechanic diagnose the problem and determine whether a cost-effective repair can be made.
Old Fuel
It’s best to consume fresh gasoline within 30 days of purchase. This is because gas can begin to degrade after this time.
Old fuel left in a string trimmer can leave behind sticky deposits and varnish. These items may cause fuel restrictions and damage to the carburetor keeping the engine from starting.
Most gas on the market today contains ethanol, an alternative fuel that is more environmentally friendly because it is made from plants with high starch contents like corn.
While ethanol may be better for the environment, it is not kind to the small engine used on your trimmer. This product naturally attracts moisture from the air causing premature corrosion and fuel restrictions.
Because of this, always use unleaded gasoline with a minimum octane rating of 89 (mid-grade) and maximum ethanol content of 10% (E10).
Never use E15 or E85 in the engine as this will damage the engine and most likely void manufacturer warranties. These fuels contain up to 15% and 85% ethanol respectively.
Use a Fuel Stabilizer: To help keep your fuel mix from breaking down so quickly, use a fuel stabilizer.
Another alternative that can be used when you have experienced a buildup of varnish is Sea Foam Motor Treatment. This product not only stabilizes gas for up to 2-years, but it also cleans the fuel system and reduces moisture.
SOLUTION: Drain old fuel in your string trimmer. Create a gas and oil mix in a fuel container. Add Sea Foam Motor Treatment to the mixture.
Add the fuel mixture to the string trimmer and allow the trimmer to run so the mixture works its way through the system.
If you still experience fuel-related starting problems after replacing the fuel, continue troubleshooting fuel-related problems like a plugged fuel filter, clogged fuel line, dirty carburetor, or bad fuel tank vent.
Bad Primer Bulb
A cracked primer bulb that won’t fill with fuel won’t function correctly to get fuel to the carburetor for starting the string trimmer.
If the primer bulb appears in good condition and it still won’t fill, check for a clogged fuel filter or fuel line.
SOLUTION: Replace with a new primer bulb.
Plugged Fuel Filter
The fuel filter used on a string trimmer can be found in the fuel tank. This filter is used to strain fuel before it enters the fuel pick-up tube. It is designed to keep dirt and debris out of the fuel system.
Just like the air filter, the fuel filter can become plugged when it isn’t changed out regularly. I recommend replacing the fuel filter at least once a year while checking it periodically.
If you find your fuel is dirty, you may need to drain the fuel tank, fill it with fresh fuel and replace the fuel filter.
When the fuel filter becomes clogged with dirt, it will restrict the amount of fuel that is able to flow through the filter and into the fuel lines. This can keep your engine from getting the fuel it requires to start and run.
SOLUTION: Locate the fuel filter inside the fuel tank and replace the plugged filter. Replace the fuel if you find it dirty as well.
- Wipe around the fuel cap to remove any loose dirt so it doesn’t fall into the fuel tank once you remove the cap.
- Gain access to the filter. A clean bent wire works well to “fish” the filter and pull it out of the tank.
- Remove the old filter from the fuel line. Be careful not to lose the retaining ring securing the line to the filter.
- Install the new fuel filter securing the fuel line to the filter using the retaining ring.
- Place the filter back inside the fuel tank and reinstall the fuel cap.
Clogged Fuel Line
Old fuel sitting in your string trimmer can leave a gummy sticky deposit behind that restricts fuel flow. This can clog the fuel line and restrict the fuel flow your string trimmer requires to start.
SOLUTION: Replace a fuel line on your trimmer when it is cracked, kinked, or clogged.
Plugged Fuel Tank Vent
There must be a way to vent the fuel tank to allow air to pass in and out of it. Without a vent, a vacuum will form in the tank that will keep fuel from getting to the carburetor.
STIHL uses a small fuel tank vent part to allow the tank to vent. When this part is clogged and no longer works, your trimmer will begin to run sluggish and possibly die.
A good indication you may have a fuel tank vent problem is when your trimmer runs for a short period and then runs sluggishly and shuts down. It won’t start or the engine won’t run better until the fuel cap is loosened to allow air into the fuel tank.
When you tighten the cap and allow it to run and the string trimmer dies and fails to start until the cap is loosened, you most likely have a fuel vent problem.
SOLUTION: Replace a plugged fuel tank vent.
Dirty Carburetor
The carburetor regulates the amount of fuel that is mixed with air to create combustion in the cylinder. Old fuel will gum up and clog the carburetor so it no longer functions properly.
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.
If the carburetor does not function after being cleaned, you may need to rebuild it or replace it with a new carburetor.
Depending on the model and age of the string trimmer you use, the price of a carburetor, and the cost of labor, it may be best to invest in a new string trimmer rather than put money towards replacing a carburetor on an old trimmer.
Bad Recoil Starter
Your trimmer uses a recoil to start the engine. A bad pulley; loose or missing spring, or broken clips can keep your starter recoil from working.
SOLUTION: You can attempt to replace the spring and restring the recoil. If it does not work because other components in your recoil are damaged, such as the clips or the pulley, you are better off just replacing the recoil assembly.
Plugged Spark Arrestor
You will find a spark arrestor in your string trimmer that can prevent it from starting. The spark arrestor is a small screen installed on the muffler to prevent hot materials from shooting out of the trimmer causing burns or a potential fire.
The spark arrestor can get plugged with soot. When this happens, the string trimmer will fail to start.
SOLUTION: Disconnect the spark plug boot. Let the muffler cool. Remove the spark arrestor screen cover, gaskets, and spark arrestor from the muffler. Clean the screen with a wire brush to remove the soot. Reinstall.
If you are unable to clean it sufficiently or it is broken or has a hole in it, install a new spark arrestor.
Flooded Engine
I have had customers bring their string trimmer to the repair shop because they can’t get it started. Many times it’s due to a flooded engine which isn’t too serious.
The engine can become flooded when the choke is in the closed position and the starter rope was pulled many times allowing too much gas into the carburetor.
It can also happen with the switch off and the starter rope being pulled multiple times or when the primer bulb is pushed too many times.
SOLUTION: Use the following procedure to “unflood” your string trimmer so the engine gets the correct fuel-to-air ratio required to start and run.
Start a Flooded Engine Option 1:
- Turn the choke knob to the off/open position.
- Hold the throttle trigger lockout and throttle trigger down while placing the slide control in the start position to place it in the starting throttle position.
- Pull the starter rope. This can take anywhere between 10 and 20 pulls before it starts. Your chainsaw engine will sputter first. Continue to pull 2 to 3 more times and it should start.
Start a Flooded Engine Option 2:
- Place the slide control in the stop position.
- Remove the spark plug.
- Pull the starter rope 6 to 8 times.
- Reinstall the clean dry spark plug.
- Start the chainsaw with the choke knob in the off/open position.
STIHL String Trimmer Starting Problems & Solutions (Quick Reference Chart)
| Problems | Solutions |
|---|---|
| Wrong Choke Setting | Ensure the choke knob is on/closed to restrict airflow when starting a cold engine. Remove the choke once the engine has warmed so it continues to run. |
| Old Fuel | Replace fuel that is older than 30 days with fresh fuel. Use a fuel stabilizer. |
| Incorrect Fuel | Drain the fuel. Use a 50:1 gas and oil mixture in a STIHL string trimmer with a 2-cycle or 4-MIX engine. |
| Plugged Air Filter | Remove a plugged air filter and clean it. Replace the filter with a new one if it is very dirty or damaged. |
| Bad Spark Plug | Replace a bad spark plug. Make sure the electrode gap is correct and the spark plug wire is securely in place. |
| Bad Fuel Tank Vent | Replace a clogged fuel tank vent. |
| Plugged Fuel Filter | Remove the old filter from the fuel tank and install a new fuel filter. |
| Weak Primer Bulb | Replace a cracked primer bulb. |
| Clogged, Dry, or Cracked Fuel Line | Remove a clogged fuel line and remove the restriction. Replace a fuel line if the clog can’t be removed or the line is dry or cracked. |
| Dirty Carburetor | Remove and clean the carburetor. Rebuild any damaged parts or install a new carburetor assembly. |
| Bad Recoil Starter | Replace a spring and restring the recoil starter. Replace the recoil starter when it is damaged. |
| Plugged Spark Arrestor | Remove the spark arrestor from the muffler. Clean it using a wire brush and reinstall it. Replace a damaged spark arrestor. |
| Flooded Chainsaw | Fix this by using one of the methods above to remove excess fuel and start the chainsaw. |