It’s hard to get all of your yard work done when the string trimmer won’t stay running. I’ve put together a list of common items that can cause a trimmer to die so you can get it up and running again.
A STIHL string trimmer starts, stalls, and dies when it isn’t getting sufficient air, fuel, or spark due to a plugged air filter, wrong choke setting, plugged spark arrestor, clogged fuel line, plugged fuel filter, dirty carburetor, bad fuel tank vent, or a fouled spark plug.
Follow the safety precautions in the STIHL operator’s manual. Wait for the engine to cool and remove the spark plug wire before performing repairs.
10 Reasons a STIHL String Trimmer Starts Then Dies
Incorrect Choke Setting
You will find a choke knob on a string trimmer. This is to open and close the choke to control airflow.
Because more fuel and less air are required to start a cold engine, the choke must be placed in the on/closed position to restrict airflow.
Once the engine gets warm, the choke knob must be adjusted to the off/open position to allow sufficient air for the engine to continue to run. If the choke isn’t adjusted, the trimmer will begin to sputter and die.
Plugged Air Filter
A lot of dust and debris gets tossed into the air when using a trimmer. An air filter is used to protect the engine by keeping this dirt from entering the carburetor throat.
When the air filter isn’t regularly cleaned or replaced to keep it in good condition, the filter can become so dirty that sufficient airflow isn’t able to pass through the filter.
The engine will be starved of clean air and will shut down.
I recommend replacing the air filter annually and checking it regularly to keep it in good condition. A filter that is very dirty, damaged, or wet must be replaced with a new filter. When it is a little dirty, remove and clean it.
How to Clean a STIHL 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.
Plugged Cooling System
The engine can die when it gets too hot. Air is used to keep a trimmer engine cool.
First, wait for the engine to cool. Check the cooling system and remove all dirt and debris from around the engine cover, the air intakes, the outside of the cylinder, and the cylinder fins that may prevent air circulation around the engine.
Continue cleaning the trimmer to make sure cool air can circulate around the engine.
Most STIHL trimmers run 2-cycle engines while a few models run a 4-MIX engine. Both types of engines require a fuel mixture of gas and 2-cycle oil at a ratio of 50:1.
Mistakenly using the wrong type of fuel or the wrong fuel mix can cause engine problems. Never run a straight gas through a STIHL trimmer. This is a sure way to cause the engine to seize due to a lack of lubrication.
Additionally, letting old gas sit in a string trimmer for long periods can damage fuel components.
STIHL string trimmers require gasoline and oil mixed at a ratio of 50:1. Mix unleaded gasoline of a minimum octane rating of 89 and a maximum ethanol content of 10% with a premium 2-cycle oil that is ISO-L-EGD and JASO M345 FD certified.
Always use fuel with an ethanol content of 10% or less. Non-ethanol fuel is best, but it’s the more expensive choice.
You can purchase STIHL 2-cycle engine oil at your local Ace Hardware or independent STIHL dealership. STIHL recommends using a 2-cycle oil like their STIHL High Performance or STIHL HP Ultra.
Because gas can begin breaking down as soon as 30 days after purchase. Add a fuel stabilizer to fresh fuel to make it last a little longer without breaking down. A fuel stabilizer can help minimize fuel issues from running old gas.
Keep in mind some 2-cycle oils include a fuel stabilizer. Even though the bottle of oil states it includes a stabilizer, read the fine print to see how long the additive will keep the fuel stable.
Read more about choosing the right gas and how to care for it in STIHL String Trimmers Use This Type of Gas and Oil Fuel Mix.
Plugged Fuel Filter
To keep dirt and debris from entering the fuel system, you will find a small filter located inside the fuel tank. It is attached to the end of the fuel line. The filter can become plugged when it isn’t replaced regularly.
The fuel restriction caused by the plugged filter can keep your trimmer from running. I recommend changing the fuel filter annually or more often when needed. You may have to replace it as often as every 3 months when using it for commercial purposes.
Replace a fuel filter:
- 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 Lines
The deposits left behind by old fuel can cause a buildup in the fuel line that will prevent a good flow of fuel from running through them.
When you find a clogged fuel line, remove the line and try to remove the restriction using carburetor cleaner to loosen the clog and compressed air to remove the clog.
If the fuel line is dry, cracked, or damaged, you should replace it with a new fuel line.
Plugged Fuel Tank Vent
The fuel tank must be able to vent to allow air to pass through it. When the vent gets plugged, the tank will form a vacuum. This vacuum prevents fuel from getting to the carburetor.
You most likely will experience the trimmer bogging down and running rough. It may also just shut down to a lack of fuel.
If you are not getting fuel to the carburetor, but you are running a good fuel filter, the carburetor is in good condition and you don’t have any clogs in the fuel lines, you may have a clogged fuel vent.
You can test this by placing your string trimmer on a level surface. Make sure you are in a well-ventilated area
Loosen the fuel cap to allow air into the tank and start the trimmer. Don’t allow gas to spill out of the fuel tank. When the trimmer starts and runs, replace the fuel cap with the trimmer still running.
Continue to let it run for a while to try to replicate the problem. If it sputters and dies with the cap on and then starts and runs once the cap is removed, you most likely have a problem with the fuel tank vent.
Replace the vent with a new one.
The carburetor regulates the amount of fuel that is mixed with air to form combustion in the cylinder. When the carburetor fails to function properly, the engine doesn’t get the gas it requires to keep running.
A carburetor may no longer work when old fuel leaves behind varnish that clogs the passageways.
The crusty deposit left behind by this fuel can cause internal parts to stick and fail to allow fuel to fill the carburetor bowl.
When the carburetor fails to work, you can attempt to clean it to remove the deposits and clean up the parts. I recommend taking photos through the disassembling process so you can be sure you put the carburetor back together correctly.
If you have removed and cleaned the carburetor and it still doesn’t function, you may need to rebuild it or replace it with a new carburetor.
Before replacing a carburetor, I recommend comparing the cost of the carburetor and labor expense to the cost of a new trimmer. Depending on the model, cost, and age of your string trimmer, you may be better off purchasing a new one.
Dirty or Bad Spark Plug
When the spark plug is worn or becomes dirty, it may provide an intermittent spark that may cause the engine to die. Remove the spark plug and inspect its condition.
If you find the tip is very dark in color, the electrode is burnt or the porcelain is cracked, it’s time to replace the spark plug. If it’s just a little dirty, you can try to clean it with a wire brush.
However, if I were you and you suspect the spark plug may be the problem, I would just go ahead and replace it with a new one. The spark plug is an inexpensive maintenance part that plays a major role in how well the trimmer performs.
Check the electrode gap and make sure it meets STIHL’s specifications. Once you have confirmed the spark plug is good and the gap is correct, reinstall the spark plug. Wait to attach the spark plug wire until you have completed all repairs.
Plugged Spark Arrestor
The spark arrestor screen can become plugged with soot and no longer allow airflow to exhaust. The buildup on the screen can make your string trimmer run sluggish and quit
The spark arrestor screen is a small metal screen installed on the muffler. This is required by law in many countries to keep hot exhaust material from shooting out of the trimmer and causing injury or starting a fire.
Clean a spark arrestor screen:
- Disconnect the spark plug wire.
- Allow the muffler to cool.
- Remove the spark arrestor screen cover, gaskets, and spark arrestor screen for the muffler.
- Clean the screen with a wire brush or a commercial solvent.
- Replace the screen with a new one if it is damaged, has a hole in it, or isn’t able to be sufficiently cleaned.
Contact your local STIHL dealer if you are having trouble locating the spark arrestor screen or if you are continuing to experience problems with your string trimmer.