Using your string trimmer to add the finishing touches to your lawn care by trimming around the landscaping, the fence posts and your pond makes your home’s exterior look complete.
It is also a great tool for those “hard to mow” places and tall grasses. When your string trimmer dies, your need to get it fixed.
A string trimmer will start and then die when it isn’t getting fuel to the engine due to running old or dirty fuel that clogs the fuel filter, fuel line, fuel vent, or carburetor. It will also quit when the air filter is plugged, the spark arrestor is clogged or the choke setting is incorrect.
Before repairing your string trimmer, disconnect the spark plug and wait for all moving parts to stop.
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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.
This is Why Your String Trimmer Starts Then Dies
1. Incorrect Choke
A string trimmer has a choke to restrict airflow to the engine so a higher concentration of fuel is allowed to start a cold engine.
When the engine warms up, the choke must be turned off to allow a normal rate of air to be combined with fuel to keep running.
When the choke isn’t properly adjusted after the engine warms, the engine will die when it can’t get proper airflow.
SOLUTION: Ensure the choke is adjusted to the off position once the is warm.
2. Plugged Air Filter
Just like the engine will die from a lack of air when the choke is left on, it can quit from a lack of air when the air filter becomes plugged.
Dirt and debris get tossed into the air when using a string trimmer. These dirty conditions can plug the air filter and cause the trimmer to die.
It’s good practice to replace the air filter annually when completing a full annual service. Periodically check the air filter to clean it and make sure it is in good condition.
Replace it if it becomes very dirty or damaged. Never run your string trimmer without an air filter, even if it’s only for a short time while waiting for a new filter.
SOLUTION: If you find your air filter is plugged, I recommend replacing the filter. The small air filter is usually not very expensive. It is an important component when it comes to protecting the engine.
Clean the string trimmer air filter:
- A foam, fabric, or felt-style filter can be cleaned using mild detergent and water. Wash the filter to remove as much dirt as possible. Rinse until clear. Allow to dry completely. Some FOAM filters must be coated in engine oil to help trap dirt.
- A paper filter can be cleaned by knocking it against a solid surface to remove as much dirt as possible. Replace it with a new one if it is extremely dirty or damaged.
Because there are so many different types of filters used which varies by manufacturer and model of the trimmer, refer to your operator’s manual for steps to clean the style of air filter in your string trimmer.
3. Plugged Cooling System
When the engine gets too hot, the string trimmer may shut down. To keep the engine cool, it requires air to circulate around the engine.
SOLUTION: Remove all grass clippings, dirt, and debris from around the air intakes and cooling fins that may be preventing air circulation. To do this, first, remove the spark plug and wait for the engine to cool.
Remove the engine cover and remove debris from the cover and around the outside of the cylinder. Clean the cylinder fins and reinstall the engine cover. Continue cleaning the trimmer to make sure cool air can circulate around the engine.
4. Old or Bad Fuel
Gasoline that has been sitting around for over 30 days can begin to break down and lose some of its combustible properties.
Old fuel attracts moisture that can leave behind varnish and sticky deposits that clog the fuel system and cause string trimmer components to fail. Ethanol-based gasoline can have negative effects on your string trimmer.
Use unleaded gasoline with a minimum octane rating of 89 and a maximum ethanol content of 10%. A low ethanol content or ethanol-free fuel is best for your fuel system and engine.
Add a fuel stabilizer like Sea Foam Motor Treatment when you are unable to use fuel within 30 days of purchase.
Most string trimmers today use a 2-cycle (2-stage) engine that requires gasoline and oil to be mixed before filling the fuel tank.
You will also find manufacturers beginning to release 4-cycle (4-stage) string trimmers to the market. Oil and gas are not mixed for these types of engines.
- 2-cycle engines require gas and oil to be mixed before adding to the fuel tank.
- 4-cycle engines require straight gas. There are separate fill ports for gas and oil. (The exception to this is STIHL’s 4-MIX engines that use a gas and oil fuel mix).
Know what type of engine is on your string trimmer so you don’t ruin your string trimmer. Adding the wrong type of fuel to your string trimmer can damage the unit beyond repair.
Read this article for more information on identifying the engine type on your string trimmer and the fuel to use.
SOLUTION: Drain the fuel tank. Mix fresh fuel and a fuel additive in an approved gas container. Add the mixture to the fuel tank.
5. Plugged Fuel Filter
A fuel filter that strains fuel to keep dirt and debris from entering the fuel system can become plugged when it isn’t changed out regularly.
A plugged filter will restrict the amount of fuel that is allowed to flow to the engine causing it to start and then quit. You will find the filter inside the fuel tank.
SOLUTION: Replace a string trimmer fuel filter.
- Wipe around the fuel cap to remove dirt and debris so they don’t fall into the tank.
- Remove the cap.
- Take note of the placement of the filter so you install the new filter in the correct position.
- Pull the fuel filter out of the fuel tank. A clean bent wire works well to retrieve the filter.
- Once the filter is out of the tank, remove it from the fuel line. Be careful not to lose the retaining ring. Keep the ring on the fuel line.
- Attach the new fuel filter by inserting the male end into the fuel line and securing the line to the filter using the retaining ring.
- Place the fuel filter back inside the fuel tank.
- Install the fuel cap.
6. Clogged Fuel Lines
The fuel lines can become clogged with dirt and sticky deposits left behind by old or bad fuel. Look for a clog or kink in the fuel line.
While looking for clogged lines, check for punctures in the lines that can cause the engine to run lean and stall.
SOLUTION: To determine if you have a fuel restriction, remove the fuel line off the carburetor to see if you are getting good flow. Replace any fuel line that is dry, cracked, clogged, or kinked with a new fuel line.
7. Plugged Fuel Tank Vent
The fuel tank has a vent to allow air to pass through it. When the vent no longer functions correctly and becomes plugged, the fuel tank will act like a vacuum.
Fuel will no longer flow out of the tank and through the fuel lines.
If you are not getting fuel to the carburetor and you are running a good filter and you don’t have any clogs in the fuel lines, you may have a clogged fuel vent.
SOLUTION: You can test the vent by placing your string trimmer on a level surface. Remove 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.
If it sputters and dies with the cap on and then immediately 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 fuel tank vent.
8. Dirty Carburetor
The carburetor mixes the correct proportion of air and fuel required for your string trimmer to run. The passageways can become clogged and the small components can fail to function correctly.
This can cause an insufficient amount of fuel mixed with air resulting in a rough running string trimmer that may just quit.
SOLUTION: You may be able to clean your carburetor to get it working again. You may end up having to rebuild (if rebuild kits are available for your carburetor) or replace the carburetor if cleaning doesn’t help.
9. 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.
SOLUTION: 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.
Check the electrode gap and make sure it meets the manufacturer’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.
10. Bad Ignition Coil
The winding on the ignition coil can separate and short out. When this happens, the spark plug won’t get the voltage required to create a spark. This will cause your trimmer to die after it’s been running.
SOLUTION: Identify a bad ignition coil using an ohmmeter to check for a break in continuity. Replace the ignition coil if you find a break.
11. Plugged Spark Arrestor
A small metal screen keeps hot exhaust material from shooting out of the string trimmer and causing injury or starting a fire. This small screen will become plugged with a carbon buildup that will affect how the engine runs.
SOLUTION: Disconnect the spark plug wire. Remove the engine cover and the engine exhaust cover. Carefully remove the spark arrestor screen. Clean it with a wire brush. Reinstall the screen.
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.
Still Having Troubles With Your String Trimmer?
As the owner of a string trimmer, you’re going to run into problems with it occasionally. This is true of all trimmers no.
To help you quickly identify the cause of your problem and how to fix it, I’ve put together a handy reference guide. It includes information on what to do when your string trimmer stops running, won’t start, bogs down, and more.
You will find tables with problems and solutions to many common string trimmer issues along with links to information in more detail here.