Your string trimmer has been giving you some trouble. Thankfully, it will still start and run, but it’s not working as it should.
To keep it running, you now have to keep the choke on or run it at half choke. I’ll try to help give you some tips on how to find and fix your problem.
A string trimmer that only runs with the choke on is either not getting enough fuel or it is getting too much air.
This can be from running old gas, a hole in the fuel line, a plugged air filter, a dirty carburetor, a bad carburetor gasket, or a plugged fuel vent.
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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.
6 Reasons a String Trimmer Only Runs With the Choke On
Old Gas
Gas that has been sitting in your string trimmer for long periods of time can develop a varnish and leave gummy deposits that restrict the fuel flow.
It is important to use the right fuel in your string trimmer and care for that fuel to minimize fuel-related problems.
Because gas begins to break down after 30 days, it’s important to only use fresh fuel in your string trimmer.
Use gasoline with a minimum octane rating of 89 and a maximum ethanol content of 10%. Using gas with higher ethanol contents will cause engine damage.
String trimmers with 2-cycle engines require oil to be mixed with the gas. String trimmers with 4-cycle engines require straight gasoline (no oil mixed in). STIHL’S 4-MIX engine is an exception to this.
Read more about choosing and caring for the gas in your string trimmer at This is the Type of Gas and Oil String Trimmers Use.
SOLUTION: Remove old gasoline from your string trimmer. Add fresh fuel that includes an additive like Sea Foam or STA-BIL to stabilize the gas, clean the fuel system and reduce moisture.
Puncture in the Fuel Line
Look for a hole or puncture in the fuel line that is causing the string trimmer to suck in additional air. This will cause you to need to have the choke on to reduce the amount of air getting to the engine. Also, check for fuel leaks coming from the fuel line.
SOLUTION: Remove and replace any damaged or cracked fuel line.
Plugged Fuel Filter
Another item that can restrict fuel flow is a plugged fuel filter. A fuel filter is used to prevent dirt and other debris from running through the fuel system and engine.
When the filter isn’t changed regularly or you’re running very dirty fuel, it can become plugged not allowing sufficient fuel to pass through the filter. It’s best to stay on top of your fuel filter maintenance by changing it out annually.
SOLUTION: Replace a plugged fuel filter. The filter is located inside the fuel tank. Wipe around the fuel tank cap before removing it to keep dirt from falling into the tank. Pull the filter out of the tank.
A clean bent wire works well to “fish” the filter out of the tank. Remove the old filter and attach a new fuel filter to the end of the fuel line and place it back inside the fuel tank. Reinstall the fuel cap.
Bad Carburetor Gasket
The gasket that sits behind the carburetor can deteriorate and become worse over time. When this happens, it no longer seals properly allowing additional air into the system causing it to run lean.
This is when there is a higher concentration of air and less fuel than required by the engine.
SOLUTION: Gain access to the carburetor and carefully remove the linkages and bolts attaching the carburetor. Remove the carburetor and gasket.
Reinstall a new gasket and reattach the carburetor, bolt, and linkages. You may need to clean the carburetor while it is off the string trimmer.
Dirty Carburetor
The carburetor’s function is to regulate the amount of fuel that is mixed with air to form combustion to start and run the string trimmer.
A buildup of varnish and deposits can make the carburetor not function right so it isn’t able to get fuel to the engine.
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 the carburetor cleaner to remove deposits left behind from old fuel.
If the carburetor does not function after being cleaned, you may need to rebuild it or replace it with a new carburetor.
Carburetors for a string trimmer can run between $60-$140 depending on the manufacturer of your string trimmer.
Depending on the model string trimmer you run and the price of the carburetor, it may be best to invest in a new string trimmer rather than put money towards replacing a carburetor on an old trimmer.
You can also bring your string trimmer to a small engine repair shop to have the carburetor cleaned or replaced if you are not comfortable doing this. Note: the repair shop may just replace the carburetor instead of taking time to clean it.
This isn’t a bad thing. The amount of labor that will be charged to remove, clean, and reinstall a carburetor may be equal to or more than the cost of just replacing it with a new one.
Plugged Fuel Vent
The fuel tank must be vented to allow air into the tank to equalize the air pressure. Without a vent, the fuel tank will create a vacuum that will prevent fuel from flowing through the string trimmer.
A good indication you may have a fuel tank vent problem is when your string trimmer runs for a few minutes and then shuts down and won’t start until you remove the fuel cap to allow air into the fuel tank.
It then shuts down again after running for several minutes once you reinstall the fuel cap.
Most string trimmers use a fuel tank vent that is attached to the fuel line coming out of the fuel tank. You may find the vent in the fuel cap on some string trimmers.
SOLUTION: Replace the fuel tank vent or fuel tank cap (depending on your model string trimmer) so air can flow into the fuel tank.
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.