You finish mowing your lawn, but the yard isn’t complete until you trim around the trees, landscaping, and fence posts. A string trimmer is an important addition to your arsenal of yard tools to achieve a well-manicured lawn.
A string trimmer will not start when the air filter is plugged, the spark plug is bad, the carburetor is dirty, old fuel gummed up the fuel system, an incorrect fuel mix is used, the wrong oil is used, the recoil starter is bad, or the engine is flooded.
Follow all safety precautions in your operator’s manual to avoid injury while working on your string trimmer. This guide references 2-cycle and 4-cycle engines.
While most gas-powered string trimmers on the market today use 2-cycle (2-stage) engines, you will also find some 4-cycle (4-stage) engines.
Check your operator’s manual if you are unsure what type of engine you are running on your string trimmer.
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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.
13 Reasons Your String Trimmer Will Not Start
The first thing I do when I am trying to solve a starting problem on a string trimmer replaces the maintenance items: fuel filter, air filter, and spark plug. This will rule out the simple items as the cause of the problem.
Plugged Air Filter
The engine needs clean air to start and run. To get clean air, an air filter is used to prevent dirt and debris from entering the air intake. When the filter isn’t changed out regularly, it can become plugged and not allow air to pass through the filter.
Never operate your string trimmer without an air filter, even if it is just for a short period so you can finish a task. Dirt in the engine can cause significant engine damage which may result in having to buy a new string trimmer.
Solution: Remove the filter and wipe out any remaining dirt from the air filter housing. Replace a dirty air filter with a new air filter.
Bad Spark Plug
The spark plug will fail and misfire when it has a buildup of carbon and oil, cracked porcelain, or a burnt electrode. When the plug isn’t functioning properly, it won’t provide the spark required to start your trimmer and keep it running.
Solution: You can attempt to clean a dirty spark plug with a wire brush and reuse it. I prefer to replace it. It is an inexpensive part and a primary item to keep your string trimmer running.
Make sure your spark plug is properly gapped and installed correctly as this can also result in a starting issue.
Plugged Fuel Filter
The fuel filter attaches to the fuel hose located in the fuel tank. Its purpose is to strain the fuel before it enters the fuel system to keep the dirt and debris out.
This filter can become plugged from running dirty fuel and not changing the filter out regularly. This can restrict fuel flow to the engine causing your string trimmer to fail to start.
Solution: Locate the fuel filter in the fuel tank. Remove the old filter from the fuel line and replace it with a new fuel filter.
Incorrect 2-Cycle Oil Mix for a 2-Cycle Engine
Using straight gas in a 2-cycle string trimmer will damage the engine and cause it to seize up. Adding straight gas to your string trimmer is a quick way to ruin it.
The 2-cycle engine in a string trimmer uses gas and oil mixed at a ratio of 50:1, 40:1, or 32:1. For example, 50:1 mix equals 50 parts gasoline to 1 part oil.
You can find the correct mix ratio for your model in your operator’s manual. You may also find it on your fuel cap. The ratio varies by manufacturer and age of the units. Below I list the gas-to-oil mix for the current 2-cycle trimmers.
When creating this mix, 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.
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.
2-Cycle Premixed Fuel
A great option to reduce fuel problems and extend engine life is using an ethanol-free fuel mix. Many manufacturers offer their own brand of premixed fuels.
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 fuel section. Also, it’s convenient to have fuel available on your shelf when you need it.
2-Cycle Gas to Oil Mix
2-Cycle String Trimmer Manufacturer & Fuel Mix
|2-Cycle Gas to Oil Mix Ratio
|Red Armor Pre-Mix
|Red Armor Pre-Mix
|Moto Mix Pre-Mix
Incorrect or Insufficient Engine Oil in a 4-Cycle Trimmer
4-cycle engines have separate fill ports for the engine oil and the fuel. DO NOT mix the oil and fuel together if you own this type of engine. Never use 2-cycle oil in a 4-cycle trimmer.
Oil is required to keep the engine components lubricated. When the wrong type or not enough oil is used, friction can build in the engine and overheat causing your string trimmer not to start and possibly ruin the engine.
Solution: Drain the engine oil and fill it with the correct oil grade. When running your trimmer in very cold or very hot temperatures, you may have to adjust the viscosity to your ambient temperature.
If you continue to have problems, have a small engine mechanic diagnose the problem and determine whether a cost-effective repair can be made.
4-Cycle String Trimmer Manufacture and Engine Oil Viscosity
|SAE30 (Recommended), 10W-30, 5W-30
|20W-50 (Recommended), SAE30, 10W-30, 10W-40
Old Fuel in a String Trimmer
Old fuel left in a string trimmer won’t only cause fuel restrictions, but it can also damage the carburetor and engine. Gasoline can begin to break down as quickly as 30 days after purchase.
The ethanol found in most types of gasoline attracts moisture from the air. This moisture and ethanol mixture gums up the fuel system and components.
Because gasoline can begin to break down and become less effective as soon as 30 days after purchase, the fuel must be consumed within this time frame.
If you are unable to use it in this amount of time, add a fuel additive to stabilize the fuel so it lasts a little longer.
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.
Solution: Drain any old fuel remaining in your string trimmer and fill it with fresh fuel. This is an oil and fuel mix for a 2-cycle engine and unleaded gasoline for a 4-cycle engine.
Add a fuel stabilizer like Sea Foam motor treatment to stabilize the fuel, clean the fuel system and reduce moisture in the fuel.
Bad Primer Bulb
A cracked primer bulb that won’t fill up with fuel won’t function correctly to get fuel to the carburetor for starting the string trimmer.
Solution: Replace with a new primer bulb.
Fuel Line Blocked
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 in the string trimmer when it is cracked, kinked, or clogged.
Plugged Fuel Tank Vent
The fuel tank vent allows air into the tank. Without a vent, the fuel tank will create a vacuum that won’t allow fuel to flow 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 with the fuel cap in place.
Solution: Replace the fuel tank vent so air can flow into the fuel tank. Depending on the manufacturer of your string trimmer, you will find the tank vent attached to a line coming out of the tank or in the fuel cap.
Clogged & 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.
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.
Bad Recoil Starter
Your string trimmer uses a recoil to start the engine. A bad pulley; loose or missing spring; or broken clips can keep your 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.
Bad 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 that can get plugged with soot.
Solution: Disconnect the spark plug boot. Remove the engine cover and engine exhaust cover. Remove the spark arrestor and clean it with a wire brush to remove the soot.
If you are unable to clean it sufficiently or it is broken or has a hole in it, replace it with a new spark arrestor.
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.
How to Fix a Flooded Engine on a String Trimmer
- Turn the switch on to the run position.
- Move the choke lever to the run position.
- Press the throttle trigger while pulling the starter rope over and over. This can take anywhere between 5 and 15 pulls before it starts. Your string trimmer engine will sputter first. Continue to pull a few more times and it should start.
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.