Why Won’t My String Trimmer Start? Common Causes and How to Avoid Them
You’re in “go” mode, ready to tackle that yard work. You grab your trusty string trimmer, pull the cord, and … nothing happens.
It’s frustrating when your weed wacker acts up but the good news is, most mechanical issues are easily fixable—even if you’re not an expert. Better yet, they’re preventable!
The solution may lie within the pages of your owner’s manual. It’s possible the problem is something as simple as learning that this trimmer brand or model starts differently than your previous machine. Along with the user guide, we’ve got some quick answers to help you troubleshoot why your weed whacker won’t run.
We’ll look at ways to diagnose what’s causing the hiccup and provide maintenance ideas to keep your weed whacker running like new.
Old or Bad Fuel
If your trimmer won’t start, bad fuel may be to blame. Gasoline, especially ethanol blends, breaks down over time and gets old or dirty, which can cause multiple problems that prevent your weed eater from starting.
A good rule of thumb is to use a tank of gas within 30 days, something that is rarely a problem for those who use their trimmers often. If this is you, mix one gallon of ethanol-free gas with a bottle of STIHL Ultra and you shouldn’t need to worry about stale fuel.
Not everyone uses their trimmer often enough to go through a tank of gas that quickly, though.
If you use the trimmer infrequently, skip the gas altogether and use a bottle of STIHL MotoMix instead. Because this fuel mix contains no ethanol, it remains stable for two years after it’s opened, giving you plenty of time to burn through it.
Solving the Problem
Ensure the trimmer is turned off, then open the fuel cap and dump the dirty gas into a fuel-safe container. Be sure to pump the primer bulb to fully clear the stale gas from the line, and run the carburetor dry to empty all gas from the engine.
Preventing the Problem
Use ethanol-free gas and a bottle of STIHL Ultra within 30 days, or use STIHL MotoMix if you don’t use your trimmer frequently.
Add STHIL MotoMix at the end of season before storing your trimmer.
Dirty or Clogged Air Filter
Your weed eater requires good air flow to operate, and if dust, dirt, and debris get caught in the filter, it can affect the carburetor— keeping the weed eater from starting as well as reducing performance and fuel efficiency.
Solving the Problem
Clean the filter by tapping it to dislodge dirt and dust, and then hold it up to the light. If you can’t see light through it, replace it.
Preventing the Problem
Check your air filter regularly. Heavy-duty users should check the filter monthly and those who use their trimmer less frequently should check it yearly.
Replace your air filter before you store your trimmer at the end of the season.
Spark Plug Issues
If you’re having trouble starting your string trimmer, the spark plug could be at fault. The spark plug ignites the air-fuel mixture that powers the engine, and over time, it can become fouled with carbon buildup, worn down, or damaged—preventing reliable starts.
Solving the Problem
Remove the plug according to your owner’s manual and inspect it for stubborn deposits. If it’s dirty, clean it with a wire brush and a spay-on cleaner meant for spark plugs.
If you notice the porcelain or electrodes are worn or cracked, immediately replace the plug with a new one.
Preventing the Problem
Change the spark plug. Replace the spark plug every year.
Flooded Engine
Flooding is a common problem among small engine tools like weed wackers. Sometimes dirty fuel causes engine flooding, but it can also happen if you overprime the machine.
Solving the Problem
If you have a flooded engine, you need to clear out the fuel. Hold down the throttle, turn off the choke, and pull the string quickly until the engine catches. Don’t be surprised if this takes 20-30 pulls – it depends on how much fuel got into the engine.
Note that this will produce a lot of exhaust smoke, so you should perform these steps outside or in a well-ventilated area.
Preventing the Problem
Don’t overprime the machine. Usually 5-6 times is enough to start the engine.
Keep your gasoline from going bad: Use all gas in the fuel tank within 30 days. If you know you won’t be using your trimmer for a few months, empty the fuel tank, run the carburetor dry so all gas is out of the engine, and add STHIL MotoMix before storing your machine.
Carburetor Issues
Old or dirty fuel can leave residue that gums up the carburetor. If the carb gets clogged, the engine can’t get what it needs to start and you’re left with a weed eater that won’t run. This happens most often when gas sits in the trimmer over the winter months, but carburetor problems can also result from dirty air filters.
Solving the Problem
If you suspect the carburetor is the reason your weed wacker won’t catch, clean any residue around the outside of the carburetor with a wire brush, removing any debris. Next, spray some carburetor cleaner into the passages and jets and give it a few minutes to work. Finally, spray compressed air into the jets and passages until they’re clear. You may need to repeat this sequence more than once.
Preventing the Problem
Drain the fuel before putting your weed wacker away for the winter.
Clear out any gas in the carburetor if it will go unused for long periods of time.
Add STIHL MotoMix to prevent gummy deposits on the carburetor, engine, and other fuel system parts.
Fuel Line or Filter Blockage
Cracked fuel lines or clogged fuel filters will restrict fuel flow, preventing your string trimmer from starting. These issues can arise due to deterioration over time and a build-up of debris in the line or the filter.
Solving the Problem
Remove the air filter and the carburetor. Separate the connector and remove the fuel line from the machine. Check the fuel filter for signs of dirt or damage and replace it if needed. If the filter is clear, attach it to a new fuel line, and insert the pieces back into the fuel tank.
While this is not an overly complicated job, there are required tools and several steps to complete the process. Consider allowing professionals to do this for you—it is an inexpensive service and can save you valuable time.
Preventing the Problem
Keep the fuel filter clean.
Replace the fuel filter. Do this yearly if you use your string trimmer commercially. Homeowners can replace it less frequently.
Inspect the fuel lines seasonally and replace them if you notice cracks or if the filter is blocked.
Get It Running (and Keep It That Way)
A dependable string trimmer is a must-have to manage your landscape. If your machine doesn’t start up right away, remember, most starting issues are resolved easily and can be prevented with regular maintenance and seasonal checks.
Carl’s Mower & Saw is here to help you get the most out of your outdoor power equipment, whether you’re a homeowner or commercial landscaper. When you need assistance with diagnostics, tune-ups, spark plugs, MotoMix, and more, our experts are here with advice.
Thank You for Reading!
While we’ve provided lots of information in this blog, you may still have questions about string trimmer issues and maintenance. We are happy to help! Come see us at Carl’s Mower & Saw, where we can answer your questions about weed eaters and preventative maintenance to help you power your outdoors.
About
Established in 1990 by Carl and Beth Levien, Carl’s Mower & Saw remains a family-owned and locally operated business to this day. We’re committed to providing an exceptional customer experience to both contractors and homeowners. Whether you’re looking for sales, service or rentals, you’ll find quality outdoor power equipment brands known for their durability and dependability, including Exmark, Gravely, Honda, Husqvarna, STIHL, and BCS America. For inquiries, feel free to reach out by calling 360-384-0799 or emailing info@carlsmower.com.
