Common problems and repair guides for weed eaters
String trimmers and weed eaters from Stihl, Echo, Husqvarna, and Ryobi use two-stroke engines that share the same fuel-mixture and carburetor issues as chainsaws. Common problems include won't start, string won't feed, trimmer head won't spin, and engine bogs under load. The fuel system is the most frequent culprit — stale gas-oil mix, clogged fuel filter, or a diaphragm carburetor that needs a rebuild kit. Our weed eater guides walk you through each fix.
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Weed Eater String Won't Auto Dispense
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Most trimmer head feeding issues stem from centrifugal force failure caused by internal spool friction, improper line winding, or damaged bump-knob components. When the line welds itself together inside the spool due to engine heat or poor quality monofilament, the auto-feed mechanism lacks the force to advance the string. Addressing this requires a thorough cleaning of the head assembly, inspecting for debris, and ensuring the line is wound with proper tension and orientation.
If your engine is running but the trimmer head is not spinning, the problem is almost certainly a mechanical disconnect within the power transmission system. This is usually caused by a sheared drive shaft spline, a worn or broken clutch assembly, or a failure in the gear head box. Inspecting the drive cable and clutch housing will generally reveal the specific point of failure.
A Stihl weed eater that won't stay running is typically suffering from a disrupted fuel-air ratio, usually caused by a restricted fuel filter, a clogged carburetor, or a vacuum leak. These engines rely on precise pulse-pressure to drive the fuel pump; if the air intake or fuel delivery is compromised, the engine will die under load or at idle. Start by verifying fuel quality and checking the spark arrestor, as these represent the most common points of failure in two-cycle equipment.
When a weed eater primer bulb won't fill, won't hold fuel, or stays empty after you press it, the cause is almost always an air leak on the suction side of the fuel circuit — not the bulb itself. The three most common culprits, in order, are: hardened or cracked fuel lines that have lost their vacuum seal, a split or pinhole-leaking primer bulb, and stuck internal check valves inside the carburetor gummed up by old fuel varnish. A bulb that pushes fuel through but won't draw it back usually means the return line is cracked or a check valve is stuck open. Replacing the bulb alone will not fix the problem if the real leak is in a fuel line or the carburetor check valves — diagnose the suction side first.
When a weed eater starts and then quickly dies, it is usually struggling to get fuel or air. The most likely culprits are old fuel clogging the carburetor or a dirty air filter choking the engine.
The most common reason Ryobi trimmer line won't feed is 'line welding,' where heat causes the string to fuse together on the spool, or a worn bump knob that can no longer index the internal drive lugs. To fix this, disassemble the head, clear any melted or tangled line, and inspect the bump knob for rounded plastic teeth. Ensure you are using the correct line diameter (typically .080" or .095") and that the internal compression spring is not missing or collapsed.
When an Echo trimmer cranks but won't fire, the most frequent culprit is a clogged spark arrestor screen or hardened carburetor diaphragms caused by ethanol fuel. First, remove the muffler's small screen to see if it's blocked with carbon, then check for a strong blue spark at the plug (gap 0.024-0.026 inches). If you have spark and exhaust flow, your issue is likely a fuel delivery failure within the Zama or Walbro carburetor metering chamber.
To diagnose a weed eater that pulls but won't fire, first verify spark with a tester and then perform a 'prime test' by adding a teaspoon of fuel directly into the carburetor throat. If it fires briefly, you have a fuel delivery issue caused by clogged carb diaphragms or cracked fuel lines. Additionally, check the spark arrestor screen in the muffler for carbon blockage, which is a common but overlooked cause of no-start conditions in 2-stroke engines.
On the Stihl FS 91, the 4-MIX engine is extremely sensitive to valve clearance and exhaust restriction. First, remove the spark arrestor screen from the muffler and burn off any carbon buildup with a torch; a clogged screen is the #1 cause of 'no-start' symptoms. If it still won't fire, you must check the valve lash—if the clearance is not exactly 0.10 mm (0.004 in) at Top Dead Center, the decompression shelf won't function correctly, making it nearly impossible to start.
The most common reason trimmer line won't feed is 'line welding,' where heat from friction causes the nylon strands to fuse together on the spool. First, disassemble the head and inspect the line for flat spots or melted sections, then check the bump knob for worn indexing teeth. If the line is wound in the wrong direction or the eyelets are grooved, the centrifugal force will lock the line in place instead of releasing it.
The most common causes for a Husqvarna string trimmer not staying running are fuel system issues, a clogged air filter, or an electrical problem. Start by checking the fuel line and filter, as well as the air filter, to ensure they are not blocked or dirty.
A weed eater that bogs down when it hits grass usually suffers from fuel delivery issues, a dirty air filter, or an old spark plug. Start by checking your air filter and trying fresh fuel, as these are the most common culprits for performance loss.
Most Craftsman trimmers fail to start due to degraded Tygon fuel lines or a carbon-clogged spark arrestor screen in the muffler. First, verify spark by grounding the plug against the engine block while pulling the cord. If spark is present, inspect the fuel lines for cracks and the primer bulb for suction; if those are intact, clean the spark arrestor screen to allow the engine to breathe.
String trimmer line usually breaks because it has become brittle from age or because you are hitting hard objects like fence posts and rocks. Start by soaking your new trimmer line in water to restore flexibility and ensuring you are using the correct diameter for your specific model.
Most starting issues on a Stihl FS 56 RC are caused by old fuel clogging the carburetor or a fouled spark plug. Start by checking for fresh fuel, ensuring the engine switch is set correctly, and inspecting the spark plug for buildup.
The most likely causes for a Shindaiwa weed eater not staying running are clogged fuel lines, a dirty carburetor, or a faulty spark plug. Start by inspecting the fuel supply and cleaning the carburetor for optimal performance.
When an Echo engine revs but the head fails to spin, the mechanical link between the clutch and the gear head is broken. Most commonly, this is a snapped flexible drive cable or a stripped trimmer head spline. Remove the gear head and verify if the internal drive cable rotates when the engine is running; if it doesn't, the issue lies in the centrifugal clutch or the drum adapter.
Most-needed maintenance items to keep your equipment running.
Quality trimmer line breaks less and cuts cleaner.
Find on AmazonFresh fuel mix prevents carb diaphragm hardening and starting issues.
Find on AmazonA clogged spark arrestor causes power loss — clean or replace it.
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