What You’ll Need
- Replacement trimmer line (correct diameter for your model)
- Flathead screwdriver
- Needle-nose pliers
- Replacement bump knob spring (if spring is broken)
- Work gloves
- Clean rag or compressed air
Safety Precautions Before You Start
- Disconnect the power source — remove the battery from cordless trimmers or unplug electric models before touching the head.
- Gas trimmers: disconnect the spark plug wire to prevent accidental starts.
- Wear gloves — trimmer line edges and metal eyelets can cut skin when handled under tension.
- Let the head cool — if you just finished trimming, the head can be hot from friction. Wait 5 minutes before working on it.
Cause 1: Tangled or Fused Line on the Spool
How to Identify It
Remove the spool and unwind a few inches of line by hand. If the line is stuck together in a blob or the layers are crossed and tangled, this is your problem. Heat from trimming can cause line to partially melt and fuse to itself, especially when the head gets hot during extended use.
How to Fix It
- Press the tabs on the head housing to release and remove the spool.
- Pull all the old line off the spool completely.
- Inspect the spool for cracks or deformation. Replace if damaged.
- Wind fresh line onto the spool following your model’s winding direction (usually marked with an arrow).
- Leave 6 inches of line trailing through each eyelet before reassembling.
Use quality trimmer line rated for your climate. Cheap line fuses much more easily. If fusing is a recurring problem, consider storing your spare line in a sealed bag with a damp cloth — hydrated line feeds more reliably and resists fusing. Check out our string trimmer line types guide for recommendations by line diameter and material.
Cause 2: Bump Knob Jammed or Worn
How to Identify It
On bump-feed heads, you advance line by tapping the bottom knob on the ground while the trimmer runs. If tapping produces no result and the line isn’t fused, the bump knob mechanism is likely worn, cracked, or jammed with debris.
How to Fix It
- Remove the head housing and pull out the spool.
- Inspect the bump knob for cracks — replace it if cracked.
- Check the spring inside the bump knob assembly. If the spring is flattened, compressed, or missing, it won’t push the spool forward to release line.
- Replace the spring (usually available for $3–6 at a hardware store — search by your trimmer brand and model).
- Clean any debris from the housing before reassembly.
Cause 3: Clogged or Worn Eyelets
How to Identify It
Eyelets are the small holes in the trimmer head housing that the line exits through. If the line can’t slide through the eyelet freely, it won’t feed. Eyelets wear down over time — the edges become sharp and rough, gripping the line instead of letting it pass through. Grass pulp and debris can also pack into eyelets and block them.
How to Fix It
- Remove the spool and look at the eyelets from the inside of the housing.
- Use compressed air or a toothpick to clean out any debris packed inside the eyelet opening.
- Try threading a short piece of line through the eyelet by hand — it should slide freely with light pressure.
- If the eyelet edge is sharp or rough (you’ll feel it drag the line), replace the head housing or the eyelet insert if your model uses replaceable inserts.
Cause 4: Overfilled or Incorrectly Wound Spool
How to Identify It
If you just re-spooled the trimmer and now the line won’t feed, the issue is almost always too much line on the spool or line wound in the wrong direction. An overfilled spool binds inside the housing and can’t rotate freely to advance line.
How to Fix It
- Remove and inspect the spool. Most spools have a max-fill indicator line — don’t wind past it.
- If overfilled, remove excess line until the last wound layer sits at least 1/4 inch below the spool rim.
- Check the winding direction. Wind against the arrow if tapping the bump knob causes the spool to spin without advancing, or re-read the label on the head housing.
- Wind the line in neat, tight, non-crossing layers. Crossed layers create the exact tangle that blocks feeding.
For a full walkthrough on loading different head types, see our string trimmer head replacement guide.
Cause 5: Wrong Line Diameter
How to Identify It
Every trimmer head is engineered for a specific line diameter range — typically 0.065″, 0.080″, 0.095″, or 0.105″. Using line that’s too thick causes it to bind in the eyelets. Line that’s too thin feeds through too easily and tangles or breaks immediately.
How to Fix It
- Check your trimmer’s user manual or the label on the trimmer head for the correct line diameter.
- Check the line diameter printed on your current line spool — it’s marked on the label.
- If the diameter doesn’t match, replace with the correct size.
Common diameter by application: 0.065″–0.080″ for light residential use, 0.085″–0.095″ for medium residential use with thicker grass, 0.105″+ for commercial or heavy-duty trimming. Our trimmer line types guide covers this in detail with a compatibility chart.
Cause 6: Broken or Weak Internal Spring
How to Identify It
The spring inside the bump-feed head assembly keeps tension on the spool so line releases only when you tap. If this spring breaks or loses tension, the spool either locks up completely or spins freely without advancing line. You’ll often hear a rattling sound inside the head when the spring is broken.
How to Fix It
- Disassemble the head and remove the spool.
- Locate the spring — it sits between the spool and the bump knob.
- If the spring is broken, deformed, or very weak compared to a new spring, replace it. Springs cost $3–8 depending on the brand.
- When reassembling, make sure the spring seats correctly in its groove before pressing the head housing closed.
Quick Diagnosis Table
| Symptom | Most Likely Cause | Fix |
|---|---|---|
| Line is stuck together on spool | Fused/tangled line | Replace line; use quality line |
| Bumping does nothing after fresh spool | Overfilled spool or wrong wind direction | Reduce line amount; check wind direction |
| Bumping does nothing on old spool | Worn bump knob or broken spring | Inspect and replace bump knob/spring |
| Line won’t thread through head at all | Clogged or worn eyelets | Clean eyelets; replace head if worn |
| Line breaks immediately after feeding | Wrong diameter (too thin) | Use correct diameter line |
| Rattling sound in head | Broken internal spring | Replace spring |
Pro Tips to Prevent Line Feeding Problems
- Soak line before winding: Soaking trimmer line in water for 24 hours before spooling makes it more pliable, reduces brittleness, and significantly cuts down on fusing during use.
- Don’t overfill: The most common cause of line not feeding after re-spooling is simply too much line. Stop winding 1/4 inch before the rim.
- Replace line before it’s fully used: Running the head with very short line stubs creates extra heat and wear on the bump mechanism. Replace when line gets shorter than 3 inches per side.
- Clean the head after every use: A quick blow of compressed air clears grass pulp before it hardens into the eyelets.
- Check line annually: Old line stored in hot conditions (garage, shed) becomes brittle and fuses easily. Fresh line performs far better.
Frequently Asked Questions
Why does my trimmer line keep jamming after I just replaced it?
Almost always, the spool is overfilled or the line is wound in the wrong direction. Check that the last wound layer sits at least 1/4 inch below the rim, and confirm the wind direction matches the arrow on the head housing.
Can I use any trimmer line on my trimmer?
No. Each trimmer head is rated for a specific diameter range. Using the wrong diameter — especially too thick — will prevent feeding entirely and can damage the eyelets. Check your manual or the label on the trimmer head for the correct diameter.
How do I know if the bump knob spring is broken?
If tapping the bump knob on the ground while the trimmer is running produces zero line advancement, and the line itself isn’t fused, the spring is the prime suspect. Disassemble the head and physically check the spring — a broken spring often has a visible gap or deformation, and you’ll hear rattling inside the head.
My trimmer is brand new and the line won’t feed — what’s wrong?
On new trimmers, the most common issue is that the line slipped back inside the head during shipping. Remove the spool, pull both line ends back through the eyelets, and ensure each end has at least 5–6 inches protruding before reassembly.
How often should I replace the trimmer head entirely?
Most trimmer heads last 2–4 seasons of regular use. When the eyelets are visibly worn or cracked, the housing is cracked, or the bump mechanism repeatedly fails despite replacing the spring, it’s time to replace the whole head. See our guide on string trimmer head replacement for a full walkthrough.
Conclusion
A string trimmer that won’t feed line is almost never a serious mechanical failure — it’s almost always a fixable maintenance issue. The six causes covered here (fused line, worn bump knob, clogged eyelets, overfilled spool, wrong diameter, broken spring) cover the vast majority of feeding problems homeowners encounter. Work through the quick diagnosis table, identify your specific symptom, and you’ll have the trimmer back in service in well under 30 minutes.
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