Line that won’t come out isn’t usually a sign the head is broken — it’s almost always something jammed, wound wrong, or worn just enough to stop the mechanism doing its job. This is a fault-diagnosis guide for feed problems specifically; if you just need to know how to strip and rewind a spool from scratch, see our guide on changing the line.
Safety first. Always stop the engine, let the head come to a complete stop, and disconnect the spark plug lead (or remove the battery on cordless models) before opening any trimmer head. Never try to clear or bump-feed line with the engine running and your hands near the head.
Diagnose it fast
| Symptom | Most likely cause | Check first |
|---|---|---|
| Line won’t move at all, spool feels locked solid | Line welded together inside the spool | Open the head, look for fused/shiny line |
| Line feeds a little then jams again | Line wound crossed over itself | Unwind and re-spool cleanly |
| Bump knob feels loose or mushy, nothing releases | Worn bump knob engagement teeth | Inspect knob teeth and spool notches |
| Spool doesn’t spring back up after a bump | Weak or broken feed spring | Compare spring length/tension to a new one |
| Line comes out then instantly retracts or won’t hold | Wound in the wrong direction for the head’s rotation | Check arrow/marking on spool base |
| Grinding or gritty feel when turning the spool by hand | Grass, dirt or grit packed inside the housing | Strip and clean the head cavity |
| Line snaps off short instead of feeding | Line dry-rotted or brittle from age/UV | Flex a test length; replace if it cracks |
| Auto-feed head does nothing even with fresh line | Internal weight/clutch mechanism failed | Inspect or replace the spool cartridge |
Work down this list roughly in order — welded line and wrong winding cause the large majority of “won’t feed” complaints, and both are free to fix.
1. Line welded inside the spool
Nylon trimmer line softens well below its melting point, and enough friction heat builds up inside a spool — especially on hot days, dense cutting, or a head that’s been running low on line — to fuse adjacent wraps together. Once fused, the line can’t unwind even though the mechanism itself is fine.
How to check:
- Remove the spool cover (usually a quarter-turn or two release tabs, occasionally a centre bolt).
- Lift the spool out and look at the wound line for shiny, glazed patches or wraps that are stuck to each other rather than sitting in loose, separate coils.
- Try to peel a few centimetres of line off by hand — welded line will resist or tear rather than unwind smoothly.
The fix: unwind the whole spool, discard any section that shows fusing or flat/glazed spots (it’s weakened even where it hasn’t fully welded), and rewind with fresh line at the correct diameter for your head. If this keeps happening on the same machine, it’s usually a sign the head is running too hot from cutting dense material for long stretches without a break, or that you’re using line thicker than the head is rated for — check your manual for the maximum line diameter (commonly 2.0-2.7 mm on domestic heads, up to 3.0 mm on heavier semi-pro units).
2. Line wound too tight or crossed
Even undamaged line won’t release if it’s been wound unevenly. Overlapping wraps bind against each other under centrifugal load, and the spool effectively locks up even though nothing is broken.
Signs: the spool turns freely by hand with no line loaded, but jams again as soon as you rewind it; you can see wraps crossing over each other rather than lying in neat, flat layers.
The fix:
- Fully unwind the line from the spool.
- Re-wind in the direction marked on the spool (usually an arrow or “wind this way” moulding) keeping tension even and each wrap laid flat against the last — don’t let wraps cross or bunch to one side.
- Leave the last 10-15 cm free and clip it into the retaining notch before refitting, so it doesn’t unravel while you fit the spool back into the housing.
- Don’t overfill — most spools have a fill line or a recommended length (commonly 4-8 m per spool half depending on head size); overfilling is one of the most common causes of binding.
3. Wrong winding direction
Every bump-feed and auto-feed head is designed to unwind in one rotational direction only, matching the direction the head spins. Line wound the opposite way will bind tighter with every bump instead of releasing, or fling straight off the spool.
Check: look for a small moulded arrow or “L”/“R” marking on the spool or housing. If your head has two line ports (dual-line heads), each side of the spool usually winds in a mirrored direction — don’t wind both sides the same way.
The fix: unwind and redo it following the marked direction. If there’s no visible marking, wind so the line comes off the outside of the spool in the same direction the head rotates when viewed from underneath with the engine running (check briefly and safely, or consult your manual).
4. Worn bump knob
The bump knob’s job is to let the spool drop slightly against spring tension, disengage its retaining teeth for a fraction of a second, and let centrifugal force pull a set length of line out before it re-engages. Worn or rounded teeth on the knob or the spool’s mating notches mean the disengagement never fully happens.
Signs it’s the knob:
- The knob depresses with little resistance and no clear “click” when it releases.
- Line feeds inconsistently — sometimes a little comes out, often nothing does — even with fresh, correctly wound line.
- Visible rounding or wear on the teeth where the knob meets the spool.
The fix: the bump knob is a low-cost part (typically €3-8) and rarely worth trying to repair — replace it as a unit. Make sure the replacement matches your head’s thread pitch and tooth pattern; universal knobs vary between brands even when the head diameter looks the same.
5. Broken or weak feed spring
The spring beneath the spool does two jobs: it holds the spool up against the housing so the retaining teeth stay engaged during normal cutting, and it pushes the spool back into that engaged position after each bump. A spring that’s lost tension, or has snapped a coil, leaves the spool sitting too low (feeding line constantly and erratically) or too high (never releasing at all).
Signs:
- Spool feels loose or rattly when you lift the assembly out.
- Compared side-by-side with a new spring, the old one is visibly shorter or the coils sit closer together.
- Line either won’t feed at all, or feeds excessively and unravels on its own during cutting.
The fix: springs are cheap (typically €2-6) and there’s no reliable way to “stretch” one back to spec — replace it. Seat it correctly over the shaft or retaining post before refitting the spool; a spring that’s sitting crooked will bind the spool off-centre and cause the same symptoms all over again.
6. Debris packed inside the head
Grass, dirt, and fine grit work their way past the housing seal over a season of use, especially in damp or overgrown ground, and pack in around the spool and spring. This adds enough friction and physical obstruction to stop the spool moving freely even when line, knob, and spring are all fine.
The fix:
- Strip the head fully — spool, spring, and knob out.
- Clean the housing cavity with a stiff brush and, if heavily packed, a blast of compressed air or a rinse (dry thoroughly before reassembly if you use water).
- Check the housing’s retaining teeth for packed grit and clear them individually — this is often the actual point of failure, not the spool teeth.
- Reassemble dry, don’t grease the mechanism — grease attracts more debris here than it prevents friction.
7. Line dry-rotted or brittle
Trimmer line degrades with age and UV exposure even sitting unused in storage — it loses flexibility, develops fine surface cracks, and snaps off at a short length instead of paying out normally, which can look like a feed fault when it’s actually the line itself.
Check: flex a length of the line sharply between your fingers. Good line bends and springs back; degraded line shows white stress lines or cracks and snaps cleanly rather than bending.
The fix: discard it and rewind with fresh line. Store spare line indoors out of direct sun, and where possible in a sealed bag — line kept in a hot shed or garage for a couple of seasons dries out noticeably faster than line stored cool and dark.
Auto-feed and semi-auto heads: what’s different
Auto-feed heads add an internal mechanism — usually a weighted arm or small centrifugal clutch — that releases a metered length of line automatically as engine revs drop, without any bumping. The checks above for welded line, winding direction, and debris all still apply first, since those cause the large majority of feed failures on these heads too. If line is correctly wound and clean but the head still won’t feed, the internal weight arm or its pivot has likely broken or seized — this isn’t something to force apart repeatedly, and the usual fix is a replacement spool cartridge rather than trying to source the individual internal part.
Quick checklist
- Stop the engine, disconnect the plug lead or battery, and let the head cool before opening it.
- Pull the spool and look first for welded or crossed line — this covers most cases.
- Confirm winding direction against the marking on the spool, and re-spool cleanly if in doubt.
- Check the bump knob teeth and compare the spring against a new one if the knob feels mushy or unresponsive.
- Clean out any packed debris from the housing and retaining teeth while it’s apart.
- Flex-test old line and replace it if it’s brittle, regardless of how much length is left on the spool.
When to call a dealer
Everything above is a straightforward DIY job with the head off and no special tools beyond a screwdriver and maybe pliers. The only time this becomes a dealer job is if the housing’s internal retaining teeth are cracked or stripped, the spool’s drive splines are rounded off, or an auto-feed cartridge’s replacement isn’t sold separately for your model — in those cases you’re pricing a new head against a repair, and given heads are generally inexpensive, replacement is usually the more sensible route once it’s gone beyond spring, knob, and line.