String Trimmer Head Jammed or Won't Advance Line

Common Symptoms

• Tapping the head on the ground doesn't advance new line
• Head assembly spins freely without resistance when engine is off
• Line breaks at the head eyelet repeatedly
• Head makes grinding noise but no line comes out
• Line is fully depleted but head still clicks when bumped
• Line welds together inside the head and won't pull out

Most Likely Causes

1. 1

   Tangled or Cross-Wound Spool (Most Common)

   When reloading line, if the two strands cross over each other or wind in opposite directions, the spool locks up. The bump mechanism tries to advance line but the tangle grips tighter under centrifugal force. This also happens when line from one strand wraps over the other during use. Remove the head and pull the spool — untangle and rewind correctly.

2. 2

   Broken Bump Knob

   The bump knob (the bottom cap of the head) contains the bump mechanism. The knob can crack at its retention tabs, or the inner post that contacts the spool can snap off. When the knob is broken, tapping the head doesn't transfer force to the spool. Inspect the knob by removing it — any visible crack or broken tab means replacement ($5–$12).

3. 3

   Worn or Missing Advance Spring

   Inside the trimmer head is a coil spring that allows the spool to rotate slightly during the bump, then returns it to its locked position. If this spring breaks or falls out (common when the head is opened outdoors), the spool has no return tension and either won't advance line or doesn't lock back in place. The spool then spins freely rather than engaging the cutting circle.

4. 4

   Fused / Melted Line

   High-speed friction generates heat at the cutting head. If you repeatedly engage the trimmer against surfaces that stall the line, heat builds and the line strands fuse together where they exit the eyelets. The line appears to be there but won't pay out. Cut away the fused section and pull fresh line out from inside the head.

5. 5

   Wrong Line Diameter

   Using line that is thicker than specified causes the spool to over-fill and jam. Using undersized line allows it to slip under other strands and tangle. Most residential trimmers use 0.065" or 0.080" line. Check the head or manual for the correct diameter — using 0.095" in a head rated for 0.065" will jam every time.

Quick DIY Checks

1. 1Remove the head from the trimmer shaft (usually a left-hand thread — turn CLOCKWISE to remove). Set it on a workbench. Press the bump knob tab (or squeeze the retention tabs on some STIHL heads) and remove the bump knob. The spool should slide out freely.
2. 2Inspect the advance spring inside the head body. Hold the head over a white surface so you'll see if the spring falls out. Confirm the spring is present, not broken, and seated correctly in its groove. Take a photo of the spring orientation before removing anything.
3. 3Unwind the spool completely. Look for crossing strands, fused sections, or line that's packed so tightly it's binding. Also check the spool arbor (center post) for wear or cracks. Reload with the correct diameter line, winding in the direction indicated by the arrow on the spool.

1. 4When reloading: cut two equal lengths of line, insert each end through the anchor hole in the center of the spool, and wind each strand in opposite directions following the arrows. Wind under tension with no crossing. Leave 5–6 inches of line extending out each eyelet before snapping the bump knob back on.
2. 5Test the bump advance on the bench before running the engine: hold the head assembly and bump the knob on your palm firmly. The spool should rotate slightly (you'll hear a click) and 1–2 inches of line should advance. No advancement means a broken spring or broken knob post.
3. 6Reinstall the head on the trimmer shaft — left-hand thread, so turn counter-clockwise to tighten. Run the trimmer at full throttle for 5 seconds, then bump to advance fresh line. If it still won't advance, the automatic clutch spring inside the drive coupling may be the issue — inspect the drive arbor for a worn notch.

Repair vs Replace

Recommended Tools & Parts

• STIHL AutoCut C5-2 Trimmer Head

  OEM-compatible replacement head for STIHL FS 40, FS 50, FS 55, FS 56, and FS 70 trimmers. Uses 0.080" line, bump-feed advance.

  $18–$28

  Buy on Amazon →

• Echo Speed-Feed 400 Head

  Rapid-load trimmer head for Echo SRM series trimmers and most universal 10mm left-hand threaded shafts. Line loads from outside without disassembly — no tangling.

  $20–$35

  Buy on Amazon →

• Trimmer Line 0.080 inch (1 lb Spool)

  Round cross-section trimmer line for most residential gas and cordless trimmers. 0.080" diameter for standard residential use. 200+ feet per pound.

  $8–$14

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

Related Repairs