Why does my bump head only feed line on one side?

One line end may be trapped, crossed, or wound unevenly on the spool. Remove the spool, check both eyelets, and rewind the line evenly in the marked direction. Replace the spool if one side is cracked or distorted.

Can I use thicker line in a bump head?

Only if the trimmer and head are rated for that diameter. Thicker line can bind in the eyelets, overload the motor, or prevent the spool from turning. Use the line size listed in the owner’s manual or on the head.

Why does the line come out but not advance when I bump the head?

The bump knob may not be moving the spool, the spring may be damaged, or debris may be blocking the mechanism. Remove the power source, clean the head, and inspect the knob, spring, and spool for wear.

Should I replace the whole trimmer if the bump head will not feed line?

Usually not. A replacement spool, bump knob, spring, or compatible head often solves the problem. Consider replacing the trimmer only when the head and drive system are damaged, parts are unavailable, or repair costs approach the value of the machine.

Bump head will not feed line

If your string trimmer bump head will not feed line, remove the spool and check for stuck, crossed, overheated, or incorrectly sized line. Then inspect the bump knob, eyelets, spring, and head for damage before looking at the trimmer itself.

If a bump head will not feed line, the problem is usually in the spool or head—not the trimmer motor. Unplug the trimmer or remove its battery, then check the line, spool, eyelets, bump knob, and head in that order. Most feed problems can be fixed by clearing debris, rewinding the spool correctly, or replacing a worn spool or head.

Start with safety

Turn off and unplug a corded trimmer.
Remove the battery from a cordless trimmer.
Disconnect the spark plug wire on a gas trimmer if the manufacturer recommends doing so before servicing the head.
Wear gloves when handling the line or removing packed grass.

Never work on the head while the trimmer could start accidentally. Follow the owner’s manual for your specific head design.

1. Check the line and spool

Remove the bump-head cover and take out the spool. Look for these common problems:

Line fused together: Heat from friction can weld adjacent wraps into a solid mass.
Crossed or overlapping wraps: These can prevent the line from leaving the spool.
Line wound too tightly: The spool may not rotate freely inside the head.
Too much line: Excess line can jam against the housing.
Wrong line diameter: Line that is too thick may not feed through the eyelets; line that is too thin may not work correctly with the head’s feed mechanism.
Old or brittle line: Dried-out line can break inside the head or fail to slide smoothly.

Remove damaged line and rewind the spool with the diameter and length specified in the trimmer manual or on the spool. Wind it evenly in the marked direction, keeping each wrap beside the previous one instead of crossing over it. Leave the correct amount of line extending through the retaining slots.

If the line repeatedly fuses, use the specified line size and avoid packing the spool beyond its stated capacity. Some line can also be softened by storing it according to the line manufacturer’s instructions, but replace line that is badly brittle or fused.

2. Clear the eyelets and head

Grass, dirt, and melted line can block the small guide holes where the line exits the head. Clean both eyelets and the inside of the head with a brush or a suitable tool recommended by the manufacturer.

Do not enlarge the eyelets with a drill or sharp tool. A damaged or oversized opening can increase vibration, make feeding inconsistent, or create a safety hazard.

Check that the spool sits fully in the housing and that the line is correctly positioned in the eyelets before reinstalling the cover.

3. Inspect the bump knob and spring

The bump knob should press and release smoothly. With the spool removed, inspect the bump knob, spring, hub, and retaining points for:

A cracked or worn bump knob
A missing, bent, or collapsed spring
Packed debris preventing movement
A worn center hub or spool contact surface
A cover that does not lock securely

If the knob will not move or does not return after being pressed, clean the assembly and check the spring. Replace the damaged part rather than forcing it. Some heads use a replaceable spool and bump knob; others require replacement of the complete head.

4. Confirm the head is assembled correctly

A bump head may fail to feed if the spool is upside down, the spring is installed on the wrong side, or the line is not captured in the spool’s retaining slots. Compare the assembly with the diagram in the manual.

Make sure:

The spring and spool are in the correct order.
The line ends pass through the eyelets without crossing or binding.
The cover is fully locked.
The bump knob can rotate or press as the design requires.
The head is firmly attached to the trimmer shaft.

Do not assume every “universal” head fits every trimmer. Thread direction, mounting hardware, shaft size, guard clearance, and operating speed all matter.

5. Check for a full spool or bag-related setup problem

Some trimmers use a prewound spool, while others use a refillable spool. If a prewound spool is empty, tangled, or damaged, replace it with the correct compatible spool. If the trimmer has a line-loading feature, use the loading method specified by the manufacturer rather than forcing line through the head.

Also check that the cutting guard is installed correctly. A missing or incorrectly positioned guard can interfere with line length and is unsafe to operate. The guard’s line-cutting blade should trim excess line after feeding; do not remove or bypass it.

6. Test the feed by hand

With the power source removed, pull the two line ends gently to confirm they can slide out. Press or tap the bump knob against a firm surface only as described in the manual. The spool should release a small amount of line without wobbling or sticking.

Do not test the head by holding it in your hand while the trimmer is running. Reinstall the guard and keep people, pets, and loose objects away before performing any powered test.

7. Consider the trimmer drive or electrical system last

If the spool, eyelets, bump mechanism, and assembly are correct but the head does not rotate, the issue may be elsewhere:

A cordless trimmer may have a depleted battery, poor battery contact, or an overload shutdown.
A corded model may have a power, switch, or extension-cord problem.
A gas model may have a clutch, drive shaft, or engine issue.
The head may be loose or the drive adapter may be damaged.

Stop using the trimmer if the head wobbles, makes unusual noise, smells hot, or vibrates heavily. Do not bypass electrical safety devices or disassemble a motor, clutch, or gearbox unless the manufacturer specifically permits it and you are qualified to do so.

When to replace the spool or head

Replace the spool when it is cracked, warped, badly worn, or repeatedly jams even with the correct line installed. Replace the bump knob or spring when those parts are available separately and the rest of the head is sound.

A complete replacement head is more reasonable when the housing, eyelets, hub, or mounting hardware is damaged, or when replacement parts are unavailable. Choose a head listed as compatible with your trimmer and verify the required line diameter, mounting system, rotation direction, and guard clearance.

If a correctly assembled replacement head still will not rotate or feed, the trimmer may have a drive or electrical fault. At that point, consult the manufacturer or a qualified repair professional rather than continuing to operate it.