A strimmer that runs fine but will not cut properly is almost always a trimmer head problem — wrong line type, incorrect winding, worn eyelets, or a head that has taken one too many knocks against paving. Paul fixes more jammed and under-performing heads in summer than almost any other strimmer complaint after starting issues.
This guide sits alongside Paul's popular strimmer troubleshooting posts on starting and power loss. If the engine fires up and holds power but the cutting head misbehaves, work through these six causes in order.
1. Wrong Line Diameter or Type
Every strimmer head is rated for a specific line thickness — usually 2.0 mm, 2.4 mm or 2.7 mm. Feed in line that is too thick and it will bind in the eyelets. Too thin and it breaks constantly or flies out under centrifugal force.
Symptoms: - Line will not feed on bump heads - Line snaps every few seconds - Head feels tight when you try to pull line through by hand
DIY fix: Check the label inside the head or your manual. Match the diameter exactly. Paul recommends round line for general grass and square or twisted line for tougher scrub — but only if the head allows it.
Paul's tip: "A customer from Denton brought in a Stihl FS 55 that 'threw line everywhere'. Someone had loaded 3 mm cord into a head rated for 2.4 mm. Fresh correct line and it behaved perfectly — five-minute fix."
2. Line Wound Incorrectly in the Head
Bump-feed and auto-feed heads are fussy about winding direction, layer count and leaving enough free end. Cross-wound line jams the spool and stops advance.
Symptoms: - Bump feed does nothing - Line feeds unevenly — one side long, one short - Head makes a grinding noise when the engine revs
DIY fix: - Wind in the direction shown by the arrow on the spool - Keep layers neat and even — do not overfill - Leave 10–15 cm of each end through the eyelets before closing the cap - Use two equal lengths on dual-line heads
If the spool is cracked or the spring is missing, replace the spool kit rather than fighting it.
3. Worn Eyelets, Cap or Bump Knob
Plastic eyelets wear into oval shapes. A cracked cap lets line escape at random. The bump knob spring weakens so the feed mechanism never engages.
Symptoms: - Line flies off without hitting anything - Bump knob feels mushy or stays depressed - Visible wear or cracks around the line exit holes
DIY fix: Replace eyelets and caps — they are cheap parts. Paul stocks head rebuild kits for Stihl AutoCut, Husqvarna T45X and common universal heads.
4. Blade Instead of Line — Dull or Damaged Metal Blade
Brushcutters fitted with three-tooth or grass blades need sharpening and balancing like a lawnmower blade. A chipped or blunt blade tears rather than cuts, loads the engine, and feels like the machine is "jamming" in thick growth.
Symptoms: - Machine bogs in light scrub it used to handle - Torn ragged cuts instead of clean slices - Vibration through the handles
DIY fix: Inspect for chips and bends. Sharpen with a file maintaining the factory bevel. Replace if bent or cracked — never straighten a bent brushcutter blade.
Paul's tip: "Landscapers from Ashton-under-Lyne often run metal blades on Honda UMK brushcutters through bramble season. Paul sharpens and balances blades same day — a dull blade makes the clutch and gearbox work twice as hard."
5. Clutch or Gearbox Drag (Feels Like Jamming Under Load)
Sometimes the head is fine but the clutch shoes are glazed or the flex shaft / gearbox is dry. The cutting head stalls while the engine revs — which owners describe as jamming.
Symptoms: - Head stops spinning when you touch vegetation even though the engine screams - Burning smell from the clutch area - Excessive heat at the gearbox after ten minutes
DIY check: With the engine off, the head should spin freely when the clutch is disengaged. Grating or tight rotation points to gearbox grease or coupling wear.
When to call Paul: Clutch rebuilds and gearbox greasing are workshop jobs on most professional machines. See Paul's full guide on strimmers that cut out or lose power for the engine-side causes.
6. Cutting Technique and Safety Guard Issues
A missing or shortened deflector guard changes how line behaves — and it is dangerous. Cutting too low into stones and paving shatters line and damages the head.
Better technique: - Keep the head parallel to the ground for edging, angled slightly for scrub - Do not bury the head in soil — it wears line and risks hitting hidden objects - Let the tip of the line do the cutting, not the full arc
Brand-Specific Notes
Stihl FS series: AutoCut heads are reliable when wound correctly. Paul sees a lot of FS 55 and FS 91 machines with worn bump knobs after commercial use.
Husqvarna 525LST and T45X heads: Professional landscapers run these hard. Keep spare eyelets and spools in the van — downtime costs money.
Honda UMK 435: Excellent engines, but the metal blade boss and clutch drum wear faster when operators run blunt blades in brambles.
When to Bring It In
If fresh correct line, a clean rewind, and new eyelets do not fix the problem — or the clutch and gearbox feel rough — bring the machine to Paul. Head rebuilds, clutch work and blade sharpening are same-day jobs for most models at the Hyde workshop.
Related guides: Strimmer won't start · Strimmer cuts out or loses power · Two-stroke oil guide
Book Paul: Call 07342 239878 or WhatsApp with your head type (bump feed, blade, or fixed line) and Paul will quote parts and labour before you drop off.
