Stihl FS-series strimmers share the same 2-MIX engine architecture across the range — which means the starting and power-loss faults Paul sees on FS 38 domestic models are often identical to the problems on professional FS 91 and FS 111 machines. The difference is usually how hard the machine has been worked and whether the carb diaphragms have hardened from old fuel.
Paul services more Stihl strimmers and brushcutters than any other handheld brand at the Hyde workshop. Landscapers from Denton, tree surgeons passing through Glossop, and homeowners in Stockport all bring the same FS machines with the same preventable fuel-system failures.
This guide covers petrol Stihl FS strimmers and brushcutters only. Paul does not work on battery or cordless models.
1. Stale Fuel and Varnished Carburettor (The Number One Cause)
Stihl 2-MIX engines run lean and clean when the fuel is fresh. Leave 50:1 mix in the tank for more than a few weeks and the jets varnish — especially on machines stored in sheds over winter.
Symptoms: - Will not start after storage - Starts then dies within minutes - Runs cold but cuts out warm
DIY fix: Drain the tank and carb bowl. Mix fresh fuel with quality two-stroke oil at 50:1 — Stihl recommends HP Ultra or equivalent JASO FD oil at 50:1 for current FS models. Fit a new spark plug if the old one is black or wet.
Paul's tip: "An FS 55 from Ashton-under-Lyne would not fire in March. Fuel smelled like varnish. Carb strip, new diaphragm kit, fresh mix — running in twenty minutes. That machine had sat since October with fuel still in the tank."
2. Air Filter Housing Seal Failure (FS-Series Specific)
Stihl FS machines use a clip-on air filter housing with a rubber seal. When the seal hardens or the clip loosens, the engine pulls unmetered air and will not idle or tune correctly.
Symptoms: - Rev hunts at idle - Will not start with choke unless you cover the air intake - Runs lean and hot under load
DIY fix: Remove the filter cover, inspect the seal for cracks, clean the foam filter, lightly oil it, and ensure the housing clips shut positively.
3. Throttle Trigger Return Spring and Cable
A common FS 55 and FS 38 fault. The throttle does not return to idle, or the cable seizes after damp storage.
Symptoms: - Engine revs on its own - Hard to start because throttle is partially open - Trigger feels sticky
DIY fix: Check the trigger returns freely with engine off. Lubricate the cable sparingly or replace if frayed. Paul replaces seized cables same day on most FS models.
4. Primer Bulb, Fuel Lines and In-Tank Filter
Stihl primer bulbs and fuel lines harden in UK sheds. A flat bulb cannot prime the carb. Cracked lines let air in.
Symptoms: - Primer stays depressed - Fuel smell but engine starves - Starts with starter fluid then dies
Fix: Replace bulb, lines and in-tank filter as a set. Paul keeps ethanol-resistant hose and Stihl-compatible bulbs in stock.
5. Carburettor Diaphragm Hardening (Professional FS 91 / FS 111)
Commercial FS machines that run daily are reliable until the carb diaphragms age. Hardened diaphragms will not pulse fuel correctly — the engine surges or cuts out under load.
Symptoms: - Fine at idle, dies in long grass - High-speed screw adjustment has no effect - Machine worked perfectly last season
Fix: Carb rebuild with genuine diaphragm and gasket kit. Paul rebuilds Stihl carbs weekly — tuning to factory spec matters on 2-MIX.
6. Clutch Drum Wear and Drive Shaft Coupling (Loses Power, Not Starting)
If the FS starts and idles but the head will not spin under load, the clutch or flex shaft coupling is next.
Symptoms: - Engine screams, head barely turns - Burning smell from clutch area - Rattling from the shaft tube
DIY check: With engine off, head should spin freely when clutch is disengaged. Drag or grinding points to clutch or gearbox issues.
Model Notes from the Workshop
Stihl FS 38 / FS 40 (domestic): Light use but often neglected over winter. Fuel and primer bulb failures dominate.
Stihl FS 55 / FS 56 (mid-range): Throttle cable and air filter seal issues are common. Reliable once fuel system is sorted.
Stihl FS 91 / FS 111 (professional): Carb diaphragms and clutch wear from heavy use. Landscapers from Droylsden and Hyde run these hard — Paul services them between contracts.
Stihl FS-KM combi systems: Power head faults often trace back to the engine unit — same carb and fuel rules apply to the KM backbone.
Real Workshop Stories
A landscaper from Denton brought two FS 91 machines that both cut out after ten minutes on site. Same fault on both: hardened carb diaphragms and weak clutch springs. Paul rebuilt both carbs and fitted new clutch components — back on the job next morning.
A homeowner in Stockport had an FS 55 that would not start after winter. Primer bulb was cracked, fuel line perished, plug fouled. Parts swap and carb blow-through — started on the third pull.
When to Call Paul
Stihl parts are dealer-sourced and carbs need correct diaphragm kits for each model family. If fresh fuel, new plug, new bulb and lines do not fix it, bring the machine in. Paul specialises in Stihl 2-MIX carb work and keeps common FS consumables at the Hyde workshop.
Related guides: Strimmer won't start · Strimmer cuts out or loses power · Strimmer line jamming · Two-stroke oil guide
Book Paul: 07342 239878 or WhatsApp with your FS model number from the data plate.
