Stihl Trimmer Problems | Weed Eater Repair Guide

Stihl makes some of the most popular string trimmers worldwide. From homeowner FS models to commercial brush cutters, these trimmers are built to last. When problems occur, this guide helps diagnose and fix common issues.

Table of Contents

• Model Overview
• Starting Problems
• Running Issues
• Fuel System
• Head and Line Issues
• Maintenance

Model Overview

Gas Trimmers

Model	Engine	Use
FS 38	27cc	Homeowner
FS 56	27cc	Homeowner+
FS 91	28cc	Professional
FS 111	31cc	Professional
FS 131	36cc	Heavy duty
FS 240	37cc	Brush cutter

Battery Trimmers

Model	Battery	Use
FSA 57	AK series	Homeowner
FSA 86	AP series	Professional
FSA 130	AP series	Professional

[NEED REAL IMAGE: Stihl trimmer lineup]

Starting Problems

Won’t Start At All

Check basics first:

• Fresh fuel mix (50:1)
• Fuel valve open
• Choke in correct position
• Spark plug connected

Flooded engine: Symptoms: Smells strongly of fuel, plug wet

Fix:

1. Remove spark plug
2. Pull cord 10+ times to clear
3. Let sit 10 minutes
4. Reinstall plug (or new one)
5. Start with choke OFF

Hard Starting

Correct starting procedure:

1. Prime bulb 5-10 times until fuel visible
2. Full choke
3. Pull until engine fires (may not run)
4. Half choke
5. Pull until running
6. Move to run position when warm

Common causes:

• Old fuel (over 30 days)
• Clogged fuel filter
• Carburetor needs adjustment
• Spark arrestor clogged
• Air filter dirty

Starts Then Dies

Dies immediately:

• Choke not opening
• Fuel delivery issue
• Air leak at carburetor

Runs briefly then dies:

• Fuel tank vent blocked
• Fuel filter clogged
• Carburetor adjustment needed

[NEED REAL IMAGE: Stihl trimmer starting position]

Running Issues

Bogs at Full Throttle

Causes:

1. High-speed mixture too lean
2. Fuel filter clogged
3. Fuel lines cracked
4. Spark arrestor blocked
5. Air filter restricted

Fix:

• Check air filter first
• Inspect fuel lines and filter
• Clean spark arrestor
• Adjust carburetor H screw (1/4 turn richer)

Idles Rough or Dies

Causes:

• Idle speed too low
• Low-speed mixture off
• Air leak at intake
• Dirty carburetor

Adjustment:

• LA screw = idle speed
• L screw = low speed mixture
• Turn screws 1/8 turn at a time

Engine Surges

Causes:

• Carburetor needs adjustment
• Fuel delivery inconsistent
• Air leak
• Governor diaphragm (if equipped)

Excessive Vibration

Causes:

• Clutch drum damaged
• Drive shaft bearings
• Engine mounts loose
• Head imbalanced

Fuel System

Fuel Mix

Stihl 2-stroke engines require 50:1 mix.

Mixing:

• 2.6 oz oil per gallon gas
• Use Stihl or quality 2-stroke oil
• Mix in clean container
• Shake before filling

Fuel age: Use within 30 days or add stabilizer

Fuel Lines

Stihl trimmers have pickup and return lines inside the tank.

Symptoms of bad lines:

• Hard starting
• Won’t stay primed
• Engine dies under load
• Visible cracks

Replacement:

1. Drain fuel
2. Remove fuel cap
3. Use hooked wire to pull lines from tank
4. Note filter position
5. Install new lines and filter
6. Route through grommets

Fuel Filter

In-tank filter on pickup line.

Replace when:

• Starting problems
• Power loss
• Fuel delivery issues
• Annually with lines

[NEED REAL IMAGE: Stihl fuel line routing]

Carburetor

Stihl uses Zama or Walbro carburetors.

Cleaning:

1. Remove air filter cover
2. Disconnect fuel lines
3. Remove carburetor
4. Disassemble carefully
5. Clean all passages with carb cleaner
6. Check diaphragms and gaskets
7. Reassemble with rebuild kit

See Zama Carburetor Guide for detailed instructions.

Head and Line Issues

Line Won’t Feed

Bump feed heads:

• Line tangled in head
• Line too thick for head
• Springs worn
• Bump knob damaged

Fix:

1. Remove head cover
2. Clear tangled line
3. Verify correct line size
4. Check spring tension
5. Replace worn parts

Line Breaks Constantly

Causes:

• Line too thin
• Running in rocky/hard areas
• Head speed too high
• Old/brittle line

Solutions:

• Use heavier line (.095 or .105)
• Reduce throttle in hard areas
• Use fresh line
• Try different line brand

Head Won’t Turn

Causes:

• Clutch failure
• Drive shaft broken
• Head seized
• Gear box failure (loop handle)

Clutch test:

• With engine off, head should spin freely
• At idle, head should not turn
• At throttle, clutch engages

[NEED REAL IMAGE: Stihl trimmer head]

Electrical

No Spark

Causes:

• Bad spark plug
• Kill switch grounded
• Ignition module failure
• Wiring damage

Test:

1. Remove plug
2. Ground against block
3. Pull cord and watch for spark
4. No spark = module or switch

Spark Plug Specs

Model	Plug	Gap
FS 38/56	NGK BPMR7A	0.020”
FS 91/111	NGK BPMR7A	0.020”
FS 131+	NGK CMR6H	0.020”

Maintenance

Every Tank

• Check air filter
• Check line condition
• Inspect head

Every 25 Hours

• Clean air filter
• Check spark plug
• Inspect fuel filter
• Clean cooling fins

Every 50 Hours

• Replace spark plug
• Replace fuel filter
• Replace air filter
• Check fuel lines

Annually

• Replace fuel lines
• Rebuild or clean carburetor
• Check clutch
• Replace head if worn

Storage

Before storing:

1. Run fuel out or add stabilizer
2. Remove and clean air filter
3. Clean entire unit
4. Store in dry location
5. Remove battery (FSA models)

Related Guides

• String Trimmer Won’t Start
• Zama Carburetor Repair
• Walbro Carburetor Repair
• Fuel Line Replacement
• Stihl Chainsaw Problems

Summary

Stihl trimmers are reliable but need regular fuel system maintenance. Old fuel and cracked fuel lines cause most problems. Use fresh 50:1 mix and replace lines annually. Carburetor adjustment can restore performance when cleaning alone doesn’t help. These trimmers will last many years with proper care.