STIHL FS 74 Repair Manual

The STIHL FS 74 is a professional-grade string trimmer/brushcutter designed for demanding clearing and trimming tasks. This manual provides comprehensive repair, maintenance, and troubleshooting guidance for the FS 74, covering its 2-stroke engine, fuel system, cutting head, drive shaft, gearbox, and safety systems. Below are key sections for safety, specifications, disassembly, component repair, assembly, and troubleshooting.

Safety Precautions

Before performing any repair, ensure the unit is completely stopped, cool, and the spark plug boot is disconnected. Wear appropriate personal protective equipment (PPE).

Hazard	Precaution
Fuel Fire/Explosion	Drain fuel tank before repair. Work in well-ventilated area away from ignition sources.
Sharp Components	Wear cut-resistant gloves when handling cutting blades or trimmer heads.
Spring Tension	Use caution when disassembling components under spring tension (e.g., starter).
Hot Surfaces	Allow muffler and cylinder to cool completely before handling.
Chemical Exposure	Use gloves when handling fuel, oil, or cleaning solvents.

Specifications & Technical Data

Key technical specifications for the STIHL FS 74.

• Engine Type: STIHL 2-MIX, single-cylinder, 2-stroke, air-cooled.
• Displacement: 40.2 cc (2.45 cu. in.).
• Power Output: 1.8 kW (2.4 hp).
• Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent ISO-L-EGD certified oil).
• Idle Speed: 2,800 rpm.
• Max Engine Speed: 12,500 rpm (with cutting attachment mounted).
• Drive System: Flexible drive shaft with bevel gearbox.
• Cutting Attachments: Compatible with STIHL PolyCut heads, circular saw blades, and grass blades.
• Weight (without attachment): Approx. 6.2 kg (13.7 lbs).

Tools & Workshop Requirements

Recommended tools for effective repair.

1. STIHL special tools: Flywheel puller, clutch holder, piston stop.
2. Standard tools: Torx screwdrivers (T27), socket set (8mm-13mm), feeler gauges, torque wrench.
3. Measuring tools: Compression tester, tachometer, multimeter.
4. Cleaning supplies: Parts cleaning solvent, compressed air, shop towels.
5. Workshop: Clean, well-lit bench with vise and soft jaws.

WARNING! Use only STIHL-approved replacement parts to maintain performance and safety.

Engine System Repair

Procedures for cylinder, piston, crankshaft, and bearings.

1. Cylinder Removal: Remove muffler, intake manifold, and cylinder cover. Unbolt and lift cylinder.
2. Piston Inspection: Check for scoring, ring wear, and pin bearing play. Replace if out of spec.
3. Crankshaft Seals: Inspect for leaks. Replace using seal puller and driver.
4. Bearings: Check crankshaft bearings for smooth rotation and no play. Press out/in as needed.
5. Compression Test: Should read a minimum of 110 psi (7.6 bar) for proper operation.

CAUTION! Label all parts during disassembly. Clean mating surfaces thoroughly before reassembly.

Fuel System Repair

Fuel tank, lines, filter, and cap.

Component	Procedure
Fuel Tank	Inspect for cracks. Clean with solvent. Ensure vent in cap is clear.
Fuel Lines	Replace if cracked or hardened. Route correctly to prevent kinks.
Fuel Filter	Replace annually or if clogged. Use STIHL part # 0000 350 3501.
Impulse Line	Check for leaks between carburetor and crankcase. Replace if brittle.

Ignition System Repair

Spark plug, ignition module, and stop switch.

1. Spark Plug: Gap to 0.020" (0.5mm). Replace if electrodes are worn or fouled.
2. Ignition Module (Coil): Check air gap (0.30mm / 0.012") with feeler gauge. Test resistance per manual.
3. Stop Switch: Ensure wire is intact and switch grounds properly to kill engine.
4. Flywheel: Inspect for damage on magnets and keyway. Torque nut to 40 Nm (30 ft-lb).

Tip: A weak spark is often caused by a faulty ignition module or improper air gap.

Drive Train & Gearbox Repair

Drive shaft, flexible cable, and bevel gearbox.

1. Drive Shaft Removal: Remove cutting attachment and handle. Withdraw shaft from tube.
2. Shaft Inspection: Check for kinks, wear on square ends, or damaged flexible core.
3. Gearbox Disassembly: Drain grease. Check gears for chipping and bearings for smooth rotation.
4. Grease Replacement: Pack with STIHL gearbox grease (approx. 50g). Reassemble with new seals.
5. Bearing Replacement: Press out old bearings; press in new ones using appropriate drivers.

WARNING! Never operate unit with a damaged drive shaft, as it can fail catastrophically.

Cutting Attachment Repair

PolyCut head, blades, and mounting.

PolyCut Head: Replace worn cutting line. Ensure bump feed mechanism operates smoothly. Replace entire head if cracked.

Metal Blades: Sharpen with file maintaining original angle. Check for cracks or excessive wear. Balance blade after sharpening.

Mounting: Ensure retaining nut is torqued to specification (40-50 Nm). Use correct washer and guard.

Carburetor Adjustment

STIHL Zama C1Q-S carburetor standard settings.

1. Initial Settings: Turn H (high) and L (low) screws gently clockwise to seat, then back out: L - 1 turn, H - 1 turn.
2. Idle Adjustment (LA): Set to achieve 2,800 rpm idle speed with cutting attachment stopped.
3. Final Tuning: Warm up engine. Adjust L for smooth acceleration. Adjust H for max power without "four-stroking" at full throttle no-load.
4. Check: Re-check idle speed after H/L adjustments. Chain/cutter must not move at idle.

CAUTION! Excessive leaning (turning screws too far counter-clockwise) can cause engine damage.

Assembly & Final Checks

Reassembly sequence and operational verification.

1. Follow disassembly steps in reverse order, applying appropriate lubricants and sealants.
2. Torque all fasteners to specifications (e.g., cylinder bolts: 10 Nm).
3. Refill gearbox with grease and fuel tank with fresh, properly mixed fuel.
4. Start unit and check for leaks (fuel, exhaust, gearbox).
5. Verify smooth operation at all throttle positions and proper engagement of cutting attachment.

Final Step: Perform a functional test in a safe area before returning unit to service.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, flooded, fuel issue	Check spark plug, ignition gap. Dry plug. Ensure fresh fuel and clear lines.
Engine starts then dies	Clogged fuel filter, carb adjustment, air leak	Replace fuel filter. Adjust carburetor. Check crankcase seals and intake gaskets.
Loss of power	Clogged air filter, exhaust screen, low compression	Clean/replace air filter. Clean spark arrestor screen. Perform compression test.
Excessive vibration	Unbalanced blade, bent drive shaft, loose mounting	Balance blade. Inspect drive shaft. Tighten all mounting bolts.
Cutting head doesn't turn	Sheared drive shaft key, gearbox failure	Inspect shaft square ends and gearbox gears. Replace damaged components.

Technical Support: For specific parts diagrams and technical bulletins, consult STIHL STIHL iMan or contact an authorized STIHL servicing dealer.