STIHL FS 450 Workshop Manual

The STIHL FS 450 is a professional-grade, high-power brushcutter/clearing saw designed for demanding vegetation management tasks. This workshop manual provides comprehensive technical information for maintenance, repair, and overhaul procedures. It covers the engine, fuel system, ignition, cutting attachments, gearbox, and safety systems. Below are key sections for specifications, maintenance schedules, disassembly/assembly, troubleshooting, and parts identification.

1. Specifications & Technical Data 2. Safety Information & Symbols 3. Maintenance Schedule 4. Fuel & Lubrication System 5. Engine Overhaul 6. Ignition System 7. Gearbox & Drive Shaft 8. Cutting Attachments 9. Carburetor Adjustment 10. Electrical System 11. Troubleshooting & Diagnostics

Specifications & Technical Data

Key technical parameters for the STIHL FS 450 engine and cutting unit.

Feature	Specification
Engine Type	2-stroke, single-cylinder, air-cooled
Displacement	45.4 cm³ (2.77 cu. in.)
Power Output	2.5 kW (3.4 hp)
Fuel/Oil Mix Ratio	50:1 (STIHL HP Ultra oil)
Idle Speed	2800 rpm
Max. Engine Speed	12,500 rpm (with cutting attachment)
Ignition System	Electronic, capacitor discharge (CDI)
Carburetor	STIHL Zama diaphragm carburetor
Starting System	STIHL ErgoStart (optional), manual recoil
Drive Shaft	Solid, with anti-vibration system
Gearbox Ratio	1.27:1 (bevel gear)
Recommended Cutting Tools	Circular saw blades, brush knives, trimmer heads

Safety Information & Symbols

Critical safety warnings and explanation of symbols found on the unit and in the manual.

Personal Protective Equipment (PPE): Always wear safety helmet with face shield, hearing protection, gloves, protective trousers, and sturdy boots.
Workshop Safety: Ensure proper ventilation when working with fuel. Disconnect spark plug boot before any service.
Fuel Handling: No smoking. Use approved fuel containers. Wipe up spills immediately.
Sharp Tools: Handle cutting attachments with extreme care; use protective covers.

WARNING! Failure to follow safety instructions can result in serious injury or death. Always refer to the original operator's manual for safe operation procedures.

Maintenance Schedule

Recommended service intervals based on operating hours or time periods.

Service Item	Every 10 hrs / Daily	Every 50 hrs	Every 100 hrs / Annually
Air Filter Cleaning	✓ (Check)	✓ (Clean/Replace)	-
Spark Plug Check	-	✓ (Clean/Gap/Replace)	-
Fuel Filter Check	-	✓ (Replace)	-
Carburetor Adjustment	-	✓ (If necessary)	-
Gearbox Grease	✓ (Check level)	✓ (Refill STIHL grease)	-
Drive Shaft Inspection	-	✓ (Check for damage)	✓ (Detailed inspection)
Vibration Dampers	-	-	✓ (Inspect for wear)
Complete Unit Inspection	-	-	✓ (All nuts, bolts, safety devices)

Fuel & Lubrication System

Details on fuel tank, lines, filter, and lubrication points.

Fuel Tank: Capacity approx. 0.95 liters. Remove tank for cleaning if contaminated.
Fuel Filter: Located in tank. Replace by pulling fuel line with filter attached.
Fuel Lines: Inspect for cracks, hardening, or leaks. Replace as a set.
Gearbox Lubrication: Fill with STIHL Gearbox Grease (approx. 20g). Remove plug to check/fill.
Drive Shaft Splines: Lubricate with STIHL Special Grease during assembly.

CAUTION! Use only STIHL HP Ultra 2-stroke oil at a 50:1 mix ratio with fresh, unleaded gasoline (min. 89 octane). Old or incorrect fuel mix is a common cause of engine failure.

Engine Overhaul

Step-by-step guide for cylinder and piston disassembly, inspection, and reassembly.

Disassembly: Remove muffler, cylinder cover, and cylinder bolts. Carefully lift cylinder off crankcase.
Piston Inspection: Check for scoring, ring groove wear, and pin bearing play. Measure piston clearance.
Cylinder Inspection: Inspect bore for scoring or glaze. Check decompression valve function.
Crankcase & Crankshaft: Check bearings for smooth rotation and side play. Inspect seals.
Reassembly: Use new gaskets and seals. Lubricate piston and rings. Torque cylinder bolts to spec (10 Nm).

Note: Special tools (piston stop, ring compressor, flywheel puller) are recommended for this procedure.

Ignition System

Testing and servicing the electronic ignition module, spark plug, and stop switch.

Spark Plug: BOSCH WSR 6 F or equivalent. Gap: 0.5 mm (0.020 in.).
Ignition Module (CDI): Test with STIHL ignition tester or multimeter according to resistance values in wiring diagram.
Stop Switch: Test for continuity between black wire and ground when switch is pressed.
Flywheel & Magneto: Check for cracks and secure mounting. Air gap is non-adjustable.

Symptom: No Spark. Check: 1) Spark plug. 2) Boot and wire. 3) Stop switch circuit. 4) Ignition module. 5) Flywheel key.

Gearbox & Drive Shaft

Servicing the bevel gearbox and drive shaft assembly.

Gearbox Disassembly: Remove cutting attachment and circlip. Withdraw output shaft.
Gear Inspection: Check bevel gears for pitting, chipping, or excessive wear.
Bearing & Seal Replacement: Press out old bearings and seals. Press in new ones using appropriate sleeves.
Drive Shaft Removal: Detach from engine and gearbox. Inspect for straightness and spline wear.

Anti-Vibration Elements: Check rubber dampers for tears or hardening. Replace in sets.

Tip: Always clean gearbox housing thoroughly before reassembly and fill with fresh grease.

Cutting Attachments

Installation, balancing, and maintenance of blades and trimmer heads.

Attachment Max. Speed Fastening Torque Notes

Circular Saw Blade (e.g., 255mm) 6,000 rpm 40-50 Nm Use correct blade washer and guard. Check for cracks.

Brush Knife (4-tooth) 8,500 rpm 40-50 Nm Sharpen symmetrically to maintain balance.

STIHL AutoCut C 5-2 Trimmer Head Max. engine speed Hand-tighten + 1/4 turn Use only STIHL nylon line. Do not overfill.

WARNING! Always ensure the cutting attachment is properly secured, balanced, and the correct guard is installed before operation.

Carburetor Adjustment

Procedure for adjusting the STIHL Zama carburetor (standard settings).

Initial Settings: Gently turn H (high speed) and L (low speed) screws clockwise to stop, then open: L = 1 turn open, H = 1 turn open.

Idle Adjustment (LA): Start engine and warm up. Adjust LA screw to achieve stable idle at 2800 rpm (chain not moving).

Low-Speed (L) Adjustment: From idle, accelerate quickly. If engine hesitates, open L screw slightly (richer). If engine smokes/loads up, close slightly (leaner).

High-Speed (H) Adjustment: Under full load (cutting), adjust H screw for maximum power without "leaning out" (high, steady rpm). Use tachometer (target: 12,500 rpm under load).

Final Idle Check: Re-check idle speed and response. Ensure cutting attachment stops completely.

CAUTION! Incorrect adjustment, especially too lean, can cause severe engine damage. Final adjustment should be performed under actual load conditions.

Electrical System

Overview of wiring, ErgoStart system (if equipped), and switch testing.

Wiring Harness: Inspect for pinched or frayed wires, especially where harness passes through handles.

ErgoStart (EasyStart): System reduces pulling force. If malfunctioning, check starter spring, pulley, and one-way clutch in starter assembly.

Throttle Trigger & Switch: Test for smooth operation and correct micro-switch engagement/interlock function.

Troubleshooting & Diagnostics

Common problems, causes, and solutions for the FS 450.

Symptom Possible Cause Corrective Action

Engine will not start 1. No spark
2. No fuel
3. Flooded 1. Check plug, wire, ignition module.
2. Check fuel flow, filter, tank vent.
3. Hold throttle wide open and pull starter.

Engine starts but dies 1. Clogged fuel filter/vent
2. Dirty carburetor
3. Faulty ignition 1. Replace fuel filter, clear tank vent.
2. Clean carburetor, replace diaphragms.
3. Check for intermittent spark.

Loss of power 1. Clogged air filter
2. Exhaust port blocked
3. Incorrect carb adjustment 1. Clean/replace air filter.
2. Clean spark arrestor screen in muffler.
3. Adjust H and L screws under load.

Excessive vibration 1. Unbalanced blade
2. Bent drive shaft
3. Worn gearbox bearings 1. Balance or replace cutting tool.
2. Inspect and replace drive shaft.
3. Replace gearbox bearings.

Gearbox overheating 1. Low grease
2. Damaged gears/bearings
3. Overloading 1. Fill with correct grease.
2. Disassemble and inspect.
3. Use correct tool for task.

Reset: For electronic issues (if applicable), disconnect spark plug wire for 1 minute to reset module.

Service: For technical support and genuine STIHL parts, contact an authorized STIHL servicing dealer.

STIHL FS 450 Workshop Manual © - All Rights Reserved 2025

Attachment	Max. Speed	Fastening Torque	Notes
Circular Saw Blade (e.g., 255mm)	6,000 rpm	40-50 Nm	Use correct blade washer and guard. Check for cracks.
Brush Knife (4-tooth)	8,500 rpm	40-50 Nm	Sharpen symmetrically to maintain balance.
STIHL AutoCut C 5-2 Trimmer Head	Max. engine speed	Hand-tighten + 1/4 turn	Use only STIHL nylon line. Do not overfill.

Symptom	Possible Cause	Corrective Action
Engine will not start	1. No spark 2. No fuel 3. Flooded	1. Check plug, wire, ignition module. 2. Check fuel flow, filter, tank vent. 3. Hold throttle wide open and pull starter.
Engine starts but dies	1. Clogged fuel filter/vent 2. Dirty carburetor 3. Faulty ignition	1. Replace fuel filter, clear tank vent. 2. Clean carburetor, replace diaphragms. 3. Check for intermittent spark.
Loss of power	1. Clogged air filter 2. Exhaust port blocked 3. Incorrect carb adjustment	1. Clean/replace air filter. 2. Clean spark arrestor screen in muffler. 3. Adjust H and L screws under load.
Excessive vibration	1. Unbalanced blade 2. Bent drive shaft 3. Worn gearbox bearings	1. Balance or replace cutting tool. 2. Inspect and replace drive shaft. 3. Replace gearbox bearings.
Gearbox overheating	1. Low grease 2. Damaged gears/bearings 3. Overloading	1. Fill with correct grease. 2. Disassemble and inspect. 3. Use correct tool for task.