Stihl SR 450 Manual

The Stihl SR 450 is a professional-grade string trimmer/brush cutter designed for demanding clearing and trimming tasks. It features a powerful 2-stroke engine, robust cutting head options, anti-vibration system, ErgoStart technology for easy starting, and a comprehensive safety system. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: Powerful 2-stroke engine, ErgoStart system, anti-vibration handles, quick-stop inertia chain brake, modular shaft system, and multiple cutting head compatibility.

Feature	Description
Engine Type	2-stroke, single-cylinder, air-cooled
Displacement	45.6 cc
Power Output	High torque for demanding vegetation
Starting System	ErgoStart for reduced pulling force
Anti-Vibration	Damped handles to reduce operator fatigue
Safety System	Quick-stop inertia chain brake (if equipped)
Shaft	Modular solid or flexible shaft options
Cutting Attachments	Compatible with trimmer heads, brush knives, circular blades
Fuel Tank Capacity	Approx. 0.95 liters
Weight (powerhead)	Approx. 7.5 kg (without attachments)

Safety Information

Read and understand all instructions before use. Always wear appropriate PPE.

1. Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, sturdy gloves, long pants, and cut-resistant boots.
2. Work Area Safety: Clear area of bystanders, especially children and pets. Be aware of hidden obstacles like wires or stones.
3. Machine Handling: Always hold unit firmly with both hands. Keep cutting attachment below waist level. Never operate with one hand.
4. Fuel Safety: Refuel in a well-ventilated area with engine off and cool. Use only STIHL MotoMix or properly mixed fuel (50:1 with STIHL HP Ultra oil).

WARNING! Risk of severe injury or death from blade contact, thrown objects, or fire. Never operate without proper guards installed.

Assembly & Preparation

Assemble according to instructions. Check all fasteners are tight.

1. Handle Assembly: Attach handlebar to shaft as per diagram. Secure bolts to specified torque.
2. Cutting Attachment: Install chosen trimmer head, brush knife, or circular blade. Ensure guard is correctly mounted and secure.
3. Drive Shaft: Connect powerhead to shaft (if modular). Engage coupling securely.
4. Fuel: Fill with fresh, properly mixed fuel. Check chain oil reservoir if using a saw blade attachment.

CAUTION! Incorrect assembly can lead to loss of control. Follow diagrams precisely.

Specifications

• Engine: 2-stroke, 45.6 cc displacement.
• Powerhead Weight: Approx. 7.5 kg.
• Fuel: Gasoline/oil mix (50:1 with STIHL HP Ultra oil).
• Fuel Tank Capacity: 0.95 liters.
• Sound Pressure Level: Typically 100 dB(A) – hearing protection required.
• Vibration Levels: Front/Rear handle: < 2.5 m/s² (with anti-vibration system).
• Recommended Cutting Attachments: AutoCut C 5-2 trimmer head, circular saw blades, brush knives.
• Warranty: Refer to STIHL warranty statement.

Controls and Components

Main controls located on the handle and engine housing.

Handle Controls: Throttle trigger with lockout, stop switch (I/O), choke lever, primer bulb, and throttle trigger lockout.

Engine Housing: Spark plug, air filter cover, muffler, fuel tank cap, and starter grip.

Safety Features: Quick-stop inertia chain brake (activates on kickback), front hand guard, and debris deflector.

Adjustments: Handlebar angle and D-loop handle position can be adjusted for operator comfort.

Fuel & Lubrication

Use only high-quality, unleaded gasoline (min. 89 octane) mixed with STIHL HP Ultra 2-stroke oil at a 50:1 ratio.

1. Mixing: Pour specified amount of oil into clean container, add gasoline, close and shake well.
2. Refueling: Clean area around fuel cap. Use funnel to avoid spills. Tighten cap securely.
3. Lubrication: If using a saw blade attachment, fill separate chain oil reservoir with STIHL chain oil. Check flow before use.
4. Storage: For storage over 30 days, either drain fuel system completely or add STIHL Fuel Stabilizer.

Tip: STIHL MotoMix pre-mixed fuel is recommended for optimal performance and reduced maintenance.

Starting & Operation

Follow cold or warm starting procedure. Operate with controlled, sweeping motions.

1. Cold Start: Set stop switch to "I". Press fuel primer bulb 5-10 times. Set choke lever to closed position. Pull starter rope sharply until engine 'fires'. Move choke lever to open position. Pull starter rope again to start.
2. Warm Start: Choke lever in open position. Pull starter rope.
3. Operation: Allow engine to warm up at idle for 30 seconds. Use a smooth, swinging arc motion from right to left. Keep cutting attachment parallel to the ground.
4. Stopping: Release throttle trigger. Allow engine to idle briefly. Set stop switch to "O".

WARNING! Never drop-start or "throw" start the unit. Always keep firm footing and balance.

Cutting Attachments

Select attachment based on vegetation type. Ensure correct guard is installed.

Attachment	Best For	Precautions
AutoCut C 5-2 Trimmer Head	Grass, weeds, light brush	Advance line with bump feed. Avoid hard surfaces.
Circular Metal Blade	Heavy brush, saplings up to 4"	Use extreme caution. Ensure blade is sharp and balanced.
Brush Knife (4-tooth)	Dense weeds, tough vegetation	Check for secure mounting and guard alignment.

Installation: Disconnect spark plug boot. Use appropriate wrench to secure attachment nut. Reconnect boot.

Maintenance Schedule

Regular maintenance ensures performance and longevity.

Interval	Task
Daily/Before Use	Check fasteners for tightness; inspect for damage; clean air intake and exterior.
Every 10 Hours	Clean air filter (replace if damaged); check spark plug condition.
Every 50 Hours	Replace spark plug; clean cooling fins; check fuel filter; inspect anti-vibration elements.
Every 100 Hours	Clean exhaust system/spark arrestor screen (if equipped); check clutch and drive shaft.
As Needed	Sharpen or replace cutting blades; replace trimmer line; clean/replace fuel filter.

CAUTION! Always disconnect spark plug boot before performing any maintenance.

Storage

Prepare unit properly for short or long-term storage.

1. Short-term (under 30 days): Run carburetor dry or add fuel stabilizer.
2. Long-term: Drain all fuel from tank and carburetor. Run engine until it stops. Remove spark plug, add a teaspoon of engine oil into cylinder, pull starter slowly 2-3 times, reinstall plug. Clean entire unit. Store in a dry, secure place.

CAUTION! Fuel left in the carburetor can form gums and varnish, causing starting problems.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, bad spark, clogged filter	Check fuel; wait if flooded; check spark plug/air filter; use fresh fuel.
Engine runs poorly/dies	Dirty air filter, old fuel, clogged fuel filter	Clean/replace air filter; drain old fuel; replace fuel filter.
Excessive vibration	Loose fasteners, unbalanced/damaged blade	Tighten all nuts/bolts; inspect blade for damage/balance.
Cutting attachment stops under load	Dull blade, worn clutch, drive shaft issue	Sharpen/replace blade; have clutch inspected by dealer.
Chain brake activates unexpectedly	Inertia mechanism sensitive or faulty	Have dealer inspect and service the quick-stop system.

Service: For repairs beyond basic maintenance, contact an authorized STIHL servicing dealer.

Support: Visit www.stihlusa.com or contact STIHL customer service.