STIHL BT 121 Manual

The STIHL BT 121 is a professional-grade backpack blower designed for demanding landscaping and cleanup tasks. It features a powerful 2-stroke engine, ergonomic harness system, variable speed throttle, cruise control, vibration damping, and a large air volume for efficient debris removal. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

1. Features at a Glance 2. Safety Information 3. Assembly & Preparation 4. Specifications 5. Controls and Operation 6. Starting & Stopping Procedure 7. Maintenance Schedule 8. Fuel & Lubrication 9. Storage 10. Regulatory Information 11. Troubleshooting

Features at a Glance

Key components: Powerful 2-stroke engine, Backpack harness, Throttle trigger with lock, Cruise control lever, Air filter, Muffler, Blower tube, and Fuel tank.

Feature	Description
Engine Type	2-stroke, single-cylinder, air-cooled
Displacement	64.8 cm³ (3.95 cu. in.)
Power Output	High torque for demanding applications
Air Volume	Up to 1,200 m³/h
Air Speed	Up to 300 km/h
Fuel Tank Capacity	1.3 liters (0.34 US gal)
Sound Pressure Level	Complies with relevant noise regulations
Weight (dry)	Approx. 10.5 kg (23.1 lbs)
Harness System	Padded, adjustable for operator comfort
Throttle Control	Variable speed with cruise control lock

Safety Information

Read and understand all instructions before operation. Wear appropriate PPE.

Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, long pants, sturdy boots, and gloves.
Operating Area: Keep bystanders, especially children and pets, at least 15 meters away.
Fuel Safety: Refuel in a well-ventilated area with engine stopped and cool. Use fuel mix specified in manual.
Hot Surfaces: Avoid contact with muffler and cylinder during and immediately after operation.
Debris Hazards: Do not point blower tube at people, animals, or windows. Be aware of flying debris.

WARNING! Risk of fire, explosion, and serious injury. Follow all safety instructions.

Assembly & Preparation

Unpack all parts. Assemble according to instructions.

Attach Harness: Connect shoulder straps and waist belt. Adjust for a snug, comfortable fit.
Install Blower Tube: Push the tube firmly onto the blower housing until it clicks into place.
Fuel Mix: Prepare a 50:1 mixture of unleaded gasoline (min. 89 octane) and STIHL HP Ultra 2-stroke oil. Fill fuel tank.
Check Air Filter: Ensure air filter is clean and properly seated.
Inspect Unit: Check for loose screws, damaged parts, or fuel leaks before starting.

CAUTION! Use only STIHL-approved parts and accessories. Incorrect assembly can cause malfunction.

Specifications

Model: STIHL BT 121.
Engine: STIHL 2-MIX engine technology.
Displacement: 64.8 cm³.
Powerhead Weight: Approx. 10.5 kg (23.1 lbs).
Sound Pressure Level (LpA): Typically below 115 dB(A).
Vibration Levels (ahv): Front handle & rear handle values comply with standards.
Fuel/Oil Mix: 50:1 (Gasoline to STIHL HP Ultra oil).
Warranty: Limited warranty as per STIHL policy.

Controls and Operation

Main Controls: Throttle trigger, Throttle trigger lockout, Master Control lever (START/STOP/RUN), Cruise control lever, Choke knob.

Operation: Wear harness. Start engine (see Starting Procedure). Squeeze throttle trigger for variable speed. Engage cruise control lever for constant speed during extended use. Use sweeping motions for efficient debris clearing.

Adjustments: Harness straps for comfort. Blower tube angle for task.

Starting & Stopping Procedure

Ensure unit is on flat, stable ground with clear area around muffler.

Cold Start: Set Master Control lever to START/CHOKE position. Press fuel pump bulb 5-10 times. Pull starter rope sharply until engine fires. Move Master Control to RUN position. Pull starter rope again to start.
Warm Start: Set Master Control to RUN position. Pull starter rope.
Stopping: Release throttle. Set Master Control lever to STOP position.

Tip: Do not pull starter rope to its full extent or let it snap back.

Maintenance Schedule

Regular maintenance ensures performance and longevity.

Interval	Task
Before each use	Check for loose fasteners, leaks, damage. Clean air filter if dirty.
Every 25 hours	Clean spark plug, check electrode gap (0.5 mm).
Every 50 hours	Clean cooling fins, check fuel filter, inspect spark arresting screen in muffler.
Every 100 hours	Replace air filter, fuel filter. Check all rubber mounts and hoses.
Annually	Have unit inspected by an authorized STIHL servicing dealer.

WARNING! Perform maintenance only when engine is off and cool. Disconnect spark plug boot for safety.

Fuel & Lubrication

Use fresh, unleaded gasoline with a minimum octane rating of 89 (RON). Mix with STIHL HP Ultra 2-stroke oil at a 50:1 ratio. Shake container thoroughly before refueling.

CAUTION! Never use fuel mix older than 30 days. Do not mix oil and gasoline directly in the fuel tank. Store fuel in an approved container.

Storage

For storage longer than 30 days, prepare the unit to prevent fuel system issues.

Drain fuel tank completely and run engine until it stops to clear carburetor.
Remove spark plug, add a few drops of engine oil into cylinder, pull starter rope slowly, reinstall plug.
Clean entire unit, especially cooling fins and air filter.
Store in a dry, secure place, out of reach of children.

Regulatory Information

This product complies with relevant EPA and CARB emissions standards for handheld engines. It meets applicable noise regulations. The spark arresting muffler is compliant with USDA Forest Service standards. STIHL is a registered trademark. For genuine parts, accessories, and service, contact an authorized STIHL dealer.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, spark plug	Check fuel level. Wait if flooded. Check/clean/replace spark plug.
Engine starts then dies	Clogged air/fuel filter, vent	Clean/replace air filter. Check fuel filter and tank vent.
Loss of power	Dirty air filter, exhaust screen	Clean air filter and spark arresting screen in muffler.
Excessive vibration	Loose parts, damaged impeller	Tighten all fasteners. Inspect impeller for damage.
Unusual noise	Foreign object, bearing wear	Stop engine immediately. Check for debris in impeller housing. Contact dealer.

Service: For repairs beyond basic maintenance, contact an authorized STIHL servicing dealer. Find a dealer at www.stihlusa.com or call 1-800-467-8445.