The Stihl FS 90 is a professional-grade brushcutter/trimmer designed for demanding clearing and trimming tasks. This service manual provides comprehensive information for maintenance, troubleshooting, and repair of the FS 90 model, including its engine, fuel system, cutting head, drive shaft, and safety features. Below are key sections covering safety, specifications, maintenance procedures, troubleshooting, and parts information.
Key components: 2-stroke engine, loop handle with controls, polycut head or optional blade, drive shaft, anti-vibration system, and decompression valve.
| Feature | Description |
|---|---|
| Engine Type | Single cylinder, 2-stroke, air-cooled |
| Displacement | 27.2 cm³ (1.66 cu. in.) |
| Power Output | 0.9 kW (1.2 bhp) |
| Fuel/Oil Mix | 50:1 (STIHL HP Ultra oil) |
| Cutting Attachment | Standard Polycut 2-4 head; accepts blades |
| Drive System | Flexible drive shaft |
| Anti-Vibration | Integrated AV system in handle and shaft |
| Starter | Rewind starter with decompression valve |
| Ignition System | Electronic, capacitor discharge (CDI) |
| Carburetor | Diaphragm carburetor (Zama C1Q) |
Always wear appropriate PPE: safety glasses, hearing protection, gloves, long pants, and sturdy boots. Clear work area of bystanders.
WARNING! Failure to follow safety procedures can result in serious injury or death. Always refer to the original operator's manual for safe operation.
Access OSD via TV MENU or SETUP (DVD). Front Panel: POWER, OPEN/CLOSE, AUTO/DPMS, MUTE.
Daily/Before Use: Check for loose fasteners, inspect cutting attachment, check fuel for contamination.
Every 50 Hours: Clean air filter, check spark plug, inspect fuel filter, clean cooling fins.
Every 100 Hours: Clean spark arrestor screen (if equipped), check drive shaft lubrication, inspect anti-vibration elements.
Annually/Seasonally: Replace fuel filter, clean carburetor (if needed), inspect/replace starter rope, check clutch and drum.
Cylinder & Piston: Inspect for scoring. Measure compression (min. 110 psi).
Ignition: Check spark plug gap (0.5 mm / 0.020"). Test ignition coil with multimeter (primary 0.2-0.3 Ω, secondary 8-10 kΩ).
Decompression Valve: Test by blowing through valve; should seal when closed.
Carburetor (Zama C1Q): Adjustments: L (low speed), H (high speed), LA (idle). Initial settings: L = 1 turn open, H = 1 turn open.
Tip: Use only fresh, properly mixed fuel (50:1). Stale fuel is a common cause of starting and running issues.
Supports Polycut head, circular blades, and grass blades.
WARNING! Always ensure cutting attachment is properly secured and guard is in place before operation.
Closed Captions: Enable in DVD setup (On/Off). Parental Controls: Block by movie/TV ratings in FEATURES menu (password 9999).
| Symptom | Possible Cause | Corrective Action |
|---|---|---|
| Engine will not start | No spark, fuel issue, flooded | Check spark plug, fuel quality, choke operation. Dry plug if flooded. |
| Engine starts then dies | Clogged fuel filter, dirty carb | Replace fuel filter, clean carburetor, check tank vent. |
| Loss of power | Clogged air filter, exhaust screen, carb adjustment | Clean/replace air filter, clean spark arrestor, adjust H needle. |
| Excessive vibration | Unbalanced blade, bent shaft, loose parts | Balance or replace blade, inspect drive shaft, tighten all fasteners. |
| Cutting head does not turn | Broken drive shaft, clutch worn, gearhead failure | Inspect shaft for breakage, check clutch shoes, disassemble gearhead. |
| Hard starting when hot | Ignition coil failing, decomp valve leak | Test coil resistance hot/cold, replace decompression valve. |
Reset: Menu > System > Reset if available.
Service: 1-877-467-4289; www.insigniaproducts.com
Note: Use a calibrated torque wrench. Do not overtighten.
Refer to official STIHL parts catalog for FS 90. Common replacement parts:
Warranty & Support: Contact authorized STIHL dealer for genuine parts and warranty service. Manual revision: 2025.