The Stihl KM 90 R is a professional-grade KombiMotor powerhead, part of Stihl's multi-tool system. This repair manual covers essential procedures for maintenance, troubleshooting, and repair of the KM 90 R engine and its attachments. It includes safety warnings, technical specifications, step-by-step disassembly/assembly instructions, fuel system servicing, ignition system checks, carburetor adjustment, and common fault diagnosis. Always use genuine Stihl parts and follow all safety precautions.
Before attempting any repair, read all safety instructions. The KM 90 R is a powerful tool; improper service can cause serious injury or damage.
| Hazard | Precaution |
|---|---|
| Fuel Fire/Explosion | Work in well-ventilated area away from sparks or flames. Drain fuel tank before service. |
| Hot Surfaces | Allow engine to cool completely before disassembly. |
| Sharp Tools/Components | Wear cut-resistant gloves when handling cutting attachments or internal components. |
| Spring-Loaded Parts | Use appropriate tools to contain springs during disassembly (e.g., starter spring). |
| Electrical Shock | Disconnect spark plug boot and secure away from plug before any work. |
WARNING! Always use genuine Stihl replacement parts. Non-genuine parts may void warranty and cause unsafe operation.
Basic mechanic's tools plus Stihl-specific tools are required for proper repair.
CAUTION! Using incorrect tools can damage components. Invest in proper Stihl tools for critical operations like flywheel removal.
Regular maintenance prevents major repairs. Follow the schedule below.
| Interval (Operating Hours) | Task | Procedure Reference |
|---|---|---|
| Before each use | Check for loose fasteners, leaks, damage. Clean air filter if dirty. | Visual inspection; clean with soapy water, dry thoroughly. |
| Every 25 hours | Clean spark arrestor screen (if equipped). Check spark plug condition/gap. | Remove muffler cover; clean screen with wire brush. Regap or replace plug. |
| Every 50 hours | Replace fuel filter. Clean carburetor external surfaces. Grease drive shaft splines. | Use fuel line clamp; replace in-tank filter. Apply Stihl grease to attachment interface. |
| Every 100 hours | Decarbonize exhaust port and muffler. Check piston/cylinder through exhaust port for scoring. | Remove muffler; scrape carbon deposits carefully. |
Tip: Keep a maintenance log. Use only Stihl-approved lubricants and cleaners.
Common issues: hard starting, poor performance, stalling. System includes tank, lines, filter, carburetor.
WARNING! Fuel is highly flammable. No smoking. Drain system completely before opening fuel lines or carburetor.
Components: Flywheel, ignition module (coil), spark plug, stop switch, wiring.
Testing: 1. Spark Test: Remove spark plug, reconnect boot. Hold plug base against cylinder head (metal). Pull starter cord—look for bright blue spark. No spark indicates faulty coil, plug, or wiring. 2. Coil Air Gap: Should be 0.2-0.4 mm (0.008-0.016 in.). Adjust by loosening coil mounting screws and using a feeler gauge. 3. Stop Switch: Disconnect wire from coil; retest for spark. If spark appears, stop switch is faulty. 4. Flywheel Key: Sheared key causes timing loss. Remove flywheel; inspect woodruff key. Replace if damaged.
Note: Ignition modules are non-repairable; replace if defective.
Zama C1Q-S carburetor has three adjustment screws: L (low speed), H (high speed), LA (idle speed).
CAUTION! H screw affects engine speed and temperature. Over-leaning (turning H in too far) can cause engine seizure.
Major repair requires splitting the crankcase. Follow this sequence.
Disassembly Order: 1. Remove all external components: fuel tank, carburetor, air filter cover, starter assembly, muffler, ignition coil. 2. Remove cylinder cover. Remove cylinder base nuts. 3. Use piston stop tool to hold piston; remove flywheel nut. Use flywheel puller to remove flywheel. 4. Remove clutch assembly (if equipped for certain attachments). 5. Separate crankcase halves by removing all screws. Gently tap with plastic mallet. 6. Remove crankshaft, bearings, seals from case halves.
Reassembly Tips: - Replace all crank seals and gaskets. - Lubricate bearings and seals with 2-stroke oil during assembly. - Torque crankcase screws in a crisscross pattern to specification (typically 10-12 Nm). - Check crankshaft endplay after assembly.
WARNING! This is an advanced procedure. Incorrect assembly will destroy the engine.
| Symptom | Possible Cause | Corrective Action |
|---|---|---|
| Engine will not start | No spark, no fuel, flooded | Check spark, fuel delivery, carburetor. Choke off, throttle open, pull repeatedly to clear flood. |
| Engine starts then dies | Clogged fuel filter, dirty carb, bad fuel | Replace fuel filter, clean/rebuild carburetor, use fresh fuel mix. |
| Loss of power | Clogged air filter, exhaust port carbon, improper H adjustment | Clean/replace air filter, decarbonize, adjust H screw. |
| Excessive vibration | Bent crankshaft, unbalanced attachment, loose components | Inspect crank; check attachment; tighten all fasteners. |
| Engine overheats | Lean fuel mix, clogged cooling fins, improper H setting | Check fuel mix ratio; clean fins; enrich H screw slightly. |
| Oil from exhaust | Too much oil in mix, worn piston/rings | Verify 50:1 mix; check compression; rebuild if low. |
Compression Check: Should be > 100 psi (6.9 bar). Below indicates worn rings, cylinder, or leaking gasket.
Always reference the official Stihl KM 90 R parts diagram (IPL) for part numbers. Key assemblies:
Ordering: Use the 10-digit Stihl part number. Contact an authorized Stihl dealer or visit www.stihlusa.com for parts lookup.
Stihl KM 90 R typically has a limited warranty for personal use. Repairs by non-authorized personnel may void warranty.
Support Contacts: - Stihl USA Customer Service: 1-800-467-8445 - Website: www.stihlusa.com - Dealer Locator: Available on website for technical support and genuine parts.
Disclaimer: This manual is for informational purposes. Stihl is not responsible for repairs performed incorrectly based on this guide. For complex repairs, consult a Stihl authorized service dealer.