STIHL MS 390 Service Manual

The STIHL MS 390 is a durable saw designed for demanding cutting tasks, but optimal performance depends on regular maintenance conducted with attention to safety and precision. Before performing any service, ensure the engine is cool, the spark plug is disconnected, and the saw is on a stable surface to prevent accidental movement. Establish a clean workspace with good lighting and ample ventilation, and lay out a designated tool tray to keep fasteners and parts organized. Proper maintenance begins with a routine that aligns with the manufacturer’s specifications and requires diligent inspection of wear items such as the bar, chain, and air filter. Documenting service intervals helps track when next maintenance is due and supports consistent performance over the life of the machine.

Safety basics for outdoor power equipment encompass personal protective equipment, operational awareness, and hazard control. Always wear cut-resistant gloves, eye protection, hearing protection, and sturdy footwear when servicing the saw. Work in a clear area free of bystanders, especially children and pets, to minimize distractions. Disconnect the spark plug boot or remove the battery on electric models to prevent accidental starting, and use a torque-limiting screwdriver or proper wrenches to avoid over-torquing fasteners. Understand the chain brake function and keep the chain in a neutral position during adjustments to reduce the risk of injuries. Adhering to lockout/tagout procedures during inspections further enhances safety by preventing unexpected energization of the system.

Key components and their functions should be understood to diagnose issues effectively. The bar guides the chain and is critical for efficient cutting; inspect for wear, bending, or cracks and ensure the bar groove is clean and free of debris. The chain transfers the engine’s power to the wood; check for sharpness, tension, and proper lubrication, adjusting the chain tension with the bar nose higher than the drive sprocket. The air filter protects the engine from dust; a clogged filter reduces performance and increases fuel consumption, so clean or replace it at recommended intervals. The carburetor, ignition system, and cooling fins all play essential roles in engine reliability, so observe for signs of fuel blowback, misfiring, or overheating and address promptly. The drive sprocket, clutch assembly, and fuel system also require periodic inspection to prevent loss of power or abrupt engine shutdowns during operation.

Recommended tools and work environment emphasize having the right equipment and a suitable workspace. A dedicated set of metric and standard wrenches, a Torx or screwdriver assortment, a scrench (screwdriver/adjuster) tool, and a torque wrench help perform accurate and safe maintenance. Use non-abrasive cleaners, a chain file or sharpener, a bar groove cleaning tool, and lubricants specified by STIHL to maintain an efficient cutting edge and smooth operation. A stable bench, anti-fatigue mat, and appropriate clamps or vices keep the saw steady while performing maintenance tasks. Maintain a clean, organized storage area for consumables like air filters, fuel, bar oil, and replacement chains to streamline future service. Always follow the manufacturer’s service limits and disposal recommendations for worn components to ensure the longevity and safety of the STIHL MS 390.

Identification of routine service intervals

Regular maintenance of the STIHL MS 390 is essential to ensure reliable operation, safe performance, and extended engine life. Establishing a clear schedule for daily, weekly, and periodic checks helps detect wear early and prevents unexpected downtime. Start each operation by performing a concise pre-operation inspection to confirm that critical components are in good condition and properly secured. Document any observations and address them before proceeding with work to maintain optimal saw performance and operator safety.

Daily pre operation inspection focuses on immediate, visible criteria that influence safety and function. Check the chain brake, clutch cover, and bar for secure mounting and proper tension. Inspect the air filter, fuel level, and bar oil reservoir to ensure there is adequate supply and no leaks. Examine the chain for sharpness and correct tension, and look for damaged teeth or cracks in the guide bar. These quick checks help prevent seized motors, kickback scenarios, and inefficient cutting that could lead to excessive wear or injury.

Periodic service schedule by hours of operation typically aligns with the manufacturer's recommendations for the MS 390. Establish a routine that scales with usage, often using a tiered approach such as every 20, 40, or 100 hours depending on operating conditions. At these intervals, perform more in-depth inspections of the ignition system, fuel system, and carburetor, and replace wear-prone components before failure occurs. A well-planned interval program reduces the likelihood of sudden downtime, maintains cutting efficiency, and preserves engine reliability over the long term.

When to replace parts and what signs indicate wear are critical for maintaining performance and safety. Replace the air filter when it shows heavy clogging or damage, and always replace the fuel filter if it appears degraded or obstructed. Chain and guide bar wear should be monitored for excessive play, chipped teeth, or uneven wear patterns that could affect cutting accuracy or increase the risk of kickback. Inspect the spark plug for signs of fouling, wear, or improper gap, and replace as needed to sustain reliable starting and smooth engine operation. By recognizing these indicators early and adhering to replacement guidelines, you minimize downtime and sustain efficient, safe operation of the STIHL MS 390.

The fuel system and engine basics for the STIHL MS 390 are foundational for reliable operation and long-term durability. Proper fuel handling begins with using fresh, clean gasoline mixed with the correct amount of high-quality two-stroke oil as specified by the manufacturer. Store fuel in approved containers, away from heat sources and direct sunlight, and avoid keeping fuel for extended periods. When preparing a fresh fuel mix, measure precisely to the recommended 50:1 or as specified by the engine’s year and model, and always shake or stir gently to achieve a uniform blend. Regularly inspect fuel lines, seals, and the carburetor intake for signs of wear, cracking, or hardening, and replace any components that show deterioration to prevent air leaks or fuel delivery issues. Keeping a clean air filtration system and a properly tuned carburetor will minimize fuel odor, reduce emissions, and improve throttle response.

Carburetor adjustment and tuning require a methodical approach to achieve optimal fuel-air mixture across different operating conditions. Start with the factory baseline settings and document any adjustments you make for future reference. When tuning, adjust the low-speed and high-speed needles in small increments, allowing time for the engine to stabilize after each change. If the saw runs lean, indicated by high engine temperatures, popping sound, or overheating, adjust toward a richer mixture by reducing the needle gap or re-tuning the high-speed and idle circuits as necessary. Conversely, a rich condition may cause black exhaust smoke, fouled spark plugs, or sluggish acceleration; in that case, lean the mixture slightly but avoid detuning beyond recommended limits. Ensure the air filter is clean and that the carburetor base gasket remains intact to prevent air leaks that would skew mixture readings. After tuning, perform a wide-open throttle pull to confirm smooth operation through the RPM range and verify stable idle when returning to idle speed.

Ignition system checks and spark plug care are essential for reliable starting and consistent power delivery. Begin with a visual inspection of the spark plug for color, insulator cracks, and electrode wear; a porcelain insulator that appears dry and light tan is typical for correctly gapped plugs. Use the correct gap specification, usually around 0.65 to 0.75 mm for many two-stroke STIHL models, and adjust with care to avoid electrode damage. Perform a spark test by grounding the spark plug and cranking the engine to confirm a strong, consistent spark; if a weak or inconsistent spark is observed, inspect the ignition coil, kill switch wiring, and flywheel magnets for alignment and wear. Ensure the ignition timing remains accurate by confirming that the flywheel key is intact and not sheared, which could cause timing drift and hard starting. Regular maintenance of the ignition system, including replacing worn gaskets and seals, helps maintain efficient combustion and reduces the likelihood of misfires during operation.

Air filtration is a critical line of defense for the STIHL MS 390’s engine, guarding against dust, dirt, and debris that can cause premature wear and loss of performance. Regular inspection of the air filter housing, seals, and filter element ensures optimal airflow and combustion efficiency. When inspecting, remove the cover carefully to avoid letting foreign material drop into the carburetor, and look for signs of oil saturation, tears, or deformation in the filter element. A clean, dry filter is essential for maintaining peak power, fuel efficiency, and emissions compliance. If the filter shows any damage or excessive soiling, replace it with an OEM filter and inspect the surrounding gaskets for integrity to prevent unfiltered air ingress. Maintain a routine that aligns with operating conditions—dusty jobs require more frequent checks and replacements to protect the engine’s longevity.

Carburetor air intake considerations focus on delivering clean, unrestricted air to the combustion chamber. A properly seated air filter and intact intake boots prevent unmetered air from entering the system, which can upset the air-fuel ratio. Inspect the carburetor mounting and air intake hose for cracks, looseness, or signs of wear that could cause air leaks. When service is performed, ensure the clips, clamps, and seals are secured to factory specifications, as improper sealing can lead to lean or rich running conditions that increase emissions and reduce performance. For performance and emissions stability, verify that the air intake path remains unobstructed by debris such as pine needles or sawdust, and clean the intake tract with appropriate tools. Regularly assess the intake grommets and fuel/air mixture performance, particularly after exposure to dusty environments or after replacing the muffler or exhaust components.

Exhaust and emissions related maintenance requires careful attention to components that directly influence combustion byproducts and overall engine efficiency. The exhaust system should be inspected for leaks, which can adversely affect backpressure and cause fuel mixture disturbances, impacting both power delivery and emissions. Check the muffler, catalytic elements (if equipped), and exhaust gaskets for signs of corrosion, cracking, or carbon buildup, removing any obstructions that could alter flow. Ensure that the spark arrestor screen, if present, is clean and free from obstructions to maintain proper exhaust flow and reduce potential heat buildup. When performing maintenance, verify that the exhaust bolts and clamps are torqued to the manufacturer’s specifications to prevent leaks and ensure consistent exhaust scavenging. Regular emissions-related checks support efficient operation, reduce fuel consumption, and help maintain compliance with applicable environmental standards for outdoor power equipment.

Bar and chain lubrication requirements: The STIHL MS 390 relies on a consistent lubrication system to maintain cutting efficiency and extend bar life. Ensure the oil tank is filled with clean, high-quality bar and chain oil that meets the manufacturer's viscosity recommendations for your climate and operating conditions. Regularly inspect the oil pump line and oil supply passages for any signs of wear, leaks, or blockages, and replace damaged hoses promptly to prevent inadequate lubrication. When starting the saw, verify that the oiler is delivering oil to the bar by observing a light oil sheen on a test surface or the bar during operation at idle or light throttle. Avoid running the engine with an empty oil supply, as insufficient lubrication accelerates bar wear, chain pin and rivet deformation, and can lead to increased friction and heat buildup. Periodic oiling system maintenance should include cleaning the oil tank breather to prevent pressure changes that could reduce oil flow and inspecting the oil filter screen for clogging, replacing it if necessary to maintain steady lubrication.

Chain tensioning and sharpening procedures: Proper chain tension is critical for safe operation and optimal cutting performance. When the chain is properly tensioned, it should have a slight sag at the midpoint of the drive-link, with the drive links fully seated in the bar groove and no kinking or binding when the bar is turned by hand. Always perform tension checks with the bar and chain cool, and after the bar and chain have cooled, recheck tension to account for any thermal expansion. If the chain is too loose, it can derail from the bar or cause excessive vibration; if it is too tight, it will increase bearing load and reduce cutting efficiency. For sharpening, use a high-quality filing guide and a round file appropriate to the chain pitch, maintaining the original tooth shape and angle. File at the correct pitch direction, ensuring even wear and consistent drive-link height, and regularly inspect cutters and depth gauges to maintain uniform cutting depth. After sharpening, recheck tension and perform a light test cut to confirm smooth operation, adjusting as needed.

Sprocket inspection and replacement guidelines: The sprocket drives the chain and must be inspected for even wear, chipped teeth, and excessive yaw or looseness in the sprocket nut. Look for hooked or uneven tooth wear, which indicates misalignment, improper lubrication, or chain binding during operation. If you notice any broken, rounded, or heavily worn teeth, replace the sprocket to preserve drive efficiency and reduce the risk of chain derailment. Before reinstalling a new or serviced sprocket, inspect the clutch drum and drive links for corresponding wear patterns and ensure the chain is the correct pitch and gauge for your bar length. When installing a new sprocket, verify proper seating with the clutch drum and ensure the chain engages smoothly with minimum play in the drive system. Regular inspection of the sprocket assembly, including the drive sprocket, clutch mechanism, and bar groove alignment, helps prevent excessive wear and contributes to safer, more reliable cutting performance.

The STIHL MS 390 uses a robust air-cooling system that relies on properly aligned cooling fins to dissipate heat generated by the engine during operation. To ensure optimal heat transfer, inspect the cylinder exterior and cooling fins for nicks, bent blades, and excessive debris buildup. Use a soft brush and compressed air to remove dust and sawdust without prying loose fins. If any fins are cracked or severely damaged, replace the cylinder or fins assembly to prevent hot spots that could lead to overheating and reduced performance. Maintain unblocked air passages around the flywheel and shrouding, as restricted airflow can significantly raise running temperatures and shorten engine life. Regular cleaning during maintenance intervals helps preserve consistent engine temperatures and prolongs service intervals between major repairs.

Compression checks provide critical information about the engine’s internal condition. Begin with a cold engine and use a calibrated compression gauge to measure peak cylinder compression, comparing results to the manufacturer’s specifications. A value significantly below spec or showing a rapid drop upon repeated cranking indicates potential issues such as worn piston rings, a scored cylinder, or a leaking crankshaft seal. If compression is uneven between the two cylinder walls, perform a wet test by adding a small amount of clean, high-quality engine oil into the spark plug hole and rechecking; an improvement suggests worn rings or valve seating problems. Always perform a leak-down test if compression readings are suspect, as this pinpoints whether the leak is past rings, valves, or gasket interfaces. Document compression results and correlate them with engine performance symptoms like hard starting, loss of power, or excessive fuel consumption to guide corrective actions.

Safety features and their proper function are essential for reliable operation and operator protection. Inspect the chain brake engagement mechanism to ensure it fully arrests the chain when engaged and returns to its resting position without binding. Test the ignition switch and run/stop controls for clean, decisive operation and verify there are no loose or exposed wiring connections that could energize components during operation. The chain catcher and Euro-style anti-kickback features should be intact and unobstructed, with mounting hardware tightened to spec to prevent failure if kickback occurs. Inspect the chain brake switch, inertia mechanism, and throttle linkages for smooth movement with no excessive play or sticking. Finally, verify that the muffler spark arrestor, guards, and cooling shrouds are securely fastened and free from fuel residues that could ignite and create a hazardous condition. Functional safety testing should be performed with the chain brake engaged and the throttle at idle to confirm that all safety interlocks behave as designed under realistic operating conditions.

Repair and service procedures

Disassembly and reassembly best practices require a methodical approach to avoid damaging components and to preserve safety. Begin by disconnecting the spark plug boot and removing the battery or accessory power where applicable, ensuring the unit is fully de-energized. Work on a clean, well-lit surface and keep fasteners organized by size and location, using labeled containers or magnetic mats. When removing covers or shields, document the order of removal and any hidden fasteners to facilitate accurate reassembly. As you progress, take clear photographs at key steps and reference the service manual to confirm torque specifications and alignment tips. Finally, revisit each fastener after reassembly to ensure all connections are secure and that no tools have been left inside the chassis.

Replacing common wear items involves identifying components that experience regular wear and impact resistance and planning timely replacements. Start with the air filter, ensuring the housing is sealed and the filter is free of contaminants before reinstalling. Inspect the fuel filter and lines for cracks or leaks, replacing any aged sections to maintain optimal fuel delivery. For the chain and guide bar, examine drive links for stretching and tension, and replace the chain if there are excessive signs of wear or damaged drive sprockets. Check the spark plug for fouling or wear and replace according to service intervals. Verify the muffler and exhaust components for blockages or tailpipe restrictions, replacing gaskets as needed to preserve proper exhaust flow. Lastly, inspect consumable items like starter components and anti-vibration mounts, replacing any signs of deterioration to maintain smooth operation and user comfort.

Troubleshooting common faults and corrective actions requires a systematic diagnostic approach to isolate issues and implement effective repairs. If the engine does not start, confirm the absence of fuel, verify spark at the spark plug, and inspect the air and fuel delivery systems for blockages or leaks. For power loss during operation, assess the carburetor adjustment, reed valve condition, and exhaust restrictions, restoring proper airflow and fuel metering as needed. If there is excessive vibration or unusual sounds, inspect the bar and chain for proper tension and sharpening, check the chain brake, and examine anti-vibration mounts for wear. When overheating occurs, verify cooling fins, air intake cleanliness, and the presence of clean fuel; ensure there are no obstructions around the cooling path. In all cases, consult torque figures and alignment specifications to ensure components are reassembled correctly and to prevent recurring faults.

Diagnostics and troubleshooting guides

Symptom based troubleshooting begins with careful observation of the STIHL MS 390’s performance issues and a methodical approach to isolating probable causes. Start by noting when the symptom occurs, such as during cold starts, under load, or after extended operation. This helps differentiate between fuel system, ignition, or air intake problems. Gather baseline information including last service date, fuel type, and any recent maintenance work. A systematic checklist ensures that common failure points such as dirty air filters, clogged carburetors, and worn spark plugs are evaluated first. By structuring the diagnostic process around symptoms, you can reduce unnecessary disassembly and pinpoint the most likely culprits with efficiency and accuracy.

Common symptoms include hard starting, poor acceleration, misfiring, excessive exhaust smoke, and reduced cutting performance. For hard starting, verify the carburetor’s fuel delivery, inspect the choke operation, and check for air leaks in the intake boot. Misfiring or loss of power often points to ignition issues or a fouled spark plug; inspect the spark plug condition, check the ignition coil resistance, and confirm proper gap. Excessive smoke can indicate rich fuel mixtures, degraded fuel, or a faulty carburetor diaphragm; perform a float height and diaphragm inspection, and examine fuel lines for cracks. Reduced cutting performance may stem from a dull chain, improper chain tension, or insufficient engine cooling; assess bar and chain maintenance, as well as air intake cleanliness. Document each finding to support future maintenance decisions and pattern recognition for recurring symptoms.

Using diagnostic tools and test methods involves applying precise measurements and safe testing practices to verify component function. Use a spark tester to confirm a strong, consistent spark and inspect the ignition timing visually if the model allows. A carburetor diagnostic requires observing fuel flow, checking for air leaks with soapy water around gaskets, and confirming proper fuel pressure where applicable. A compression test can reveal cyclinder wear and overall engine health; perform a controlled test with a properly warmed engine and compare readings to the manufacturer’s specifications. Electrical system checks should include battery condition if the tool relies on an electric start, harness continuity checks, and fuses inspection. Record the results of each diagnostic step, including readings and any corrective actions taken, to build a troubleshooting history that supports faster service in the future.

Record keeping and maintenance logs are essential for tracking performance, preventive maintenance, and recurring issues. Maintain a log that captures service dates, parts replaced, fuel and oil type, and operating hours at the time of service. Include notes on observed conditions such as fuel quality, filter cleanliness, and chain tension, along with any test results and adjustments made. A well-maintained log helps identify trends, such as gradual compression loss or chronic carburetor leaks, enabling proactive maintenance rather than reactive repairs. Use standardized entries to facilitate handoffs between technicians and to provide a reliable reference for warranty claims or future diagnostics. Regularly review these records to optimize maintenance schedules, parts inventories, and tool calibration routines, ensuring the STIHL MS 390 remains reliable and performing at peak efficiency.

Proper storage techniques are essential to extend the life of your STIHL MS 390 chainsaw and ensure safe operation when you next use it. Begin by cleaning the bar, chain, and chassis to remove sawdust, oil, and sap that can corrode components over time. Apply a light coat of storage oil or bar-and-chain oil to the chain and bar to prevent rust, and ensure the chain is loosened slightly to avoid bearing load on the bar during long-term storage. Store the saw in a cool, dry location away from direct sunlight, solvents, and extreme temperature fluctuations, which can degrade seals and gaskets. If possible, keep the saw in its sheath or protective case to shield the bar and chain from moisture and accidental contact. Regularly inspect the air filter, spark plug boot, and fuel cap seal during storage intervals to identify any signs of deterioration that could compromise starting and performance.

Seasonal preparation and rust prevention require a proactive approach to maintain readiness across off-season periods. Drain or stabilize the fuel according to manufacturer recommendations to minimize gum formation in the carburetor and fuel lines; if storage is anticipated for several months, use a reputable fuel stabilizer and run the engine briefly to circulate stabilized fuel before long-term shutoff. Remove the spark plug and apply a light coating of anti-seize compound or engine oil to the cylinder walls before reseating the spark plug to deter rust formation inside the combustion chamber. Clean and lubricate moving parts, including the chain tensioner, drive sprocket, and throttle linkage, using the appropriate lubricants specified by STIHL. Inspect the bar groove for wear and deburr any sharp edges that could cause uneven chain wear; replace the bar if the groove or rails are significantly worn. Maintain proper humidity control where the tool is stored; silica gel packs or a dehumidifier in the storage area can help prevent internal corrosion on metal components.

Transport safety and secure mounting are critical to prevent damage to the chainsaw and to ensure safe handling during moves. Transport the MS 390 in an upright or horizontal position according to the manufacturer’s guidelines, using the original carrying case or a protective container to guard against impact and moisture ingress. Remove loose items such as nuts, bolts, or debris that could damage the carburetor, air intake, or cooling fins during movement, and secure the saw to a stable surface with straps or tie-downs that do not compress critical controls. When mounting for vehicle transport, protect the chain brake mechanism and chain, keeping the chain cover in place and ensuring the chain is properly tensioned or secured to prevent rotation. For air transport or storage in a vehicle, ensure fuel is properly contained in an approved container and the fuel cap is closed tightly to minimize vapor leakage and safety hazards. Regularly verify that mounting hardware—bolts, screws, and clamps—remains tight after transport, as vibration can loosen fasteners and compromise safety and performance.

The STIHL MS 390 service manual appendix provides essential resources to support accurate repair and maintenance procedures. This section consolidates parts diagrams and specification references to help technicians verify component fit and compatibility during reassembly. When referencing diagrams, ensure you match part numbers to the exact serial range and model variant, as small differences can affect performance and safety. Use the specification references to confirm correct replacement parts, tolerances, and assembly sequences, reducing guesswork and downtime in the shop. Having clear diagrams on hand streamlines diagnostics and supports precise torque and alignment checks during reassembly. Verify that all prints are the most current revisions from STIHL, and cross-check any alternate part numbers with the manufacturer’s current catalog to avoid incorrect substitutions.

Torque specs and fastener information are central to restoring optimal performance and ensuring operator safety. This appendix lists precise torque values for all critical fasteners, including cylinder head, bar nut, flywheel, and chain brake components, along with recommended lubricants and thread-locking approaches where appropriate. Follow the specified torque sequence meticulously to prevent warping or uneven clamping force, which can lead to leaks, misalignment, or component failure. When reassembling, use calibrated torque wrenches and column gauges as required, and record the exact values in the service log for future maintenance references. In cases where fasteners are thread-locked or coated, observe the manufacturer’s curing times and any temperature considerations that may affect tightening requirements during installation. Document any deviations from standard torque values with justification, including observed wear patterns or damaged threads, and replace compromised fasteners as necessary.

Warranty and service center guidance within this appendix outlines the eligibility criteria for coverage and the proper procedures for submitting claims. It highlights which repairs are considered warranty events versus user-caused damage, including common issues such as improper maintenance or unauthorized alterations. The guidance emphasizes using approved service centers and maintaining complete repair documentation, including parts lists, serial numbers, and technician notes, to support warranty determinations. Detailed steps for initiating a warranty claim, including required forms and proof of service, help streamline the process and reduce turnaround times. This section also provides contact information for authorized service centers, escalation paths for disputed claims, and reminders about warranty exclusions to prevent future misinterpretation. Keeping thorough records aligned with these guidelines ensures compliance and protects both the customer and the service provider during warranty periods.