MS Stihl 391 Chainsaw Troubleshooting (5 Expert Fixes)

For anyone looking for a reliable, workhorse chainsaw that offers excellent value for money, the MS Stihl 391 often tops the list. I’ve spent countless hours in the woods with this saw, felling trees, bucking logs, and preparing firewood, and I can confidently say it’s a machine that can handle a lot. However, even the toughest saws can run into problems. That’s why I’ve put together this troubleshooting guide, drawing on my personal experiences and technical knowledge to help you diagnose and fix common issues with your MS Stihl 391. Think of it as your go-to resource for keeping your saw running smoothly.

MS Stihl 391 Chainsaw Troubleshooting: 5 Expert Fixes

This guide focuses on five key areas where the MS Stihl 391 can experience problems. I’ll walk you through the common symptoms, potential causes, and, most importantly, the steps you can take to resolve them. Whether you’re a seasoned logger or a weekend warrior, these fixes will help you get back to work quickly and safely.

1. Starting Problems: When Your Saw Refuses to Roar

There’s nothing more frustrating than pulling the starter cord repeatedly without the engine firing. Starting problems are a common complaint with any chainsaw, and the MS Stihl 391 is no exception. Let’s dive into the potential culprits.

Common Symptoms

• Engine doesn’t start at all.
• Engine starts but stalls immediately.
• Engine is hard to start, requiring multiple pulls.
• Engine runs roughly after starting.

Potential Causes

• Fuel Issues: Stale fuel, incorrect fuel mixture, clogged fuel filter, or a blocked fuel line.
• Spark Issues: Fouled spark plug, damaged spark plug wire, or faulty ignition module.
• Air Issues: Dirty air filter, blocked air intake, or carburetor problems.
• Compression Issues: Worn piston rings, scored cylinder, or loose cylinder bolts (less common, but possible with older saws).

Expert Fixes

1. Fuel System Check and Refresh: This is always my first step. I’ve seen countless starting problems solved simply by draining the old fuel and replacing it with a fresh mixture of gasoline and two-stroke oil.

   • Action: Drain the fuel tank completely. Use fresh, high-quality gasoline with an octane rating of at least 89. Mix it with Stihl two-stroke oil at a ratio of 50:1 (2.6 fl oz of oil per gallon of gasoline). I personally prefer Stihl Ultra oil, as it’s designed to minimize carbon buildup.
   • Data Point: Using a fuel stabilizer can extend the life of your fuel mixture. I recommend adding a stabilizer if you anticipate storing the saw for more than a month.
   • Fuel Filter Inspection: Locate the fuel filter inside the fuel tank (it’s attached to the fuel line). Remove it and inspect for clogs or debris. If it’s dirty, replace it. Fuel filters are relatively inexpensive and should be replaced annually, or more frequently if you’re working in dusty conditions.
   • Fuel Line Inspection: Check the fuel line for cracks, kinks, or leaks. Replace if necessary. A cracked fuel line can allow air to enter the fuel system, causing starting problems and poor performance.

2. Spark Plug Inspection and Cleaning/Replacement: A fouled or damaged spark plug is a frequent offender.

   • Action: Remove the spark plug using a spark plug wrench. Inspect the electrode for signs of wear, carbon buildup, or damage.
   • Data Point: The spark plug gap should be 0.020 inches (0.5 mm). Use a feeler gauge to check and adjust the gap if necessary.
   • Cleaning: If the spark plug is simply fouled with carbon, you can try cleaning it with a wire brush or spark plug cleaner. However, if it’s damaged or heavily worn, replace it.
   • Replacement: The recommended spark plug for the MS Stihl 391 is a Bosch WSR 6 F or NGK BPMR7A.
   • Personal Story: I once spent an entire afternoon troubleshooting a saw that refused to start, only to discover that the spark plug insulator had a hairline crack. Replacing the spark plug instantly solved the problem.

3. Air Filter Cleaning and Inspection: A dirty air filter restricts airflow to the engine, leading to starting problems and reduced performance.

   • Action: Remove the air filter cover and the air filter element. Inspect the filter for dirt, dust, and debris.
   • Cleaning: Clean the air filter by tapping it gently to remove loose dirt. For more stubborn dirt, wash it with warm, soapy water. Rinse thoroughly and allow it to air dry completely before reinstalling.
   • Data Point: Some air filters are oiled. If yours is, re-oil it lightly with air filter oil after cleaning. This will help trap dirt and prevent it from entering the engine.
   • Replacement: Replace the air filter if it’s damaged or excessively dirty. I recommend replacing it every 3-6 months, depending on how often you use the saw and the conditions you’re working in.

4. Carburetor Adjustment (Proceed with Caution): The carburetor regulates the fuel-air mixture. If it’s out of adjustment, it can cause starting problems, rough running, and poor performance.

   • Warning: Carburetor adjustment can be tricky. If you’re not comfortable doing it yourself, it’s best to take the saw to a qualified service technician.
   • Action: Locate the carburetor adjustment screws (usually marked “L” for low-speed, “H” for high-speed, and “LA” for idle speed).
   • Initial Settings: As a starting point, turn both the “L” and “H” screws clockwise until they are lightly seated, then back them out 1 turn each.
   • Idle Speed Adjustment: Start the engine and let it warm up. Adjust the “LA” screw until the engine idles smoothly without stalling.
   • Fine Tuning: With the engine running at full throttle (use extreme caution!), adjust the “H” screw until the engine runs smoothly and doesn’t bog down. Adjust the “L” screw for smooth acceleration from idle.
   • Data Point: The optimal carburetor settings will vary depending on altitude, temperature, and humidity.
   • Case Study: I once worked on a project in the Rocky Mountains, where the high altitude significantly affected carburetor performance. I had to lean out the fuel mixture (by turning the “H” screw in slightly) to compensate for the thinner air.

5. Compression Test (For Persistent Problems): If you’ve checked the fuel, spark, and air systems, and the saw still won’t start, it’s possible that you have a compression problem.

   • Action: Use a compression tester to measure the engine’s compression. Remove the spark plug and screw the compression tester into the spark plug hole. Pull the starter cord several times and note the reading on the gauge.
   • Data Point: A healthy MS Stihl 391 should have a compression reading of at least 120 PSI.
   • Interpretation: If the compression is significantly lower than 120 PSI, it indicates a problem with the piston rings, cylinder, or valves. This is a more serious issue that may require professional repair.
   • Personal Experience: I had a saw that kept stalling after a few minutes of use. After checking everything else, a compression test revealed significantly low compression. The issue was worn piston rings, which required a complete engine rebuild.

2. Chain Oiling Issues: Keeping the Chain Lubricated

Proper chain lubrication is crucial for the performance and longevity of your chainsaw. A dry chain will quickly overheat, causing premature wear and damage to the chain, bar, and sprocket.

Common Symptoms

• Chain is smoking or overheating.
• Chain is dulling quickly.
• Chain is binding or seizing.
• Oil reservoir is full, but the chain is not being lubricated.

Potential Causes

• Empty oil reservoir.
• Clogged oil filter.
• Blocked oil line.
• Faulty oil pump.
• Incorrect oil type.
• Bar groove is clogged.

Expert Fixes

1. Oil Reservoir Check and Refill: This seems obvious, but it’s the most common cause of chain oiling problems.

   • Action: Check the oil reservoir level regularly and refill as needed.
   • Data Point: The oil reservoir capacity of the MS Stihl 391 is approximately 10.1 oz (300 ml).
   • Oil Type: Use a high-quality bar and chain oil specifically designed for chainsaws. I recommend Stihl bar and chain oil, as it’s formulated to provide excellent lubrication and reduce wear. Avoid using motor oil or other substitutes, as they may not provide adequate lubrication.
   • Tip: I always fill the fuel and oil tanks at the same time to ensure that I don’t run out of oil before fuel.

2. Oil Filter Cleaning and Inspection: A clogged oil filter can restrict oil flow to the chain.

   • Action: Locate the oil filter inside the oil reservoir (it’s usually attached to the oil line). Remove it and inspect for clogs or debris.
   • Cleaning: Clean the oil filter by washing it with solvent or carburetor cleaner.
   • Replacement: Replace the oil filter if it’s damaged or excessively dirty. I recommend replacing it annually.

3. Oil Line Inspection: Check the oil line for kinks, cracks, or leaks.

   • Action: Inspect the oil line from the oil reservoir to the oil pump. Replace if necessary.

4. Bar Groove Cleaning: The bar groove can become clogged with sawdust and debris, preventing oil from reaching the chain.

   • Action: Remove the chain and bar. Use a bar groove cleaner or a small screwdriver to clean the bar groove.
   • Data Point: The bar groove width should match the chain gauge (usually 0.063 inches for the MS Stihl 391).
   • Tip: I use compressed air to blow out the bar groove after cleaning it.

5. Oil Pump Inspection and Adjustment (Advanced): If the oil filter, oil line, and bar groove are clear, the problem may be with the oil pump.

   • Warning: Oil pump repair requires specialized tools and knowledge. If you’re not comfortable doing it yourself, it’s best to take the saw to a qualified service technician.
   • Action: The MS Stihl 391 has an adjustable oil pump. Locate the oil pump adjustment screw (usually located on the bottom of the saw). Turn the screw clockwise to increase oil flow and counterclockwise to decrease oil flow.
   • Data Point: The optimal oil flow rate will vary depending on the type of wood you’re cutting and the size of the bar and chain. As a general rule, you should be able to see a light mist of oil coming off the chain while it’s running.
   • Case Study: I once had a saw that wasn’t oiling properly, even after cleaning the filter and lines. After disassembling the oil pump, I discovered that the pump piston was worn. Replacing the piston restored proper oil flow.

3. Chain Tension Problems: Finding the Sweet Spot

Proper chain tension is essential for safe and efficient cutting. A chain that is too loose can derail and cause injury, while a chain that is too tight can overheat and damage the bar and sprocket.

Common Symptoms

• Chain is sagging or drooping.
• Chain is difficult to pull around the bar.
• Chain is overheating.
• Chain is derailing frequently.

Potential Causes

• Incorrect chain tension.
• Worn chain.
• Worn bar.
• Worn sprocket.
• Stretched chain.

Expert Fixes

1. Chain Tension Adjustment: This is the most common solution to chain tension problems.

   • Action: Loosen the bar nuts slightly. Use the chain tensioning screw to adjust the chain tension. The chain should be snug against the bar but still able to be pulled around by hand.
   • Data Point: When properly tensioned, you should be able to pull the chain away from the bar about 1/8 inch (3 mm) at the midpoint of the bar.
   • Tip: Adjust the chain tension after every few cuts, as the chain will stretch as it heats up.

2. Chain Inspection and Replacement: A worn chain will stretch and become difficult to tension properly.

   • Action: Inspect the chain for worn or damaged cutters, tie straps, and drive links. Replace the chain if it’s worn or damaged.
   • Data Point: The chain should be replaced when the cutters are worn down to the wear marks or when the tie straps are loose or damaged.
   • Chain Selection: Use a chain that is specifically designed for the MS Stihl 391. The recommended chain pitch is 3/8 inch, and the gauge is 0.063 inch.

3. Bar Inspection and Replacement: A worn bar can also cause chain tension problems.

   • Action: Inspect the bar for wear, damage, and burrs. Pay particular attention to the bar rails (the edges of the bar that the chain rides on). If the bar rails are worn or damaged, replace the bar.
   • Data Point: The bar should be replaced when the bar rails are worn down or when the bar is bent or damaged.
   • Bar Maintenance: File the bar rails regularly to remove burrs and ensure that the chain is running smoothly.

4. Sprocket Inspection and Replacement: A worn sprocket can cause the chain to skip and derail.

   • Action: Inspect the sprocket for wear and damage. Replace the sprocket if it’s worn or damaged.
   • Data Point: The sprocket should be replaced when the teeth are worn down or when the sprocket is damaged.

5. Chain Stretching: All chains stretch over time. If the chain is excessively stretched, it may not be possible to tension it properly.

   • Action: If the chain is excessively stretched, it should be replaced.
   • Tip: Use a chain breaker to remove a few links from the chain if it’s only slightly stretched. However, be careful not to remove too many links, as this will make the chain too short.

4. Cutting Performance Issues: When Your Saw Loses Its Bite

A chainsaw that’s not cutting properly can be frustrating and dangerous. Here’s how to diagnose and fix common cutting performance problems.

Common Symptoms

• Saw is cutting slowly.
• Saw is cutting unevenly.
• Saw is pulling to one side.
• Saw is smoking excessively.
• Saw is vibrating excessively.

Potential Causes

• Dull chain.
• Incorrect chain sharpening.
• Incorrect depth gauge setting.
• Pinched bar.
• Bent bar.
• Incorrect cutting technique.
• Incorrect wood type.

Expert Fixes

1. Chain Sharpening: A sharp chain is essential for efficient cutting.

   • Action: Sharpen the chain regularly using a chainsaw file or a chain grinder.
   • Data Point: The correct filing angle for the MS Stihl 391 chain is typically 30 degrees. Consult your chain manufacturer’s specifications for the exact angle.
   • Tip: Use a chain filing guide to ensure that you’re sharpening the chain at the correct angle and depth.
   • Personal Story: I once spent an entire day struggling to fell a large oak tree with a dull chain. After finally giving up and sharpening the chain, I was able to fell the tree in a matter of minutes.

2. Depth Gauge Adjustment: The depth gauges (also known as rakers) control the amount of wood that each cutter takes. If the depth gauges are too high, the chain will not cut effectively. If they are too low, the chain will grab and kick back.

   • Action: Use a depth gauge tool to adjust the depth gauges. The depth gauges should be slightly lower than the cutters.
   • Data Point: The recommended depth gauge setting for the MS Stihl 391 chain is typically 0.025 inches (0.6 mm). Consult your chain manufacturer’s specifications for the exact setting.
   • Tip: File the depth gauges evenly to ensure that the chain is cutting smoothly.

3. Bar Inspection and Repair: A pinched or bent bar can cause the chain to bind and cut unevenly.

   • Action: Inspect the bar for pinches, bends, and burrs. Remove any pinches with a bar rail closer. File down any burrs. Replace the bar if it’s bent.

4. Cutting Technique: Incorrect cutting technique can also lead to poor cutting performance.

   • Action: Use proper cutting technique. Let the saw do the work. Avoid forcing the saw into the wood. Use a sharp chain and keep the bar lubricated.
   • Tip: Use a felling wedge to prevent the bar from pinching when felling trees.

5. Wood Type: Some types of wood are harder to cut than others.

   • Action: Use a chain that is designed for the type of wood you’re cutting. For example, a chain with carbide-tipped cutters is ideal for cutting hardwoods.
   • Data Point: Hardwoods, such as oak and maple, have a higher density and require more power to cut than softwoods, such as pine and fir.
   • Wood Moisture Content: Green wood (freshly cut) is often easier to cut than dry wood. However, green wood is also heavier and more prone to splitting.

5. Overheating Problems: Keeping Your Cool

Overheating can damage your chainsaw engine and shorten its lifespan. Here’s how to prevent and address overheating problems.

Common Symptoms

• Engine is smoking excessively.
• Engine is losing power.
• Engine is stalling.
• Engine is difficult to start after running.
• Engine is emitting a burning smell.

Potential Causes

• Dirty air filter.
• Clogged cooling fins.
• Incorrect fuel mixture.
• Overloading the engine.
• Running the saw at full throttle for extended periods.
• Worn engine components.

Expert Fixes

1. Air Filter Cleaning: A dirty air filter restricts airflow to the engine, causing it to overheat.

   • Action: Clean the air filter regularly, as described in section 1.

2. Cooling Fin Cleaning: The cooling fins on the engine cylinder help dissipate heat. If the cooling fins are clogged with dirt and debris, the engine will overheat.

   • Action: Use a brush or compressed air to clean the cooling fins regularly.
   • Tip: Remove the engine cover to access the cooling fins more easily.

3. Fuel Mixture Correction: An incorrect fuel mixture can cause the engine to overheat.

   • Action: Ensure that you’re using the correct fuel mixture (50:1).
   • Tip: Use pre-mixed fuel to ensure that the fuel mixture is accurate.

4. Engine Overload Avoidance: Overloading the engine can cause it to overheat.

   • Action: Avoid forcing the saw into the wood. Let the saw do the work. Use a sharp chain and keep the bar lubricated.

5. Throttle Management: Running the saw at full throttle for extended periods can cause it to overheat.

   • Action: Vary the throttle speed. Avoid running the saw at full throttle for more than a few minutes at a time.
   • Tip: Let the saw cool down periodically, especially when working in hot weather.

6. Engine Component Inspection (Advanced): If the engine continues to overheat after checking the air filter, cooling fins, and fuel mixture, it’s possible that there is a problem with the engine components.

   • Action: Inspect the piston, cylinder, and bearings for wear and damage. If any of these components are worn or damaged, they should be replaced.
   • Warning: Engine repair requires specialized tools and knowledge. If you’re not comfortable doing it yourself, it’s best to take the saw to a qualified service technician.

By following these troubleshooting steps, I’m confident that you can keep your MS Stihl 391 chainsaw running smoothly and efficiently for years to come. Remember to always prioritize safety and consult a qualified technician if you’re unsure about any repair. Happy cutting!

Learn more