Chainsaw Stihl 391 Troubleshooting (5 Pro Fixes for Woodcutters)

Do you wrestle with a temperamental Stihl MS 391 chainsaw, watching precious daylight slip away as you troubleshoot instead of cutting wood? I get it. There’s nothing more frustrating than a chainsaw that refuses to cooperate when you have a stack of logs waiting. Over years of felling trees, bucking firewood, and milling lumber with my trusty Stihl 391, I’ve learned a thing or two about keeping it humming. Let me share some hard-won wisdom, so you can spend less time tinkering and more time making sawdust.

Key Takeaways:

• Fuel Issues are Common: Many Stihl 391 problems originate with stale fuel or incorrect fuel mixtures.
• Spark is Essential: A weak or nonexistent spark will prevent the engine from firing.
• Airflow Matters: A clogged air filter can starve the engine and cause performance issues.
• Chain and Bar Maintenance is Key: A dull chain or improperly lubricated bar can lead to overheating and slow cutting.
• Carburetor Adjustments Might Be Needed: If other troubleshooting steps fail, the carburetor might require fine-tuning.

Stihl MS 391 Troubleshooting: 5 Pro Fixes for Woodcutters

The Stihl MS 391 is a workhorse. It’s powerful enough for felling medium-sized trees, versatile enough for bucking firewood, and reliable enough to be a go-to saw for many professionals and serious homeowners. However, even the best machines can have their off days. When your Stihl 391 starts acting up, knowing where to begin troubleshooting can save you a lot of time and frustration. Here are the five most common issues I’ve encountered, along with the fixes that have consistently worked for me.

1. The Fuel Fiasco: Stale Gas and Mixture Mayhem

I can’t stress this enough: Fuel is the lifeblood of your chainsaw. Stale fuel is public enemy number one for small engines. Gasoline degrades over time, especially when mixed with 2-cycle oil. The lighter hydrocarbons evaporate, leaving behind a gummy residue that can clog fuel lines, the carburetor, and generally wreak havoc.

My Experience: I once left my Stihl 391 sitting idle for a few months with a half-full tank. When I went to start it, nothing. It would crank, but wouldn’t fire. I initially suspected a spark issue, but after checking the plug, I realized the fuel smelled… off. After draining the old fuel and replacing it with a fresh mix, the saw fired up on the second pull. Lesson learned.

Here’s what you need to know:

• The Problem: Stale fuel, incorrect fuel/oil mixture, or contaminated fuel.
• The Culprit: Gasoline starts to degrade after about 30 days, especially when mixed with 2-cycle oil. Ethanol-blended fuels are particularly prone to absorbing moisture, which can lead to corrosion and fuel separation.

• The Fix:

  • Drain the Fuel Tank: Use a fuel siphon or carefully tip the saw to empty the tank into an approved container. Dispose of the old fuel properly.
  • Check the Fuel Filter: The fuel filter is typically located inside the fuel tank, attached to the end of the fuel line. Use a small hook or pliers to pull it out. If it’s clogged or dirty, replace it. Fuel filters are inexpensive and readily available.
  • Clean the Carburetor: If the fuel has been sitting for a long time, the carburetor might be clogged. You can try using carburetor cleaner spray to dissolve deposits. If that doesn’t work, you might need to disassemble and clean the carburetor thoroughly (more on that later).
  • Mix Fresh Fuel: Use a high-quality 2-cycle oil and mix it with fresh gasoline at the correct ratio (typically 50:1 for the Stihl MS 391). Always use the correct measuring container to ensure the right ratio. I prefer using a pre-mixed fuel like Stihl MotoMix as it’s ethanol-free and has a long shelf life.
  • Pro Tip: Always use fresh fuel, especially if your saw has been sitting idle for a while. Consider using fuel stabilizer if you know you won’t be using the saw for an extended period.
  • Data Point: According to a study by the Outdoor Power Equipment Institute (OPEI), over 70% of small engine problems are related to fuel issues.

2. The Spark Saga: No Spark, No Start

If your Stihl 391 is cranking but not starting, the problem might be with the ignition system. The spark plug needs to produce a strong spark to ignite the fuel-air mixture in the cylinder.

My Experience: I once had a Stihl 391 that would start intermittently. Sometimes it would fire right up, other times it would crank endlessly. I initially suspected a fuel issue, but after checking the fuel lines and filter, I turned my attention to the spark plug. Sure enough, the spark plug was fouled with carbon deposits. After cleaning the plug and re-gapping it, the saw started consistently.

Here’s what you need to know:

• The Problem: Weak or nonexistent spark.
• The Culprit: Fouled spark plug, faulty ignition coil, or damaged spark plug wire.

• The Fix:

  • Check the Spark Plug: Remove the spark plug using a spark plug wrench. Inspect the electrode for carbon deposits, cracks, or other damage. Clean the plug with a wire brush or replace it with a new one.
  • Test for Spark: With the spark plug removed, reattach it to the spark plug wire and ground the electrode against the engine block. Pull the starter cord. You should see a bright blue spark jumping across the electrode gap. If there’s no spark, or the spark is weak and yellow, the ignition coil might be faulty.
  • Check the Spark Plug Wire: Inspect the spark plug wire for cracks, cuts, or loose connections. Replace the wire if necessary.
  • Test the Ignition Coil: Testing the ignition coil requires a multimeter. Disconnect the spark plug wire from the coil. Set the multimeter to measure resistance (ohms). Touch one probe to the spark plug wire terminal on the coil and the other probe to the engine ground. Consult the Stihl MS 391 service manual for the correct resistance range. If the resistance is outside the specified range, the ignition coil is likely faulty and needs to be replaced.
  • Adjust the Air Gap: The air gap between the ignition coil and the flywheel is crucial for proper spark generation. Consult the Stihl MS 391 service manual for the correct air gap specification. Use a feeler gauge to adjust the gap.
  • Pro Tip: Keep a spare spark plug on hand. They’re inexpensive and can save you a lot of downtime.
  • Data Point: A study by the Equipment Engine & Service Association (EESA) found that faulty ignition systems are responsible for approximately 20% of small engine failures.

3. The Airflow Anomaly: Blocked Air Filter = Bogged Down Performance

The engine needs a steady supply of clean air to run efficiently. A clogged air filter restricts airflow, causing the engine to run rich (too much fuel, not enough air). This can lead to reduced power, poor fuel economy, and excessive exhaust smoke.

My Experience: I was cutting firewood one day, and my Stihl 391 started losing power. It would bog down under load and produce a lot of black smoke. I initially thought the carburetor needed adjusting, but then I remembered I hadn’t cleaned the air filter in a while. Sure enough, the air filter was completely clogged with sawdust. After cleaning the filter, the saw ran like new.

Here’s what you need to know:

• The Problem: Restricted airflow due to a clogged air filter.
• The Culprit: Sawdust, dirt, and debris accumulating on the air filter.

• The Fix:

  • Remove the Air Filter: The air filter is typically located under a cover on top of the engine. Remove the cover and take out the air filter.
  • Clean the Air Filter: Clean the air filter by tapping it against a hard surface to remove loose debris. You can also use compressed air to blow out the filter from the inside out. If the filter is heavily soiled, wash it with warm soapy water, rinse it thoroughly, and let it dry completely before reinstalling it.
  • Inspect the Air Filter: Check the air filter for tears, holes, or other damage. Replace the filter if necessary.
  • Clean the Air Filter Housing: Wipe out the air filter housing with a clean cloth to remove any accumulated dirt and debris.
  • Pro Tip: Clean the air filter regularly, especially when cutting in dusty conditions. I recommend cleaning it after every 5-10 hours of use.
  • Data Point: According to a study by the American Society of Agricultural and Biological Engineers (ASABE), a clogged air filter can reduce engine power by up to 15%.

4. The Chain and Bar Blues: Dullness and Lack of Lube

A dull chain or an improperly lubricated bar can put a lot of stress on the engine and significantly reduce cutting efficiency. It can also lead to overheating, premature wear, and even kickback.

My Experience: I once tried to cut through a large oak log with a dull chain. The saw struggled, smoked, and took forever to make the cut. I was putting so much pressure on the saw that I was worried about damaging the engine. After sharpening the chain, the saw cut through the log like butter.

Here’s what you need to know:

• The Problem: Dull chain, improperly lubricated bar, or incorrect chain tension.
• The Culprit: Cutting dirty wood, hitting rocks or metal, or neglecting chain maintenance.

• The Fix:

  • Sharpen the Chain: Use a chainsaw file and guide to sharpen the chain. Follow the manufacturer’s instructions for the correct filing angles and depth gauge settings. Alternatively, you can take the chain to a professional for sharpening.
  • Check the Bar Lubrication: Make sure the bar oil reservoir is full and that the oiler is working properly. You should see a steady stream of oil coming from the bar when the saw is running. If not, check the oiler ports for clogs and clean them with a small wire.
  • Clean the Bar: Remove the bar and clean the groove with a screwdriver or specialized bar groove cleaner. Remove any burrs or damage to the bar rails with a flat file.
  • Adjust Chain Tension: The chain should be snug on the bar but still able to be pulled around by hand. Adjust the chain tension according to the manufacturer’s instructions.
  • Inspect the Sprocket: Check the sprocket for wear and damage. Replace the sprocket if necessary.
  • Pro Tip: Sharpen your chain frequently. A sharp chain will cut faster, smoother, and with less effort.
  • Data Point: A study by the U.S. Forest Service found that a sharp chainsaw chain can increase cutting efficiency by up to 50%.

5. The Carburetor Conundrum: Adjustments and Overhauls

The carburetor is responsible for mixing the correct amount of fuel and air for combustion. If the carburetor is not properly adjusted, the engine might run poorly or not at all.

My Experience: I had a Stihl 391 that was running rough and idling erratically. It would stall frequently, especially when it was cold. I tried adjusting the idle speed screw, but it didn’t seem to make much difference. After researching the problem online, I learned that the carburetor might need a more thorough adjustment. I carefully followed the instructions in the Stihl MS 391 service manual to adjust the high and low-speed mixture screws. After a few tweaks, the saw ran perfectly.

Here’s what you need to know:

• The Problem: Improper fuel-air mixture.
• The Culprit: Clogged jets, incorrect adjustments, or worn carburetor components.

• The Fix:

  • Adjust the Idle Speed: The idle speed screw controls the engine’s idle speed. Turn the screw clockwise to increase the idle speed and counterclockwise to decrease it. Adjust the idle speed so that the chain does not move when the engine is idling.
  • Adjust the High and Low-Speed Mixture Screws: The high and low-speed mixture screws control the fuel-air mixture at different engine speeds. Consult the Stihl MS 391 service manual for the correct adjustment procedure. Important: Before adjusting these screws, make sure the air filter is clean and the spark plug is in good condition. Start by turning both screws all the way in (clockwise) and then backing them out to the factory settings. Fine-tune the adjustments as needed to achieve smooth running and good throttle response.
  • Clean the Carburetor: If adjusting the screws doesn’t solve the problem, the carburetor might be clogged. Disassemble the carburetor and clean all the jets and passages with carburetor cleaner spray. Use compressed air to blow out any remaining debris.
  • Rebuild the Carburetor: If the carburetor is severely clogged or worn, you might need to rebuild it with a carburetor rebuild kit. The kit typically includes new gaskets, diaphragms, and needles.
  • Pro Tip: Carburetor adjustments can be tricky. If you’re not comfortable working on carburetors, it’s best to take the saw to a qualified service technician.
  • Data Point: According to a survey of small engine repair shops, carburetor problems account for approximately 15% of all repairs.

A Note on Ethanol: Ethanol-blended gasoline is a major headache for small engines. Ethanol absorbs water, which can lead to corrosion, fuel separation, and clogged fuel lines. If possible, use ethanol-free gasoline in your Stihl 391. If you must use ethanol-blended fuel, use a fuel stabilizer to help prevent problems.

Case Study: Firewood Processor Efficiency

I once worked with a small firewood processing operation that was struggling to meet demand. They were using a Stihl MS 391 as their primary bucking saw. However, the saw was constantly breaking down, which slowed down their production. After analyzing their operation, I identified several areas for improvement:

• Fuel Management: They were using stale fuel and not using fuel stabilizer.
• Chain Maintenance: They were not sharpening the chain frequently enough.
• Air Filter Maintenance: They were not cleaning the air filter regularly.

By implementing a few simple changes, such as using fresh fuel, sharpening the chain daily, and cleaning the air filter weekly, they were able to significantly increase their production efficiency. They were able to process more firewood in less time, which helped them meet their demand and increase their profits.

The Importance of Preventative Maintenance

The best way to avoid chainsaw problems is to perform regular preventative maintenance. This includes:

• Cleaning the air filter regularly.
• Sharpening the chain frequently.
• Checking the bar lubrication.
• Using fresh fuel and fuel stabilizer.
• Inspecting the spark plug.
• Cleaning the carburetor.
• Storing the saw properly when not in use.

By following these simple steps, you can keep your Stihl MS 391 running smoothly for years to come.

A Word on Safety

Chainsaws are powerful tools that can be dangerous if used improperly. Always wear appropriate safety gear, including:

• Eye protection (safety glasses or a face shield).
• Hearing protection (earplugs or earmuffs).
• Gloves.
• Chainsaw chaps.
• Steel-toed boots.

Never operate a chainsaw when you are tired, distracted, or under the influence of drugs or alcohol. Always follow the manufacturer’s instructions for safe operation.

Conclusion: Keep Your Saw Sharp and Your Woodpile Growing

Troubleshooting a Stihl MS 391 chainsaw doesn’t have to be a daunting task. By understanding the common issues and following these pro fixes, you can get your saw back up and running quickly. Remember, fuel quality, spark, airflow, chain maintenance, and carburetor adjustments are the key areas to focus on. And don’t forget the importance of preventative maintenance. A well-maintained chainsaw will not only last longer but also perform better and more safely.

So, grab your tools, get your hands dirty, and get back to cutting wood. With a little knowledge and some elbow grease, you can keep your Stihl MS 391 running like a champ for years to come. Now, what are you waiting for? Go make some sawdust!

Call to Action:

• Need a new spark plug? Check out our recommended spark plugs for the Stihl MS 391.
• Looking for a high-quality chainsaw file? We’ve got you covered.
• Ready to upgrade your safety gear? Browse our selection of chainsaw chaps, gloves, and eye protection.

Learn more