Stihl MS 391 Problems (5 Expert Fixes You Must Know)

Stihl MS 391 Problems (5 Expert Fixes You Must Know)

Let’s face it, the Stihl MS 391 is a workhorse. It’s a chainsaw that many, including myself, rely on for everything from felling small trees to bucking firewood. Its robust design and powerful engine are usually a winning combination. But even the best tools can have their hiccups. Over the years, I’ve seen my fair share of MS 391s (and worked on quite a few myself) exhibiting common problems. So, I’ve put together this guide to share five expert fixes that I’ve found consistently effective. I’ll walk you through troubleshooting and repairs, and even share some personal anecdotes and data-backed insights I’ve gleaned from years of wood processing experience.

Understanding the Stihl MS 391: A Foundation for Troubleshooting

Before diving into specific fixes, it’s crucial to understand the MS 391. This chainsaw is a mid-range model, typically equipped with a 20-inch bar (though bar sizes can vary from 16 to 25 inches depending on the user’s needs). It boasts a 64.1 cc engine, delivering a power output of around 4.4 bhp (3.2 kW). This power makes it suitable for handling logs up to 24 inches in diameter.

• Engine Displacement: 64.1 cc
• Power Output: 4.4 bhp (3.2 kW)
• Weight (without bar and chain): 6.4 kg (14.1 lbs)
• Recommended Bar Length: 16-25 inches

The MS 391 is designed for semi-professional use, meaning it’s built to withstand demanding tasks but isn’t quite as heavy-duty as a professional-grade saw like the MS 462. This translates to a good balance of power, weight, and durability – but also some limitations we need to be aware of.

Problem 1: Chainsaw Starts Hard or Won’t Start

This is probably the most common complaint I hear, and frankly, the most frustrating. You pull and pull, the engine sputters, but refuses to roar to life. Here’s my troubleshooting process:

Fix 1: Fuel System Check

• The Culprit: Often, the culprit is stale fuel, a clogged fuel filter, or a malfunctioning carburetor.
• My Experience: I once spent an entire morning wrestling with an MS 391 that wouldn’t start, only to discover the fuel filter was completely gunked up with debris. Since then, I’ve made it a habit to visually inspect the fuel filter every few months.
• The Fix:
  1. Drain Old Fuel: First, drain the old fuel. Ethanol-blended fuels can degrade quickly, especially if left sitting in the tank for extended periods. I recommend using fuel stabilizers if you know the saw will be idle for more than a month.
  2. Inspect the Fuel Filter: Locate the fuel filter inside the fuel tank. It’s usually attached to the fuel line. Remove it and inspect for clogs or debris. If it’s dirty, replace it. A new fuel filter is inexpensive and well worth the investment.
     • Pro Tip: Use a hooked wire or a specialized fuel filter removal tool to avoid damaging the fuel line.
  3. Check Fuel Lines: Inspect the fuel lines for cracks or leaks. Cracked fuel lines can introduce air into the fuel system, making starting difficult. Replace any damaged lines.
  4. Carburetor Cleaning: If the problem persists, the carburetor may be clogged. While a full carburetor rebuild is sometimes necessary, you can often clean it by spraying carburetor cleaner into the carburetor throat while the engine is off. Let it sit for a few minutes, then try starting the saw.
     • Data Point: A study by the Outdoor Power Equipment Institute (OPEI) found that over 70% of small engine problems are related to fuel issues.
  5. Carburetor Adjustment: Sometimes, the carb just needs adjusting. Locate the “L” (low-speed) and “H” (high-speed) adjustment screws on the carburetor. Using a small screwdriver, gently adjust the “L” screw. Turn it clockwise to lean the mixture (less fuel) and counter-clockwise to richen the mixture (more fuel). A slight adjustment can often make a big difference. Consult your owner’s manual for the recommended starting point for these screws.

Fix 2: Ignition System Check

• The Culprit: A faulty spark plug or a malfunctioning ignition coil can prevent the engine from firing.
• My Experience: I was once cutting firewood with a crew, and one of our MS 391s suddenly died mid-cut. After some troubleshooting, we found the spark plug was completely fouled. A quick replacement got us back in business.
• The Fix:
  1. Inspect the Spark Plug: Remove the spark plug and inspect it. Look for cracks, excessive carbon buildup, or a wet, oily appearance. A healthy spark plug should be clean and dry with a light tan color.
     • Spark Plug Gap: Check the spark plug gap using a feeler gauge. The correct gap for the MS 391 is typically around 0.5 mm (0.020 inches). Adjust the gap if necessary.
  2. Test for Spark: Reconnect the spark plug to the spark plug wire and ground the spark plug against the engine cylinder. Pull the starter cord and observe the spark plug gap. You should see a strong, blue spark. If there’s no spark or a weak, yellow spark, the ignition coil may be faulty.
  3. Replace the Ignition Coil: Replacing the ignition coil is a relatively straightforward process. Disconnect the old coil, remove it from the engine, and install the new coil. Ensure the air gap between the coil and the flywheel is correct (usually around 0.3 mm or 0.012 inches). Use a business card as a spacer to set the gap accurately.

Problem 2: Chainsaw Idles Poorly or Stalls

A chainsaw that idles erratically or stalls frequently is not only annoying but also potentially dangerous. It can make precise cuts difficult and increase the risk of accidents.

Fix 3: Carburetor Adjustment and Cleaning (Again!)

• The Culprit: An improperly adjusted carburetor or a partially clogged carburetor jet is often the cause of idling problems.
• My Experience: I’ve found that even a seemingly clean carburetor can have tiny particles obstructing the idle jet. A thorough cleaning and precise adjustment are often necessary.
• The Fix:
  1. Idle Speed Adjustment: Locate the idle speed adjustment screw on the carburetor. Turn it clockwise to increase the idle speed and counter-clockwise to decrease it. Adjust the idle speed until the chain stops moving when the throttle is released, but the engine doesn’t stall.
     • Data Point: Stihl recommends an idle speed of around 2800 RPM for the MS 391. You can use a tachometer to accurately measure the idle speed.
  2. Carburetor Cleaning (Deep Dive): If adjusting the idle speed doesn’t solve the problem, a more thorough carburetor cleaning is necessary. Remove the carburetor from the engine and disassemble it. Use carburetor cleaner and compressed air to clean all the jets and passages. Pay particular attention to the idle jet, which is often the smallest and most easily clogged.
     • Safety Note: Wear eye protection and gloves when working with carburetor cleaner.
  3. Check for Air Leaks: Air leaks can also cause idling problems. Inspect the intake manifold and carburetor mounting flange for cracks or loose connections. Use carburetor cleaner to spray around these areas while the engine is running. If the engine speed changes when you spray a particular area, you’ve likely found an air leak. Repair or replace the affected components.

Fix 4: Inspect the Clutch

• The Culprit: A worn or damaged clutch can cause the chain to spin at idle, even when the throttle is released.
• My Experience: I once had an MS 391 that would creep forward when idling, even with the chain brake engaged. The problem turned out to be a worn clutch spring.
• The Fix:
  1. Remove the Clutch Cover: Remove the clutch cover and inspect the clutch assembly. Look for worn clutch shoes, broken clutch springs, or a glazed clutch drum.
  2. Replace Worn Components: If any of the clutch components are worn or damaged, replace them. Replacing the entire clutch assembly is often the easiest and most reliable solution.
  3. Check the Clutch Drum Bearing: Ensure the clutch drum bearing is properly lubricated and spins freely. A seized or worn bearing can cause the clutch to drag, leading to idling problems.

Problem 3: Chainsaw Lacks Power or Cuts Slowly

A chainsaw that lacks power or cuts slowly is a sign of underlying issues that need to be addressed. It can put extra strain on the engine and increase the risk of kickback.

Fix 5: Chain Sharpening and Bar Maintenance

• The Culprit: A dull chain, a damaged bar, or improper chain tension are the most common causes of power loss.
• My Experience: I’ve seen countless chainsaws underperforming simply because the chain wasn’t properly sharpened. A sharp chain makes all the difference in cutting efficiency and safety.
• The Fix:
  1. Sharpen the Chain: Sharpen the chain using a chainsaw file and a sharpening guide. Ensure the cutters are sharpened to the correct angle and depth. A properly sharpened chain will pull itself into the wood with minimal effort.
     • Data Point: A study by Oregon Products found that a properly sharpened chain can increase cutting speed by up to 30%.
  2. Check Chain Tension: Ensure the chain is properly tensioned. A chain that is too loose can vibrate and cut inefficiently, while a chain that is too tight can overheat and damage the bar. The chain should be snug against the bar but still able to be pulled around by hand.
     • Pro Tip: Check chain tension frequently, especially when using a new chain.
  3. Inspect the Bar: Inspect the bar for wear and damage. Look for burrs, uneven wear, or a bent bar. Use a bar dressing tool to remove burrs and smooth the bar rails. If the bar is severely damaged, replace it.
  4. Lubricate the Chain and Bar: Ensure the chain and bar are properly lubricated. A lack of lubrication can cause excessive friction, leading to overheating and premature wear. Check the oil level in the oil tank and ensure the oiler is functioning properly.
     • Data Point: Stihl recommends using their BioPlus chain oil for optimal performance and environmental protection.
  5. Wood Type Considerations: Remember that harder woods like oak, maple, and hickory will require more power and a sharper chain than softer woods like pine or fir. Adjust your cutting technique and chain maintenance accordingly. When working with hardwoods, I often find myself sharpening the chain more frequently, sometimes even after just a few cuts.
     • Hardwood Density Data:
       • Oak (Red): 752 kg/m³ (47 lb/ft³)
       • Maple (Sugar): 705 kg/m³ (44 lb/ft³)
       • Hickory: 833 kg/m³ (52 lb/ft³)
       • Pine (Eastern White): 385 kg/m³ (24 lb/ft³)
       • Fir (Douglas): 530 kg/m³ (33 lb/ft³)

Beyond the Fixes: Preventative Maintenance

While these fixes address common problems, the best approach is preventative maintenance. Here are a few tips to keep your MS 391 running smoothly:

• Regular Cleaning: Clean the chainsaw after each use. Remove sawdust and debris from the engine, bar, and chain.
• Air Filter Maintenance: Clean or replace the air filter regularly. A clogged air filter can restrict airflow and reduce engine performance.
• Fuel Storage: Store fuel in a clean, airtight container. Use fuel stabilizer if you know the fuel will be stored for an extended period.
• Chain and Bar Lubrication: Use high-quality chain oil and ensure the oiler is functioning properly.
• Professional Service: Have your chainsaw professionally serviced at least once a year. A qualified technician can identify and address potential problems before they become major issues.

Safety First: A Personal Note

I’ve been working with chainsaws for over 20 years, and I’ve learned that safety is paramount. Always wear appropriate safety gear, including a helmet, eye protection, hearing protection, gloves, and chainsaw chaps. Never operate a chainsaw when you are tired or under the influence of drugs or alcohol. And always be aware of your surroundings and potential hazards.

Remember, a well-maintained chainsaw is a safe chainsaw. By following these expert fixes and practicing preventative maintenance, you can keep your Stihl MS 391 running smoothly and safely for years to come. Happy cutting!

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