Stihl MS 391 Muffler Removal Guide (Pro Tips For Safe Woodcutting)

Let’s face it, the gleam of a well-maintained chainsaw isn’t just about vanity; it’s a reflection of our respect for the tool and the work it enables. The Stihl MS 391, a workhorse in the woodcutting world, is no exception. A clean, properly functioning muffler contributes significantly to its performance and longevity. But sometimes, you need to get under the hood—or, in this case, behind the exhaust—to address issues like carbon buildup or damage. That’s where this guide comes in.

I’ve spent countless hours in the field, felling trees, bucking logs, and splitting firewood. Over the years, I’ve learned that a little preventative maintenance goes a long way. I remember one particularly cold winter where my old MS 391 started acting up. The engine was sputtering, and power was way down. After checking the basics (fuel, spark plug), I finally took a look at the muffler. It was almost completely clogged with carbon deposits. After a thorough cleaning, the chainsaw ran like new! That experience taught me the importance of regular muffler maintenance, and I want to share that knowledge with you.

This guide will walk you through the process of safely removing and inspecting the muffler on your Stihl MS 391. I’ll share my pro tips, learned through years of experience, to help you maintain your saw and keep it running at its best.

Stihl MS 391 Muffler Removal: A Step-by-Step Guide

Before we dive in, let’s talk safety. Chainsaws are powerful tools, and working on them requires respect and caution.

Safety First:

Always disconnect the spark plug wire: This prevents accidental starting. I can’t stress this enough. Imagine reaching into the engine compartment and the saw unexpectedly fires up!
Wear gloves: Protect your hands from sharp edges and hot surfaces.
Work in a well-ventilated area: Avoid breathing in exhaust fumes or debris.
Let the engine cool completely: Never work on a hot chainsaw.
Eye Protection: Wear safety glasses to avoid any debris getting into your eyes.

Tools You’ll Need

Socket wrench or combination wrench set: The specific size will depend on the bolts holding the muffler in place (typically 10mm or 13mm).
Screwdriver: Usually a Torx screwdriver (T27) is needed for some Stihl models.
Wire brush: For cleaning carbon deposits.
Penetrating oil (optional): To loosen stubborn bolts.
New muffler gasket (recommended): To ensure a proper seal upon reassembly.
Torque wrench (optional but recommended): To tighten bolts to the correct specification.

Step-by-Step Removal

Preparation: Place the chainsaw on a stable surface. Ensure the chain brake is engaged. Locate the muffler. It’s the metal component attached to the side of the engine, usually with a protective heat shield.
Remove the Heat Shield (if applicable): Some MS 391 models have a heat shield covering the muffler. This is usually held in place by screws. Use the appropriate screwdriver to remove these screws and carefully detach the heat shield.
Loosen the Muffler Bolts: Identify the bolts securing the muffler to the engine cylinder. Apply penetrating oil if the bolts are corroded or difficult to turn. Let the oil soak for a few minutes before attempting to loosen them.
Remove the Muffler Bolts: Using the correct size socket wrench or combination wrench, carefully loosen and remove the muffler bolts. Be careful not to strip the bolt heads. Work slowly and deliberately.
Detach the Muffler: Once the bolts are removed, carefully detach the muffler from the engine cylinder. It may be slightly stuck due to carbon buildup. Gently wiggle it back and forth to break it free.
Inspect the Muffler Gasket: The muffler gasket sits between the muffler and the engine cylinder. Inspect it for damage or wear. If it’s cracked, brittle, or torn, replace it with a new one. A faulty gasket can cause exhaust leaks and reduced engine performance.
Clean the Muffler (Optional but Recommended): If the muffler is heavily coated with carbon deposits, clean it using a wire brush. You can also soak it in a solvent specifically designed for cleaning carburetors and mufflers. Always wear gloves and eye protection when working with solvents.

Key Considerations and Pro Tips

Bolt Size: The most common bolt sizes for Stihl MS 391 mufflers are 10mm and 13mm. However, always double-check your specific model to ensure you’re using the correct size wrench.
Torx Screws: Some MS 391 models use Torx screws to secure the heat shield. Make sure you have a Torx screwdriver set on hand. A T27 Torx screwdriver is often required.
Stripped Bolts: If you encounter a stripped bolt head, try using a bolt extractor. These tools are designed to grip damaged bolt heads and allow you to remove them.
Penetrating Oil: Don’t underestimate the power of penetrating oil. Apply it liberally and let it soak for at least 15 minutes before attempting to loosen stubborn bolts.
Muffler Cleaning: While a wire brush can remove surface carbon deposits, a solvent soak is more effective for removing stubborn buildup. Be sure to follow the solvent manufacturer’s instructions carefully.
Gasket Replacement: I always recommend replacing the muffler gasket whenever you remove the muffler. It’s a cheap part, and it ensures a proper seal.
Torque Specifications: When reassembling the muffler, it’s crucial to tighten the bolts to the correct torque specification. This prevents leaks and ensures the muffler is securely attached. Refer to your Stihl MS 391 owner’s manual for the correct torque specifications. If you don’t have a torque wrench, tighten the bolts snugly but not excessively.

Muffler Inspection and Maintenance

Once the muffler is removed, it’s time to inspect it for damage and perform any necessary maintenance.

Inspecting the Muffler

Cracks and Holes: Carefully examine the muffler body for cracks or holes. These can cause exhaust leaks, reduce engine performance, and increase noise levels. If you find any cracks or holes, the muffler needs to be replaced.
Carbon Buildup: Carbon deposits can accumulate inside the muffler, restricting exhaust flow and reducing engine power. Check the inside of the muffler for excessive carbon buildup.
Spark Arrestor Screen: Many Stihl MS 391 models are equipped with a spark arrestor screen. This screen prevents sparks from exiting the muffler and potentially igniting dry vegetation. Inspect the spark arrestor screen for clogs or damage.

Cleaning the Muffler

Wire Brush: Use a wire brush to remove loose carbon deposits from the muffler body and the spark arrestor screen.
Solvent Soak: For stubborn carbon buildup, soak the muffler in a solvent specifically designed for cleaning carburetors and mufflers. Follow the solvent manufacturer’s instructions carefully.
Spark Arrestor Screen Cleaning: If the spark arrestor screen is clogged, you can clean it with a wire brush or by burning off the carbon deposits with a propane torch. Be careful not to damage the screen.

Spark Arrestor Screen: Regulations and Importance

The spark arrestor screen is more than just a piece of metal; it’s a crucial safety component, especially in areas prone to wildfires. Many regions have regulations requiring chainsaws to have a functioning spark arrestor. Failing to comply can result in fines.

US Forest Service Regulations: The US Forest Service requires all chainsaws used on National Forest System lands to be equipped with a properly functioning spark arrestor.
California Regulations: California has similar regulations requiring spark arrestors on chainsaws used in fire hazard areas.
Importance in Fire Prevention: A clogged or damaged spark arrestor screen can allow hot particles to escape from the muffler, potentially igniting dry vegetation.

Case Study: Spark Arrestor Maintenance and Fire Prevention

I once worked on a logging project in Northern California during a particularly dry summer. The fire risk was extremely high, and the US Forest Service was strictly enforcing spark arrestor regulations. One of the crew members neglected to clean his spark arrestor screen regularly. One day, a small fire started near where he was working. Fortunately, it was quickly extinguished, but it served as a stark reminder of the importance of spark arrestor maintenance.

Reassembling the Muffler

Once you’ve inspected and cleaned the muffler, it’s time to reassemble it.

Step-by-Step Reassembly

Install the Muffler Gasket: Place a new muffler gasket between the muffler and the engine cylinder. Ensure the gasket is properly aligned.
Attach the Muffler: Carefully attach the muffler to the engine cylinder. Align the bolt holes.
Install the Muffler Bolts: Install the muffler bolts and tighten them by hand.
Tighten the Muffler Bolts: Using a torque wrench, tighten the muffler bolts to the correct torque specification. Refer to your Stihl MS 391 owner’s manual for the correct torque specifications. If you don’t have a torque wrench, tighten the bolts snugly but not excessively.
Reinstall the Heat Shield (if applicable): Reinstall the heat shield and secure it with the screws.
Reconnect the Spark Plug Wire: Reconnect the spark plug wire.

Torque Specifications: Why They Matter

Torque specifications are crucial for ensuring a proper seal and preventing leaks. Overtightening the bolts can damage the muffler or the engine cylinder. Undertightening the bolts can cause exhaust leaks and reduced engine performance.

Stihl MS 391 Muffler Bolt Torque: The typical torque specification for Stihl MS 391 muffler bolts is between 10 and 12 Newton-meters (Nm). However, always refer to your owner’s manual for the exact specification.
Using a Torque Wrench: A torque wrench allows you to tighten bolts to a specific torque value. This ensures that the bolts are tightened correctly and prevents damage.
Consequences of Incorrect Torque: Overtightening can strip the threads in the engine cylinder, requiring expensive repairs. Undertightening can cause exhaust leaks, which can lead to reduced engine performance and increased noise levels.

Real-World Example: Torque Wrench Precision

I once worked with a fellow logger who consistently overtightened his muffler bolts. He didn’t believe in using a torque wrench. As a result, he frequently stripped the threads in his engine cylinder, requiring costly repairs. After several such incidents, he finally invested in a torque wrench and learned to tighten the bolts to the correct specification. He never had another problem with stripped threads.

Common Problems and Troubleshooting

Even with careful maintenance, you may encounter problems with your Stihl MS 391 muffler. Here are some common issues and how to troubleshoot them.

Exhaust Leaks

Symptoms: Increased noise levels, reduced engine performance, exhaust fumes near the operator.
Causes: Damaged muffler gasket, loose muffler bolts, cracked muffler body.
Troubleshooting:
- Inspect the muffler gasket for damage and replace it if necessary.
- Tighten the muffler bolts to the correct torque specification.
- Inspect the muffler body for cracks and replace the muffler if necessary.

Reduced Engine Power

Symptoms: Difficulty starting, poor acceleration, reduced cutting power.
Causes: Clogged muffler, excessive carbon buildup, damaged spark arrestor screen.
Troubleshooting:
- Clean the muffler using a wire brush or solvent soak.
- Clean or replace the spark arrestor screen.
- Ensure the muffler is properly attached and sealed.

Excessive Noise

Symptoms: Unusually loud exhaust noise.
Causes: Cracked muffler body, missing heat shield, damaged muffler.
Troubleshooting:
- Inspect the muffler body for cracks and replace the muffler if necessary.
- Reinstall the heat shield if it’s missing.
- Ensure the muffler is properly attached and sealed.

Case Study: Diagnosing Performance Issues

I once had a Stihl MS 391 that was experiencing a significant loss of power. I checked the usual suspects – fuel, spark plug, air filter – but everything seemed fine. Finally, I removed the muffler and found that it was almost completely clogged with carbon deposits. After cleaning the muffler, the chainsaw ran like new. This experience taught me that muffler maintenance is crucial for maintaining engine performance.

Maintaining Your Chainsaw for Optimal Performance

Maintaining your chainsaw is not just about aesthetics; it’s about safety, efficiency, and longevity. Regular maintenance can prevent costly repairs and ensure your chainsaw is always ready to tackle the job.

Key Maintenance Tasks

Sharpen the Chain: A sharp chain is essential for efficient and safe cutting. Sharpen the chain regularly using a chainsaw file or a chain grinder.
- Filing Angle: Maintain the correct filing angle (typically 30 degrees) for optimal cutting performance.
- Depth Gauges: Check and adjust the depth gauges as needed.
Clean the Air Filter: A clean air filter ensures proper airflow to the engine. Clean the air filter regularly using compressed air or warm soapy water.
- Filter Type: Use the correct type of air filter for your Stihl MS 391.
- Replacement: Replace the air filter when it becomes excessively dirty or damaged.
Check the Spark Plug: A healthy spark plug is essential for reliable starting and smooth engine performance. Check the spark plug regularly and replace it if necessary.
- Spark Plug Gap: Ensure the spark plug gap is set to the correct specification (typically 0.5mm).
- Electrode Condition: Inspect the electrode for wear or damage.
Lubricate the Chain: Proper chain lubrication is essential for reducing friction and extending the life of the chain and bar. Use a high-quality chain oil and check the oil level regularly.
- Oil Flow: Ensure the chain oiler is functioning properly and delivering adequate oil to the chain.
- Bar Maintenance: Clean the bar groove regularly to remove debris and ensure proper oil flow.
Inspect the Fuel System: Check the fuel lines, fuel filter, and carburetor for leaks or damage. Replace any damaged components.
- Fuel Filter: Replace the fuel filter annually or more frequently if necessary.
- Carburetor Adjustment: Adjust the carburetor as needed to ensure proper fuel mixture.

Data-Backed Insights: Chain Sharpness and Cutting Efficiency

Studies have shown that a sharp chainsaw chain can increase cutting efficiency by as much as 20%. A dull chain requires more force to cut through wood, which can lead to operator fatigue and increased fuel consumption.

Cutting Speed: A sharp chain can cut through a 12-inch log in half the time compared to a dull chain.
Fuel Efficiency: A sharp chain requires less engine power, resulting in improved fuel efficiency.
Operator Fatigue: A sharp chain reduces the amount of force required to cut through wood, reducing operator fatigue and the risk of injury.

Personal Anecdote: The Value of a Sharp Chain

I remember one time when I was working on a large firewood cutting project. I had been using the same chain for several days, and it had become quite dull. I was struggling to cut through the logs, and I was getting exhausted quickly. Finally, I took the time to sharpen the chain. The difference was night and day! The chainsaw cut through the logs effortlessly, and I was able to finish the project much faster and with less effort.

Wood Processing Methods: Matching the Tool to the Task

Choosing the right wood processing method is crucial for efficiency and safety. Whether you’re felling trees, bucking logs, or splitting firewood, using the appropriate technique can make a big difference.

Felling Techniques

Directional Felling: Use directional felling techniques to control the direction of the falling tree. This can help prevent damage to property and ensure the safety of the operator.
- Hinge Wood: Leave an adequate amount of hinge wood to guide the tree’s fall.
- Back Cut: Make the back cut slightly higher than the face cut to prevent kickback.
Tree Assessment: Before felling a tree, assess its size, lean, and any potential hazards.
- Widow Makers: Be aware of any dead or broken branches that could fall during the felling process.
- Power Lines: Avoid felling trees near power lines.

Bucking Techniques

Log Support: Support logs properly before bucking to prevent pinching the chain.
- Bucking Horses: Use bucking horses or other supports to elevate the logs.
- Cutting from Above: Cut from above to prevent the log from pinching the chain.
Bore Cutting: Use bore cutting techniques to relieve tension in the log and prevent pinching.
- Controlled Cuts: Make controlled cuts to avoid kickback.
- Safety Gear: Always wear appropriate safety gear, including a helmet, eye protection, and hearing protection.

Splitting Techniques

Hydraulic Log Splitter: Use a hydraulic log splitter for splitting large or difficult-to-split logs.
- Log Size: Ensure the log splitter is rated for the size of the logs you’re splitting.
- Safety Features: Use a log splitter with safety features such as two-handed operation.
Manual Splitting: Use a splitting axe or maul for splitting smaller logs.
- Stance: Maintain a stable stance and keep your feet clear of the splitting area.
- Swing Technique: Use a controlled swing and aim for the center of the log.

Data-Driven Optimization: Wood Moisture Content and Splitting Force

The ease of splitting wood is significantly affected by its moisture content. Green wood, with a higher moisture content, generally requires more force to split compared to seasoned wood.

Moisture Content Levels:
- Green Wood: Moisture content above 30%.
- Seasoned Wood: Moisture content between 15% and 20%.
Splitting Force:
- Green Wood: Requires up to 50% more splitting force compared to seasoned wood.
- Seasoned Wood: Splits more easily due to reduced fiber strength.

Case Study: Optimizing Firewood Production

I once helped a friend optimize his firewood production process. He was struggling to split green oak logs with a manual splitting axe. I suggested that he let the logs season for several months before splitting them. After seasoning, the logs were much easier to split, and he was able to increase his firewood production significantly.

Firewood Preparation: From Log to Hearth

Preparing firewood is more than just splitting logs; it’s about selecting the right wood, seasoning it properly, and storing it in a way that promotes drying and prevents rot.

Wood Selection Criteria

Hardwoods vs. Softwoods: Hardwoods like oak, maple, and ash provide more heat and burn longer than softwoods like pine, fir, and spruce.
- BTU Content: Hardwoods have a higher BTU (British Thermal Unit) content per cord compared to softwoods.
- Burning Characteristics: Hardwoods burn cleaner and produce less smoke than softwoods.
Wood Density: Denser woods provide more heat and burn longer than less dense woods.
- Weight: Denser woods are heavier per unit volume compared to less dense woods.
- Cell Structure: Denser woods have a tighter cell structure, which slows down the burning process.
Wood Moisture Content: Seasoned wood with a low moisture content burns more efficiently and produces less smoke.
- Splitting and Seasoning: Split logs and stack them in a well-ventilated area to promote drying.
- Moisture Meter: Use a moisture meter to check the moisture content of the wood before burning.

Seasoning Process

Splitting: Split logs to reduce their size and increase their surface area for drying.
- Surface Area: Splitting increases the surface area by up to 50%.
- Drying Time: Splitting can reduce drying time by several months.
Stacking: Stack logs in a single row with gaps between the logs to allow for air circulation.
- Airflow: Ensure adequate airflow around the stack to promote drying.
- Sun Exposure: Position the stack in a sunny location to maximize drying.
Covering: Cover the top of the stack to protect the wood from rain and snow.
- Water Protection: Covering prevents the wood from reabsorbing moisture.
- Ventilation: Ensure the sides of the stack remain open to allow for air circulation.

Storage Methods

Wood Shed: Store firewood in a wood shed to protect it from the elements.
- Ventilation: Ensure the wood shed is well-ventilated to prevent moisture buildup.
- Elevated Floor: Elevate the floor of the wood shed to prevent rot.
Outdoor Stacking: Stack firewood outdoors on pallets or other supports to keep it off the ground.
- Ground Contact: Avoid direct contact with the ground to prevent moisture absorption.
- Covering: Cover the top of the stack to protect the wood from rain and snow.

Data-Driven Analysis: Drying Times and Wood Species

The drying time for firewood varies depending on the wood species, log size, and climate conditions. Hardwoods typically take longer to season than softwoods.

Drying Times:
- Hardwoods: Typically require 6-12 months to season properly.
- Softwoods: Typically require 3-6 months to season properly.
Climate Conditions:
- Dry Climates: Wood seasons faster in dry climates with low humidity.
- Humid Climates: Wood seasons slower in humid climates with high humidity.

Personal Experience: The Importance of Proper Seasoning

I once burned unseasoned firewood in my wood stove. It was difficult to get the fire started, and the wood produced a lot of smoke. The stove also produced creosote, which is a fire hazard. I learned my lesson and now I always make sure to burn properly seasoned firewood.

Conclusion: Respect the Tool, Respect the Wood

Maintaining your Stihl MS 391 muffler is a small but vital part of responsible woodcutting. By following the steps outlined in this guide, you can ensure your chainsaw runs smoothly, efficiently, and safely. Remember, the key to success in wood processing is a combination of knowledge, skill, and respect for the tools and materials you’re working with. So, take care of your chainsaw, choose your wood wisely, and enjoy the warmth and satisfaction of a well-prepared fire. Happy woodcutting!