Stihl MS170 Carburetor Adjustment Guide (3 Pro Logging Tips)

As a seasoned logger and wood processing enthusiast, I understand the frustration of a chainsaw that just won’t run right. The Stihl MS170 is a popular choice for homeowners and light-duty tasks, but like any small engine, its carburetor can sometimes need adjustment to ensure optimal performance. Carburetor adjustment might seem intimidating, but with a little knowledge and patience, you can get your MS170 running smoothly again.

In this guide, I’ll walk you through the process of adjusting the carburetor on your Stihl MS170, offering pro logging tips along the way to improve performance, longevity, and overall efficiency. I’ll share insights gleaned from years of working with chainsaws in the field, focusing on practical, actionable advice you can immediately apply.

Understanding the Stihl MS170 Carburetor

Before diving into the adjustment process, it’s crucial to understand the basic components and function of the carburetor. The carburetor’s primary job is to mix air and fuel in the correct ratio to create a combustible mixture for the engine. This ratio is crucial for proper ignition, smooth running, and minimizing emissions.

The MS170, like many small two-stroke engines, typically has a simple carburetor with three main adjustment screws:

H (High-Speed Screw): Controls the fuel mixture at high engine speeds.
L (Low-Speed Screw): Controls the fuel mixture at idle and low engine speeds.
LA (Idle Speed Screw): Adjusts the engine’s idle speed.

Knowing the function of each screw is essential for making precise adjustments and diagnosing common problems.

Tools You’ll Need

Before you begin, gather the necessary tools:

Screwdriver: A small, flat-head screwdriver is essential for adjusting the carburetor screws. A specialized carburetor adjustment tool might be necessary for newer models with tamper-proof screws.
Tachometer (Optional but Recommended): A tachometer helps you accurately measure the engine’s RPM (revolutions per minute), which is crucial for setting the high-speed and idle speeds correctly.
Spark Plug Wrench: For removing and inspecting the spark plug.
Cleaning Supplies: Carburetor cleaner and a clean cloth for cleaning the carburetor and surrounding area.
Work Gloves: To protect your hands.
Safety Glasses: To protect your eyes from debris.

Having the right tools on hand will make the process smoother and safer.

Safety First: Preparing Your Chainsaw

Safety is paramount when working with any power equipment. Before you start adjusting the carburetor, take these precautions:

Turn Off the Engine: Ensure the chainsaw is completely turned off and the spark plug wire is disconnected. This prevents accidental starting.
Wear Safety Gear: Always wear safety glasses and work gloves to protect yourself from potential hazards.
Work in a Well-Ventilated Area: Carburetor cleaner and gasoline fumes can be harmful, so work in an open or well-ventilated space.
Place the Chainsaw on a Stable Surface: Make sure the chainsaw is stable and secure to prevent it from moving during adjustments.
Refer to the Owner’s Manual: Always consult your Stihl MS170 owner’s manual for specific instructions and safety guidelines.

Following these safety precautions can prevent accidents and injuries.

Step-by-Step Carburetor Adjustment Guide

Now, let’s get to the actual adjustment process. I’ll break it down into manageable steps:

Step 1: Locating the Carburetor Adjustment Screws

The carburetor adjustment screws are typically located on the side of the carburetor, near the engine. On the MS170, they are usually accessible without removing any major components. Refer to your owner’s manual for the exact location of the H, L, and LA screws.

Step 2: Initial Settings

Before making any adjustments, it’s helpful to know the factory settings for the H and L screws. These settings are a good starting point. Typically, the factory settings are around 1 to 1.5 turns out from the fully closed position (turned clockwise until gently snug). Your owner’s manual should specify the exact factory settings for your model.

To set the screws to the initial settings:

Gently turn each screw clockwise until it is fully closed. Do not overtighten, as this can damage the screw or the carburetor.
Then, turn each screw counterclockwise the specified number of turns (e.g., 1 turn, 1.25 turns, etc.) as indicated in your owner’s manual.

Step 3: Starting the Chainsaw

After setting the initial settings, start the chainsaw. It may take a few pulls to get it running. If it doesn’t start, double-check that the fuel is fresh and the spark plug is clean and properly gapped.

Step 4: Adjusting the Low-Speed (L) Screw

The low-speed screw affects the engine’s performance at idle and low speeds. Here’s how to adjust it:

Warm Up the Engine: Let the engine run for a few minutes to warm up. This ensures more accurate adjustments.
Listen to the Idle: Observe the engine’s idle speed. If it’s too low, the engine may stall. If it’s too high, the chain may spin.
Adjust the L Screw:
- If the engine stalls or runs roughly at idle, slowly turn the L screw counterclockwise (richening the mixture) until the engine runs smoothly.
- If the engine idles too fast or the chain spins, slowly turn the L screw clockwise (leaning the mixture) until the engine idles smoothly and the chain stops spinning.
Fine-Tuning: Make small adjustments (1/8 of a turn at a time) and allow the engine to respond before making further changes.

Pro Logging Tip #1: The “Lean Best Idle” Technique

One technique I’ve found particularly effective is the “lean best idle” method. Start by turning the L screw clockwise until the engine starts to stumble or slow down. Then, slowly turn it counterclockwise until the engine reaches its smoothest and fastest idle speed. This is often the optimal setting for the L screw.

Step 5: Adjusting the Idle Speed (LA) Screw

After adjusting the L screw, you may need to adjust the idle speed using the LA screw. The goal is to set the idle speed so that the engine runs smoothly without the chain spinning.

Locate the LA Screw: The LA screw is usually located near the throttle linkage.
Adjust the LA Screw:
- If the chain is spinning at idle, turn the LA screw counterclockwise to reduce the idle speed.
- If the engine stalls at idle, turn the LA screw clockwise to increase the idle speed.
Target Idle Speed: Aim for an idle speed that is just high enough to keep the engine running smoothly without the chain spinning. A tachometer can help you achieve the precise RPM specified in your owner’s manual (typically around 2,800-3,200 RPM).

Step 6: Adjusting the High-Speed (H) Screw

The high-speed screw controls the fuel mixture at full throttle. Adjusting the H screw is crucial for preventing engine damage and ensuring optimal power.

Important Note: Adjusting the H screw requires caution. Running the engine too lean (not enough fuel) at high speeds can cause it to overheat and seize, resulting in serious damage.

Full Throttle Test: With the engine warmed up, run it at full throttle.
Listen to the Engine: Listen carefully to the engine’s sound. A properly adjusted engine should sound smooth and powerful at full throttle.
Adjust the H Screw (If Necessary):
- Too Rich (Too Much Fuel): If the engine sounds sluggish, bogs down, or emits excessive smoke, it may be running too rich. Slowly turn the H screw clockwise (leaning the mixture) in small increments (1/8 of a turn at a time) until the engine runs smoothly.
- Too Lean (Not Enough Fuel): If the engine sounds high-pitched, raspy, or hesitates when you accelerate, it may be running too lean. Slowly turn the H screw counterclockwise (richening the mixture) in small increments until the engine runs smoothly.
Tachometer Verification: Use a tachometer to verify that the engine’s maximum RPM is within the manufacturer’s specifications. Exceeding the maximum RPM can damage the engine. The Stihl MS170 typically has a maximum RPM range of around 12,500-13,500 RPM.

Pro Logging Tip #2: The “Four-Stroking” Sound

A useful technique for setting the H screw is to listen for the “four-stroking” sound. At full throttle, a slightly rich mixture will cause the engine to momentarily “four-stroke” (sound like it’s missing a beat). This is a safe indication that you’re close to the optimal setting. Slowly lean the mixture (turn the H screw clockwise) until the four-stroking just disappears.

Step 7: Final Adjustments and Testing

After adjusting all three screws, make some test cuts to ensure the chainsaw is running properly under load. Pay attention to the engine’s performance, throttle response, and cutting ability.

Check Idle Speed: Recheck the idle speed and adjust the LA screw if necessary.
Test Throttle Response: Make sure the engine responds quickly and smoothly when you accelerate.
Observe Cutting Performance: Ensure the chainsaw cuts efficiently without bogging down or stalling.
Fine-Tune (If Needed): If you notice any issues, make small adjustments to the H and L screws as needed.

Step 8: Spark Plug Inspection

After making the carburetor adjustments, it’s a good idea to inspect the spark plug. The spark plug’s color can provide valuable information about the engine’s fuel mixture.

Ideal Color: A light tan or chocolate brown color indicates a properly adjusted fuel mixture.
Black and Sooty: A black, sooty spark plug indicates a rich mixture (too much fuel).
White or Light Gray: A white or light gray spark plug indicates a lean mixture (not enough fuel).

If the spark plug’s color indicates a problem, readjust the carburetor accordingly.

Troubleshooting Common Carburetor Problems

Even with careful adjustments, you may encounter some common carburetor problems. Here are a few troubleshooting tips:

Engine Won’t Start: Check the fuel, spark plug, and air filter. Make sure the carburetor is getting fuel. If the engine is flooded, remove the spark plug and pull the starter cord several times to clear the excess fuel.
Engine Stalls at Idle: Adjust the L screw counterclockwise to richen the mixture or increase the idle speed using the LA screw.
Engine Bogs Down Under Load: Adjust the H screw counterclockwise to richen the mixture. Also, check the air filter and fuel filter for clogs.
Engine Overheats: This is often a sign of a lean mixture. Immediately stop the engine and adjust the H screw counterclockwise to richen the mixture.
Chain Spins at Idle: Reduce the idle speed by turning the LA screw counterclockwise.

Pro Logging Tip #3: Preventative Maintenance is Key

The best way to avoid carburetor problems is to practice preventative maintenance. Always use fresh fuel mixed with the correct ratio of high-quality two-stroke oil. Clean the air filter regularly, and inspect the fuel filter periodically. Consider using a fuel stabilizer if you’re not going to use the chainsaw for an extended period.

Advanced Carburetor Maintenance

For more advanced carburetor maintenance, you may need to disassemble and clean the carburetor. This involves removing the carburetor from the engine, carefully disassembling it, and cleaning all the parts with carburetor cleaner.

Caution: Disassembling the carburetor can be complex, and it’s easy to lose small parts. If you’re not comfortable with this process, it’s best to take your chainsaw to a qualified service technician.

Here are some steps involved in advanced carburetor maintenance:

Remove the Carburetor: Disconnect the fuel lines and throttle linkage, and remove the carburetor from the engine.
Disassemble the Carburetor: Carefully disassemble the carburetor, noting the location of all the parts.
Clean the Parts: Clean all the parts with carburetor cleaner, paying particular attention to the jets and passages. Use compressed air to blow out any remaining debris.
Inspect the Diaphragm and Gaskets: Inspect the diaphragm and gaskets for cracks or damage. Replace them if necessary.
Reassemble the Carburetor: Reassemble the carburetor, making sure all the parts are in their correct locations.
Reinstall the Carburetor: Reinstall the carburetor on the engine, reconnect the fuel lines and throttle linkage, and adjust the carburetor as described above.

Upgrading Your Stihl MS170 for Enhanced Performance

While the MS170 is a reliable chainsaw, there are several upgrades you can consider to enhance its performance and longevity. As someone who relies on chainsaws daily, I’ve found these upgrades to be particularly beneficial:

Upgraded Chain: Switching to a higher-quality chain with improved cutting performance can significantly increase your productivity. Look for chains with features like low-vibration designs and hardened cutters.
Aftermarket Carburetor: While the stock carburetor is adequate, an aftermarket carburetor designed for performance can improve throttle response and power.
High-Performance Air Filter: A high-performance air filter can improve airflow to the engine, resulting in increased power and efficiency.
Improved Spark Plug: Upgrading to an iridium spark plug can provide more consistent ignition and improved engine performance.
Full Wrap Handle: A full wrap handle provides more versatility and control when felling trees or working in awkward positions. This is especially useful in professional logging scenarios.
Larger Sprocket: Switching to a larger sprocket can increase the chain speed and cutting performance. However, this may also require more power from the engine.
Heavy-Duty Bar: Upgrading to a heavier-duty bar can improve the chainsaw’s durability and stability, especially when cutting larger trees.
Chain Tensioner Upgrade: Some aftermarket chain tensioners offer more precise and easier chain adjustments, which is a great convenience during long workdays.
AV (Anti-Vibration) System Enhancement: While the MS170 has a basic AV system, adding aftermarket dampeners can reduce vibration further, minimizing fatigue during extended use.
Exhaust Modification: Modifying the exhaust can improve the engine’s breathing and increase power. However, this may also increase noise levels.

The Importance of Proper Fuel and Oil

Using the correct fuel and oil mixture is crucial for the health and performance of your Stihl MS170. Stihl recommends using a high-quality two-stroke oil mixed with fresh, unleaded gasoline at a ratio of 50:1 (50 parts gasoline to 1 part oil).

Fuel: Use fresh, unleaded gasoline with an octane rating of 89 or higher. Avoid using gasoline that contains ethanol, as ethanol can damage the engine’s components.
Oil: Use a high-quality two-stroke oil specifically designed for air-cooled engines. Stihl recommends using Stihl HP Ultra oil.
Mixing: Mix the fuel and oil thoroughly in a separate container before adding it to the chainsaw’s fuel tank.

Using the wrong fuel or oil mixture can lead to engine damage, reduced performance, and increased emissions.

Final Thoughts

Adjusting the carburetor on your Stihl MS170 may seem daunting at first, but with a little patience and practice, you can master the process. By following the steps outlined in this guide and incorporating the pro logging tips I’ve shared, you can keep your chainsaw running smoothly and efficiently for years to come. Remember to prioritize safety, use the correct tools and materials, and perform regular maintenance to prevent problems.

As a final thought, always remember that the best approach to chainsaw maintenance is proactive. Regularly inspect your saw, keep it clean, and address any issues before they become major problems. By doing so, you’ll not only extend the life of your saw but also ensure a safer and more productive experience in the woods. Now, get out there and make some sawdust!

Stihl MS170 Carburetor Adjustment Guide (3 Pro Logging Tips)