Stihl MS 170 Carburetor Adjustment Tips (5 Pro Tuning Hacks)

Okay, let’s dive into the world of chainsaw carburetor adjustment, specifically focusing on the Stihl MS 170. I’ll start by relating it to our furry friends, then get into the nitty-gritty details.

Have you ever noticed how a dog’s bark can tell you a lot about its mood? A happy bark is rhythmic and full of energy, while a strained bark can indicate discomfort or distress. Similarly, the sound of your Stihl MS 170 chainsaw is a direct reflection of its internal health, particularly the carburetor’s tune. A well-tuned carburetor produces a smooth, powerful hum, while a poorly adjusted one results in sputtering, stalling, and a generally unhappy saw.

Stihl MS 170 Carburetor Adjustment Tips (5 Pro Tuning Hacks)

The Stihl MS 170 is a workhorse for homeowners and small-scale firewood processors. It’s lightweight, reliable, and relatively easy to maintain. However, like any engine, its performance hinges on a properly tuned carburetor. A carburetor is responsible for mixing air and fuel in the correct ratio to ensure efficient combustion. When the carburetor is out of adjustment, the engine won’t run smoothly, leading to reduced power, increased fuel consumption, and potential engine damage.

In this guide, I’ll share five pro tuning hacks to help you optimize your Stihl MS 170’s carburetor, ensuring it runs like a champion. I’ll break down the process into simple, actionable steps, providing insights gleaned from years of experience in wood processing and firewood preparation.

Why Carburetor Adjustment Matters

Before we jump into the tuning hacks, let’s understand why carburetor adjustment is so crucial.

• Performance: A properly tuned carburetor ensures optimal engine performance. You’ll experience better throttle response, increased cutting power, and smoother operation.
• Fuel Efficiency: An incorrectly adjusted carburetor can lead to excessive fuel consumption. Tuning it correctly ensures the engine burns fuel efficiently, saving you money.
• Engine Longevity: Running an engine with an improperly adjusted carburetor can cause engine damage over time. A lean mixture (too much air, not enough fuel) can lead to overheating, while a rich mixture (too much fuel, not enough air) can foul the spark plug and cause carbon buildup.
• Emissions: A properly tuned carburetor reduces harmful emissions, contributing to a cleaner environment.
• Starting Ease: A well-tuned carburetor makes starting the chainsaw much easier, especially in cold weather.

Key Terms and Concepts

To understand carburetor adjustment, it’s essential to grasp a few key terms:

• Carburetor: The device that mixes air and fuel in the correct ratio for combustion.
• Idle Speed: The engine speed when the throttle is released.
• Idle Adjustment Screw (LA): The screw that controls the idle speed. This is sometimes labelled “T” for Tachometer.
• High-Speed Needle (H): Controls the fuel mixture at high engine speeds.
• Low-Speed Needle (L): Controls the fuel mixture at low engine speeds and idle.
• Lean Mixture: A mixture with too much air and not enough fuel.
• Rich Mixture: A mixture with too much fuel and not enough air.
• RPM (Revolutions Per Minute): A measure of engine speed.
• Green Wood: Freshly cut wood with a high moisture content (often above 30%).
• Seasoned Wood: Wood that has been dried to reduce its moisture content (typically below 20%).
• Air Filter: Filters air entering the engine, preventing dirt and debris from causing damage.
• Spark Plug: Ignites the air-fuel mixture in the cylinder.
• Choke: Restricts airflow to enrich the fuel mixture for starting a cold engine.

Safety First

Before you start any carburetor adjustment, prioritize safety:

• Wear Safety Glasses: Protect your eyes from debris.
• Wear Gloves: Protect your hands from cuts and abrasions.
• Work in a Well-Ventilated Area: Chainsaw exhaust contains carbon monoxide, which is dangerous.
• Use a Spark Plug Wrench: Avoid damaging the spark plug.
• Use a Screwdriver with Correct Size: Don’t strip the carburetor adjustment screws.
• Keep the Chain Brake Engaged: Prevent accidental chain movement.
• Never Adjust the Carburetor While the Chain is Moving: This is extremely dangerous.
• Refer to the Stihl MS 170 Owner’s Manual: This manual contains specific instructions and safety information for your chainsaw.

Tools You’ll Need

• Screwdriver: A small, flat-head screwdriver is essential for adjusting the carburetor screws. Specifically, you will need a splined screwdriver bit that is designed to fit the carburetor adjustment screws on the Stihl MS 170. These are often called “D” screwdrivers or “pacman” screwdrivers due to the shape of the tip.
• Tachometer (Optional): A tachometer can help you accurately measure the engine RPM, making it easier to set the idle speed and high-speed mixture. I highly recommend one, especially if you’re aiming for optimal performance. A Tiny Tach is a good option.
• Spark Plug Wrench: For removing and inspecting the spark plug.
• Cleaning Supplies: Carburetor cleaner, compressed air, and a clean cloth for cleaning the carburetor and surrounding area.
• Feeler Gauges: To check and adjust the spark plug gap.
• New Spark Plug (Optional): If the spark plug is fouled or worn, replace it.
• Fuel Stabilizer: To prevent fuel from going stale in the carburetor.
• Work Gloves: To protect your hands.
• Safety Glasses: To protect your eyes.

5 Pro Tuning Hacks for Your Stihl MS 170 Carburetor

Now, let’s get to the heart of the matter: the five pro tuning hacks that will transform your Stihl MS 170’s performance.

Hack #1: The Initial Setup – Foundation First

Before you even think about adjusting the carburetor, you need to ensure the engine is in good working order. This means checking and cleaning the air filter, inspecting the spark plug, and using fresh fuel. Think of it as building a house – you need a solid foundation before you can start adding the walls and roof.

Step 1: Air Filter Inspection and Cleaning

A dirty air filter restricts airflow, causing the engine to run rich.

1. Remove the air filter cover.
2. Remove the air filter.
3. Inspect the air filter for dirt and debris.
4. If the filter is dirty, clean it with warm, soapy water. Alternatively, you can use compressed air to blow out the dirt.
5. Allow the filter to dry completely before reinstalling it.
6. Reinstall the air filter and cover.

Step 2: Spark Plug Inspection and Cleaning

A fouled or worn spark plug can cause starting problems and poor engine performance.

1. Disconnect the spark plug wire.
2. Use a spark plug wrench to remove the spark plug.
3. Inspect the spark plug for fouling, carbon buildup, or damage.
4. Clean the spark plug with a wire brush or spark plug cleaner.
5. Check the spark plug gap with a feeler gauge. The correct gap for the Stihl MS 170 is typically 0.5 mm (0.020 inches). Adjust the gap if necessary.
6. If the spark plug is badly fouled or worn, replace it with a new one. I’ve found that using a high-quality spark plug, like an NGK, can make a noticeable difference in performance and longevity.
7. Reinstall the spark plug and connect the spark plug wire.

Step 3: Fresh Fuel and Fuel Stabilizer

Old or stale fuel can clog the carburetor and cause starting problems.

1. Drain any old fuel from the fuel tank.
2. Mix fresh fuel with the correct ratio of two-stroke oil. The Stihl MS 170 typically requires a 50:1 fuel-to-oil ratio (50 parts fuel to 1 part oil). Always use a high-quality two-stroke oil.
3. Add a fuel stabilizer to the fuel mixture. Fuel stabilizer prevents the fuel from going stale and helps keep the carburetor clean. I personally use Stabil or Seafoam in all my small engine equipment.

Step 4: Visual Inspection of Fuel Lines and Filter Check the fuel lines for cracks, leaks, or damage. Replace if necessary. Inspect the fuel filter inside the fuel tank and clean or replace if clogged.

Why This Matters: This initial setup is crucial because it eliminates potential problems that could be mistaken for carburetor issues. If the air filter is clogged or the spark plug is fouled, adjusting the carburetor won’t solve the underlying problem.

Hack #2: The Idle Speed Adjustment – Smooth Operator

The idle speed is the engine speed when the throttle is released. Setting the correct idle speed is essential for smooth starting and preventing the chain from spinning at idle.

Step 1: Locate the Idle Adjustment Screw (LA)

The idle adjustment screw (LA) is typically located on the carburetor body. Refer to your Stihl MS 170 owner’s manual for the exact location. It is often labelled “T”.

Step 2: Start the Engine

Start the engine and let it warm up for a few minutes. This is important because the engine’s behavior changes as it warms up.

Step 3: Adjust the Idle Speed

1. If the chain is spinning at idle, turn the idle adjustment screw counterclockwise (to the left) to reduce the idle speed.
2. If the engine stalls at idle, turn the idle adjustment screw clockwise (to the right) to increase the idle speed.
3. Adjust the idle speed until the engine runs smoothly at idle and the chain does not spin. The ideal idle speed for the Stihl MS 170 is typically around 2,800 RPM. Use a tachometer to accurately measure the idle speed.

Step 4: Fine-Tuning

Once you’ve set the idle speed, fine-tune it by making small adjustments and listening to the engine. The goal is to find the lowest possible idle speed that allows the engine to run smoothly without stalling.

A Personal Anecdote: I once spent hours trying to diagnose a starting problem on a Stihl MS 170, only to discover that the idle speed was set too low. A simple adjustment of the idle screw solved the problem instantly.

Why This Matters: Setting the correct idle speed ensures the engine starts easily, runs smoothly at idle, and prevents the chain from spinning, which is a safety hazard.

Hack #3: The Low-Speed Needle Adjustment (L) – Throttle Response

The low-speed needle (L) controls the fuel mixture at low engine speeds and idle. Adjusting the low-speed needle affects throttle response and overall engine smoothness.

Step 1: Locate the Low-Speed Needle (L)

The low-speed needle (L) is typically located on the carburetor body, near the idle adjustment screw. Refer to your Stihl MS 170 owner’s manual for the exact location.

Step 2: Initial Setting

Before making any adjustments, turn the low-speed needle clockwise (to the right) until it is lightly seated. Then, back it out (turn it counterclockwise) to the factory setting. The factory setting for the Stihl MS 170 is typically 1 to 1.5 turns out from lightly seated. Refer to your owner’s manual for the exact setting.

Step 3: Fine-Tuning

1. Start the engine and let it warm up.
2. Accelerate the engine quickly from idle to full throttle.
3. Listen to the engine’s response. If the engine hesitates or stumbles when you accelerate, it is likely running lean (too much air, not enough fuel). Turn the low-speed needle counterclockwise (to the left) in small increments (e.g., 1/8 of a turn) to richen the mixture.
4. If the engine bogs down or smokes excessively when you accelerate, it is likely running rich (too much fuel, not enough air). Turn the low-speed needle clockwise (to the right) in small increments to lean out the mixture.
5. Repeat this process until the engine accelerates smoothly and cleanly from idle to full throttle.

Step 4: The “Sweet Spot”

The goal is to find the “sweet spot” where the engine accelerates smoothly and cleanly without hesitation or bogging down. This requires patience and careful listening.

Why This Matters: The low-speed needle adjustment is crucial for throttle response and overall engine smoothness. A properly adjusted low-speed needle ensures the engine accelerates quickly and cleanly, making it easier to control the chainsaw.

Hack #4: The High-Speed Needle Adjustment (H) – Power Unleashed

The high-speed needle (H) controls the fuel mixture at high engine speeds. Adjusting the high-speed needle affects the engine’s power and performance under load.

Important Note: Adjusting the high-speed needle requires caution. Running the engine too lean at high speeds can cause engine damage due to overheating.

Step 1: Locate the High-Speed Needle (H)

The high-speed needle (H) is typically located on the carburetor body, often near the low-speed needle. Refer to your Stihl MS 170 owner’s manual for the exact location.

Step 2: Initial Setting

Before making any adjustments, turn the high-speed needle clockwise (to the right) until it is lightly seated. Then, back it out (turn it counterclockwise) to the factory setting. The factory setting for the Stihl MS 170 is typically 1 to 1.5 turns out from lightly seated. Refer to your owner’s manual for the exact setting.

Step 3: Wide Open Throttle Test

1. Start the engine and let it warm up.
2. Engage the chain brake.
3. Hold the throttle wide open.
4. Listen to the engine’s sound. If the engine sounds “flat” or “raspy,” it is likely running lean. Turn the high-speed needle counterclockwise (to the left) in small increments (e.g., 1/8 of a turn) to richen the mixture.
5. If the engine bogs down or smokes excessively, it is likely running rich. Turn the high-speed needle clockwise (to the right) in small increments to lean out the mixture.
6. Use a tachometer to measure the engine’s RPM at wide open throttle. The maximum RPM for the Stihl MS 170 is typically around 12,500 RPM. Do not exceed this limit, as it can damage the engine.

Step 4: The “Four-Stroking” Test

A useful technique for setting the high-speed needle is the “four-stroking” test. With the engine at wide open throttle, listen carefully to the engine’s sound. A properly adjusted high-speed needle will cause the engine to “four-stroke” slightly, meaning it will sound like it’s missing a beat every few cycles. This indicates that the engine is getting just enough fuel to run at its peak performance. If the engine is running smoothly without any four-stroking, it is likely running lean.

Step 5: Load Test

After adjusting the high-speed needle, perform a load test by cutting a large piece of wood. Listen to the engine’s sound under load. If the engine bogs down or stalls, it may need further adjustment.

Why This Matters: The high-speed needle adjustment is crucial for maximizing the engine’s power and performance under load. A properly adjusted high-speed needle ensures the engine has enough fuel to run at its peak performance without overheating.

Caution: Always err on the side of a slightly rich mixture when adjusting the high-speed needle. Running the engine too lean can cause engine damage.

Hack #5: The Final Check – Putting It All Together

After adjusting the idle speed, low-speed needle, and high-speed needle, it’s essential to perform a final check to ensure everything is working correctly.

Step 1: Starting Test

Start the engine and make sure it starts easily, both when cold and when warm.

Step 2: Idle Test

Let the engine idle for a few minutes and make sure it runs smoothly without stalling.

Step 3: Acceleration Test

Accelerate the engine quickly from idle to full throttle and make sure it accelerates smoothly and cleanly without hesitation or bogging down.

Step 4: Load Test

Cut a large piece of wood and make sure the engine has enough power to cut through the wood without bogging down or stalling.

Step 5: Spark Plug Inspection

After performing the load test, remove the spark plug and inspect its color. A properly adjusted carburetor will result in a spark plug with a light tan color. A black, sooty spark plug indicates a rich mixture, while a white or gray spark plug indicates a lean mixture.

Case Study: The Stubborn Oak Log

I once had a customer who was struggling to cut through a large oak log with his Stihl MS 170. He had tried everything, but the chainsaw kept bogging down and stalling. After inspecting the chainsaw, I discovered that the high-speed needle was set too lean. I richened the mixture slightly, and the chainsaw was able to cut through the oak log with ease.

Why This Matters: The final check ensures that all the adjustments are working together harmoniously and that the engine is running at its peak performance.

Additional Tips and Considerations

• Altitude: Carburetor settings may need to be adjusted for different altitudes. At higher altitudes, the air is thinner, so the engine may need a leaner mixture.
• Temperature: Carburetor settings may also need to be adjusted for different temperatures. In colder temperatures, the engine may need a richer mixture.
• Carburetor Rebuild: If you’re still having problems after adjusting the carburetor, it may need to be rebuilt. A carburetor rebuild involves disassembling the carburetor, cleaning all the parts, and replacing any worn or damaged parts.
• Professional Help: If you’re not comfortable adjusting the carburetor yourself, take the chainsaw to a qualified mechanic.

Strategic Insights

• Preventative Maintenance: Regular maintenance, such as cleaning the air filter and replacing the spark plug, can help prevent carburetor problems.
• Fuel Quality: Use high-quality fuel and two-stroke oil. Avoid using old or stale fuel.
• Storage: When storing the chainsaw for an extended period, drain the fuel tank and run the engine until it stalls to prevent fuel from going stale in the carburetor.
• Documentation: Keep a record of your carburetor settings so you can easily revert to them if necessary.

Next Steps

Now that you’ve learned how to adjust the carburetor on your Stihl MS 170, it’s time to put your knowledge into practice. Start by performing the initial setup, then adjust the idle speed, low-speed needle, and high-speed needle. Remember to prioritize safety and take your time. With a little patience and practice, you’ll be able to tune your Stihl MS 170 to perfection.

Conclusion

The Stihl MS 170 is a fantastic chainsaw, and with these five pro tuning hacks, you can unlock its full potential. Remember, a well-tuned carburetor is the key to optimal performance, fuel efficiency, and engine longevity. By following these steps and paying attention to your engine’s sound and behavior, you can keep your Stihl MS 170 running smoothly for years to come. Happy cutting!

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