Carburetor for Stihl MS170: Fix Tips (5 Pro Tuning Hacks)

Ever wrestled with a Stihl MS170 that just wouldn’t purr like it should? Let’s dive into those finicky carburetors and get your chainsaw singing again! I’ve spent years in the woods, and I can tell you, a poorly tuned MS170 can turn a productive day into a frustrating one. I’m going to share my hard-earned wisdom, along with five pro tuning hacks to get your MS170 cutting like new.

Stihl MS170 Carburetor: Troubleshooting and Tuning Guide

The Stihl MS170 is a popular chainsaw, known for its reliability and ease of use. However, like any small engine, it relies on a properly functioning carburetor to deliver the correct air-fuel mixture for optimal performance. When the carburetor is out of tune, it can lead to a variety of problems, including difficulty starting, poor idling, stalling, and reduced power.

Understanding the Stihl MS170 Carburetor

Before diving into the tuning hacks, it’s crucial to understand the basics of the MS170 carburetor. This model typically uses a Walbro or Zama carburetor, which are diaphragm-type carburetors. These carburetors use atmospheric pressure to draw fuel into the engine.

Key Components:

• Fuel Inlet: Where fuel enters the carburetor.
• Fuel Filter: Filters debris from the fuel.
• Diaphragm: A flexible membrane that pumps fuel.
• Metering Lever: Controls the fuel flow.
• Needle Valve: Regulates the fuel entering the carburetor.
• Jets (High-Speed and Low-Speed): Control the fuel mixture at different engine speeds.
• Adjustment Screws (H and L): Fine-tune the fuel mixture.

How it Works:

The engine’s vacuum pulls air through the carburetor venturi, creating a low-pressure area. This pressure difference draws fuel from the fuel tank, through the fuel filter, and into the carburetor. The diaphragm, actuated by crankcase pressure, pumps fuel to the metering chamber. The metering lever and needle valve regulate the fuel flow based on engine demand. The high-speed (H) and low-speed (L) jets then meter the fuel into the air stream, creating the correct air-fuel mixture for combustion.

Common Carburetor Problems with the MS170

I’ve seen it all in the field. Here’s a rundown of the most common culprits behind a poorly running MS170.

• Dirty Carburetor: This is the most frequent problem. Debris, old fuel, and varnish can clog the jets and passages, disrupting the fuel flow.
• Stale Fuel: Fuel that sits for extended periods can break down, leaving behind gummy deposits that clog the carburetor. I always tell people to use fuel stabilizer!
• Air Leaks: Leaks in the fuel lines, intake manifold, or carburetor gaskets can disrupt the air-fuel mixture, leading to poor performance.
• Damaged Diaphragm or Metering Lever: Over time, the diaphragm can become brittle and crack, while the metering lever can become bent or worn, affecting fuel regulation.
• Incorrect Adjustment: Improper adjustment of the high-speed (H) and low-speed (L) screws can lead to a lean or rich fuel mixture, causing performance issues.

Safety First: Before You Start

Before you even think about touching that carburetor, safety is paramount. I’ve seen too many accidents in my time.

• Wear Safety Glasses: Protect your eyes from fuel and debris.
• Wear Gloves: Protect your hands from fuel and chemicals.
• Work in a Well-Ventilated Area: Avoid breathing in fuel fumes.
• Disconnect the Spark Plug: Prevent accidental starting.
• Allow the Engine to Cool: Avoid burns from a hot engine.
• Have a Fire Extinguisher Nearby: Just in case.

Tools You’ll Need

Having the right tools makes the job infinitely easier. Don’t skimp here.

• Screwdrivers: A small flathead screwdriver and a carburetor adjustment tool (splined or D-shaped, depending on your carburetor) are essential.
• Carburetor Cleaner: A specialized carburetor cleaner is more effective than general solvents.
• Compressed Air: For blowing out debris from the carburetor passages.
• Soft Brush: For cleaning the carburetor body.
• Pliers: For removing fuel lines and other components.
• New Fuel Filter: A clogged fuel filter can restrict fuel flow.
• New Spark Plug: A fouled spark plug can mimic carburetor problems.
• New Fuel Lines (Optional): If your fuel lines are cracked or brittle, replace them.
• Carburetor Rebuild Kit (Optional): If your carburetor is severely damaged, a rebuild kit may be necessary.
• Shop Rags: For cleaning up spills.

Hack #1: The Thorough Cleaning

This is the foundation of any carburetor fix. A clean carburetor is a happy carburetor. I can’t stress this enough.

Step 1: Remove the Air Filter and Air Filter Cover. The air filter is usually held in place by a clip or screw. Remove the cover and then the filter. Inspect the filter for dirt and damage. Replace if necessary.

Step 2: Remove the Carburetor. Carefully disconnect the fuel lines from the carburetor. Note their original positions for reassembly. Disconnect the throttle linkage and choke linkage. Remove the bolts or screws that hold the carburetor to the engine.

Step 3: Disassemble the Carburetor. Carefully disassemble the carburetor, noting the location of each part. Pay attention to the diaphragm, metering lever, and jets.

Step 4: Clean the Carburetor Components. Spray all carburetor components with carburetor cleaner. Use a soft brush to remove any stubborn deposits. Pay particular attention to the jets and passages. Use compressed air to blow out any remaining debris.

Step 5: Reassemble the Carburetor. Reassemble the carburetor, ensuring that all parts are in their original positions. Install new gaskets if necessary.

Step 6: Reinstall the Carburetor. Reinstall the carburetor onto the engine. Reconnect the fuel lines, throttle linkage, and choke linkage.

Step 7: Install the Air Filter and Air Filter Cover. Reinstall the air filter and air filter cover.

Why this works: A thorough cleaning removes dirt, debris, and varnish that can clog the carburetor and disrupt the fuel flow.

My Experience: I once had an MS170 that wouldn’t start no matter what I did. After a thorough cleaning, it fired right up. Sometimes, it’s the simplest things that make the biggest difference.

Hack #2: The Fuel System Check

A clean carburetor is useless if the fuel isn’t flowing properly. I’ve seen clogged fuel filters bring down entire logging operations.

Step 1: Inspect the Fuel Tank. Check the fuel tank for dirt, debris, and water. Clean the tank if necessary.

Step 2: Replace the Fuel Filter. The fuel filter is located inside the fuel tank, attached to the fuel line. Use a small hook or pliers to remove the old filter. Install a new fuel filter.

Step 3: Inspect the Fuel Lines. Check the fuel lines for cracks, leaks, and brittleness. Replace any damaged fuel lines.

Step 4: Check the Fuel Vent. The fuel vent allows air to enter the fuel tank, preventing a vacuum from forming. A clogged fuel vent can restrict fuel flow. Clean the fuel vent with a small wire or compressed air.

Step 5: Test the Fuel Pump. If your MS170 has a fuel pump (some models do), test it to ensure it’s working properly. Disconnect the fuel line from the carburetor and crank the engine. Fuel should flow from the fuel line.

Why this works: A clean and properly functioning fuel system ensures that the carburetor receives a steady supply of fuel.

My Experience: I remember a time when my MS170 was running lean, even after cleaning the carburetor. Turns out, the fuel line had a tiny crack that was letting air in. A quick fuel line replacement solved the problem.

Hack #3: The Idle Speed Adjustment

Getting the idle speed right is crucial for smooth operation. A chainsaw that stalls at idle is a real pain in the woods.

Step 1: Locate the Idle Speed Screw. The idle speed screw is usually located near the throttle linkage on the carburetor. Refer to your MS170’s owner’s manual for the exact location.

Step 2: Start the Engine. Start the engine and let it warm up for a few minutes.

Step 3: Adjust the Idle Speed. Use a screwdriver to adjust the idle speed screw. Turn the screw clockwise to increase the idle speed and counterclockwise to decrease the idle speed.

Step 4: Set the Idle Speed. Adjust the idle speed until the engine runs smoothly without stalling. The ideal idle speed for the MS170 is typically around 2,800 RPM. You can use a tachometer to measure the RPM.

Step 5: Fine-Tune the Adjustment. Make small adjustments to the idle speed until you achieve the desired performance.

Why this works: Proper idle speed ensures that the engine runs smoothly when the throttle is released.

My Experience: I once spent hours trying to diagnose a stalling MS170. It turned out the idle speed was set too low. A simple adjustment fixed the problem.

Hack #4: The High-Speed (H) and Low-Speed (L) Adjustment

This is where things get a bit more technical, but mastering these adjustments is key to unlocking your MS170’s full potential.

Understanding the H and L Screws:

• H (High-Speed): Controls the fuel mixture at high engine speeds (full throttle).
• L (Low-Speed): Controls the fuel mixture at low engine speeds (idle and low throttle).

Important Note: Many newer MS170 carburetors have limiter caps on the H and L screws to restrict adjustment. Removing these caps may be necessary for proper tuning but can also void the warranty. Proceed with caution.

Step 1: Locate the H and L Screws. The H and L screws are usually located on the side of the carburetor. Refer to your MS170’s owner’s manual for the exact location.

Step 2: Initial Settings. Start with the factory settings. The factory settings are typically listed in the owner’s manual. If you don’t have the owner’s manual, a common starting point is 1 turn out from fully closed for both the H and L screws.

Step 3: Start the Engine. Start the engine and let it warm up for a few minutes.

Step 4: Adjust the Low-Speed (L) Screw. Adjust the L screw until the engine idles smoothly and responds quickly to throttle inputs. If the engine stalls or hesitates when you open the throttle, the mixture is too lean (not enough fuel). Turn the L screw counterclockwise to richen the mixture. If the engine smokes excessively or bogs down when you open the throttle, the mixture is too rich (too much fuel). Turn the L screw clockwise to lean the mixture.

Step 5: Adjust the High-Speed (H) Screw. Adjust the H screw while the engine is running at full throttle. Listen to the engine. If the engine sounds strained or “screaming,” the mixture is too lean. Turn the H screw counterclockwise to richen the mixture. If the engine smokes excessively or bogs down at full throttle, the mixture is too rich. Turn the H screw clockwise to lean the mixture.

Step 6: Fine-Tune the Adjustments. Make small adjustments to both the H and L screws until you achieve optimal performance. The goal is to have a smooth idle, quick throttle response, and strong power at full throttle.

Step 7: Spark Plug Check. After tuning, run the chainsaw under load for a few minutes and then check the spark plug. A light tan color indicates a good air-fuel mixture. A black, sooty plug indicates a rich mixture, while a white or light gray plug indicates a lean mixture. Adjust the H and L screws accordingly.

Why this works: Properly adjusting the H and L screws ensures that the engine receives the correct air-fuel mixture at all engine speeds.

My Experience: I spent years struggling to get my MS170 to run perfectly. It wasn’t until I truly understood the H and L adjustments that I was able to unlock its full potential. It takes practice, but it’s worth the effort.

Important Considerations:

• Altitude: Higher altitudes require a leaner fuel mixture.
• Temperature: Colder temperatures require a richer fuel mixture.
• Fuel Quality: Poor fuel quality can affect carburetor performance.
• Engine Condition: A worn engine may require different carburetor settings.

Hack #5: The Carburetor Rebuild (When All Else Fails)

Sometimes, cleaning and adjusting just isn’t enough. If your carburetor is severely damaged or worn, a rebuild may be necessary. This involves replacing the diaphragm, gaskets, and needle valve.

Step 1: Purchase a Carburetor Rebuild Kit. Make sure the rebuild kit is compatible with your MS170 carburetor.

Step 2: Disassemble the Carburetor. Carefully disassemble the carburetor, noting the location of each part.

Step 3: Replace the Components. Replace the diaphragm, gaskets, and needle valve with the new components from the rebuild kit.

Step 4: Reassemble the Carburetor. Reassemble the carburetor, ensuring that all parts are in their original positions.

Step 5: Reinstall the Carburetor. Reinstall the carburetor onto the engine. Reconnect the fuel lines, throttle linkage, and choke linkage.

Step 6: Adjust the Carburetor. Adjust the H and L screws according to the instructions in Hack #4.

Why this works: A carburetor rebuild replaces worn or damaged components, restoring the carburetor to like-new condition.

My Experience: I had an old MS170 that had been sitting in my shed for years. The carburetor was completely gummed up. A rebuild kit brought it back to life.

Troubleshooting Guide: Symptoms and Solutions

Here’s a quick reference guide to help you diagnose and solve common MS170 carburetor problems.

Preventing Future Problems

Prevention is always better than cure. Here are some tips to keep your MS170 carburetor running smoothly.

• Use Fresh Fuel: Use fresh, high-quality fuel with a fuel stabilizer. I recommend using fuel that is less than 30 days old.
• Store Fuel Properly: Store fuel in a clean, airtight container in a cool, dry place.
• Clean the Air Filter Regularly: Clean the air filter after each use.
• Inspect the Fuel Filter Regularly: Inspect the fuel filter regularly and replace it as needed.
• Run the Engine Periodically: If you’re not using your MS170 regularly, run it for a few minutes every month to keep the carburetor clean.
• Drain the Fuel Tank Before Storage: Before storing your MS170 for an extended period, drain the fuel tank and run the engine until it stalls to prevent fuel from gumming up the carburetor.
• Use a Fuel Stabilizer: Always use a fuel stabilizer when storing your MS170 for more than a few weeks.

Advanced Tuning Techniques (For Experienced Users)

For those who want to take their MS170 tuning to the next level, here are some advanced techniques.

• Using a Tachometer: A tachometer allows you to accurately measure the engine RPM, which is essential for precise tuning.
• Reading the Spark Plug: As mentioned earlier, the color of the spark plug can tell you a lot about the air-fuel mixture.
• Adjusting the Metering Lever: The metering lever controls the fuel flow into the carburetor. Adjusting the metering lever height can fine-tune the fuel mixture. This is a delicate procedure and should only be attempted by experienced users.
• Modifying the Carburetor: Some users modify their MS170 carburetors to improve performance. This can involve drilling out the jets or modifying the venturi. However, these modifications can void the warranty and may damage the engine if not done correctly.

Case Studies

Here are a couple of real-world examples of how I’ve used these hacks to fix MS170 carburetor problems.

Case Study 1: The Stalling MS170

A friend of mine was having trouble with his MS170. It would start, but it would stall immediately. I started by cleaning the carburetor and replacing the fuel filter. That didn’t fix the problem. I then checked the fuel lines and found a small crack. I replaced the fuel line, and the MS170 ran perfectly.

Case Study 2: The Lackluster Performance

Another friend was complaining that his MS170 lacked power. I cleaned the air filter and checked the spark plug. Both were fine. I then adjusted the H and L screws. The H screw was set too lean. After adjusting the H screw, the MS170 had much more power.

Conclusion

Tuning a Stihl MS170 carburetor can seem daunting, but with a little knowledge and patience, it’s a task anyone can master. By following these five pro tuning hacks, you can keep your MS170 running smoothly and efficiently for years to come. Remember to always prioritize safety and use the right tools. Don’t be afraid to experiment, but always start with the basics and work your way up. And if all else fails, don’t hesitate to consult a qualified mechanic. Now get out there and make some sawdust!

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