Stihl MS170 Won't Start (5 Pro Fixes Every Arborist Must Try)

Let’s face it, there’s almost nothing more frustrating than pulling the starter cord on your Stihl MS170, only to be met with…silence. Or worse, a cough and sputter that quickly dies. I’ve been there, countless times. Over the years, I’ve learned to troubleshoot and fix these issues myself. I’m going to share my top five fixes that every arborist, homeowner, or weekend warrior should know. These are the fixes I reach for first, and they’ve saved me a lot of time and money.

Stihl MS170 Won’t Start: 5 Pro Fixes Every Arborist Must Try

The Stihl MS170 is a workhorse. It’s lightweight, reliable (usually!), and perfect for smaller jobs like limbing, pruning, and cutting firewood. But like any machine, it can have its moments. A non-starting chainsaw can derail your entire day, especially when you’re on a tight schedule or dealing with a pile of downed trees.

1. Fuel System Check: The First Suspect

The fuel system is almost always the first place I look when a chainsaw refuses to start. Think of it like this: your saw is an engine that needs to breathe (air), eat (fuel), and spark (ignition) to run. No fuel, no go.

Key Concept: Two-Stroke Engines: The MS170 uses a two-stroke engine, meaning it requires a specific fuel mixture of gasoline and oil. This mixture lubricates the engine as it runs. Using straight gasoline or the wrong oil ratio is a surefire way to damage your saw.
Step 1: Check the Fuel Mix: This might sound obvious, but double-check that you’re using the correct fuel mixture. Stihl recommends a 50:1 ratio (50 parts gasoline to 1 part oil) using high-quality two-stroke oil. I personally use Stihl Ultra HP oil. It’s a bit more expensive, but I’ve found it provides excellent protection and helps keep my saws running smoothly.
- My Experience: I once lent my MS170 to a neighbor who, bless his heart, thought he was doing me a favor by filling it with straight gasoline. It took me hours to clean the fuel system and replace the spark plug. Lesson learned: always clearly label your fuel containers!
Step 2: Inspect the Fuel Filter: The fuel filter is located inside the fuel tank, attached to the end of the fuel line. It’s a small, often overlooked component that can cause big problems if it’s clogged.
- Procedure:
  1. Remove the fuel cap.
  2. Use a clean piece of wire (a bent coat hanger works well) to carefully hook the fuel line and pull the filter out of the tank. Be gentle, as the fuel line can become brittle over time.
  3. Inspect the filter. If it’s dirty or clogged, replace it. Fuel filters are inexpensive and readily available.
  4. Reattach the fuel line to the new filter and carefully insert it back into the tank.
  5. Ensure the filter rests at the bottom of the tank so it can draw fuel even when the tank is low.
- Tool Specification: A small hook made from a coat hanger or a specialized fuel filter removal tool.
Step 3: Check the Fuel Line: Inspect the fuel line for cracks, leaks, or kinks. Over time, the fuel line can dry out and become brittle, leading to fuel leaks and a loss of pressure.
- Procedure:
  1. Carefully examine the entire length of the fuel line, from the fuel tank to the carburetor.
  2. Pay close attention to the areas where the fuel line connects to the tank and the carburetor, as these are common areas for leaks.
  3. If you find any damage, replace the fuel line.
- Material Specs: Use fuel line specifically designed for two-stroke engines. These lines are resistant to the chemicals in gasoline and oil.
Step 4: Carburetor Issues (Beginner Check): While a full carburetor rebuild is often best left to a professional, a simple check can sometimes identify the problem. Look for obvious signs of dirt or debris around the carburetor.
- Procedure:
  1. Remove the air filter (more on that in the next section).
  2. With the air filter removed, look into the carburetor throat.
  3. Operate the throttle lever. You should see the throttle plate move smoothly. If it’s sticking or gummed up, it could be preventing the saw from starting.
- Strategic Advantage: Addressing minor carburetor issues early can prevent more serious problems down the road.

2. Air Filter Inspection: Letting Your Saw Breathe

A clean air filter is crucial for proper engine performance. A clogged air filter restricts airflow, which can lead to a rich fuel mixture (too much fuel, not enough air) and a saw that won’t start.

Key Concept: Air-Fuel Ratio: The air-fuel ratio is the ratio of air to fuel in the engine’s combustion chamber. A proper air-fuel ratio is essential for efficient combustion and optimal engine performance.
Step 1: Locate the Air Filter: On the Stihl MS170, the air filter is typically located under a cover on top of the engine.
Step 2: Remove and Inspect the Air Filter: Carefully remove the air filter and inspect it for dirt, dust, and debris. If the filter is visibly dirty, it needs to be cleaned or replaced.
Step 3: Clean or Replace the Air Filter:
- Cleaning: If the filter is only slightly dirty, you can try cleaning it. Tap the filter against a hard surface to remove loose debris. You can also use compressed air to blow out the dirt. Be sure to blow from the inside out to avoid pushing the dirt further into the filter.
- Replacing: If the filter is heavily soiled or damaged, it’s best to replace it. Air filters are relatively inexpensive and readily available.
- My Experience: I once spent an entire morning trying to start my MS170, only to discover that the air filter was completely clogged with sawdust. A quick cleaning, and the saw fired right up. Now, I make it a habit to check the air filter before every use.
Material Specs: Use air filters specifically designed for the Stihl MS170.
Strategic Advantage: Regularly cleaning or replacing the air filter can improve engine performance, fuel efficiency, and extend the life of your saw.

3. Spark Plug Check: Igniting the Power

The spark plug is what ignites the fuel-air mixture in the engine’s cylinder. A faulty spark plug can prevent the saw from starting or cause it to run poorly.

Key Concept: Ignition System: The ignition system is responsible for creating the spark that ignites the fuel-air mixture. The spark plug is a critical component of this system.
Step 1: Locate the Spark Plug: The spark plug is located under a rubber boot on the side of the engine.
Step 2: Remove the Spark Plug: Carefully pull the rubber boot off the spark plug. Use a spark plug wrench to remove the spark plug.
- Tool Specification: Spark plug wrench (typically 19mm).
Step 3: Inspect the Spark Plug: Inspect the spark plug for signs of damage, such as cracks, chips, or excessive wear. Also, check the electrode for carbon buildup or fouling.
- Visual Inspection: A healthy spark plug should have a clean, dry electrode. If the electrode is wet or oily, it could indicate a problem with the fuel mixture or engine compression.
Step 4: Clean or Replace the Spark Plug:
- Cleaning: If the spark plug is only slightly fouled, you can try cleaning it with a wire brush. Be sure to clean the electrode and the surrounding area.
- Replacing: If the spark plug is heavily fouled or damaged, it’s best to replace it. Spark plugs are inexpensive and readily available. Use the correct spark plug for the Stihl MS170 (typically a NGK BPMR7A or equivalent).
- Measurement: The spark plug gap should be set to the manufacturer’s specification (typically 0.020 inches or 0.5mm). Use a spark plug gap tool to adjust the gap.
- My Experience: I once had a Stihl MS170 that would start, but it would run rough and stall frequently. After checking everything else, I finally decided to replace the spark plug. To my surprise, the old spark plug was cracked. A new spark plug solved the problem immediately.
Strategic Advantage: Replacing the spark plug is a simple and inexpensive way to improve engine performance and ensure reliable starting.

4. Switch and Wiring Inspection: The Electrical Connection

Sometimes, the problem isn’t with the fuel or the spark plug, but with the electrical system. A faulty on/off switch or damaged wiring can prevent the saw from starting.

Key Concept: Electrical Circuit: The on/off switch and wiring form part of the electrical circuit that controls the ignition system. If this circuit is broken, the saw won’t start.
Step 1: Check the On/Off Switch: Make sure the on/off switch is in the “on” position. Sometimes, the switch can become loose or damaged, preventing it from making proper contact.
Step 2: Inspect the Wiring: Carefully inspect the wiring for any signs of damage, such as cuts, cracks, or loose connections. Pay close attention to the wires that connect to the on/off switch and the ignition coil.
- Procedure:
  1. Visually inspect the wires for any obvious damage.
  2. Gently tug on the wires to check for loose connections.
  3. Use a multimeter to check the continuity of the wires.
- Tool Specification: Multimeter.
Step 3: Test the On/Off Switch (Advanced): If you suspect the on/off switch is faulty, you can test it with a multimeter.
- Procedure:
  1. Disconnect the wires from the on/off switch.
  2. Set the multimeter to the continuity setting.
  3. Place the multimeter probes on the switch terminals.
  4. With the switch in the “on” position, the multimeter should show continuity. With the switch in the “off” position, the multimeter should not show continuity.
Step 4: Replace Damaged Components: If you find any damaged wiring or a faulty on/off switch, replace them.
- My Experience: I once had a Stihl MS170 that wouldn’t start, and after checking everything else, I discovered that the wire connecting the on/off switch to the ignition coil was broken. A simple wire repair solved the problem.
Strategic Advantage: Addressing electrical issues promptly can prevent more serious engine damage.

5. Flooding Issues: Too Much of a Good Thing

Sometimes, the problem isn’t a lack of fuel, but too much of it. Flooding occurs when the engine cylinder is filled with too much fuel, making it difficult for the spark plug to ignite the mixture.

Key Concept: Flooding: Flooding is a condition in which the engine cylinder is filled with too much fuel, preventing the engine from starting.
Causes of Flooding: Flooding can be caused by a variety of factors, including:
- Repeatedly pulling the starter cord without allowing the engine to start.
- A faulty choke mechanism.
- A leaking carburetor.
Step 1: Identify Flooding: If you smell a strong odor of gasoline coming from the engine, it’s likely flooded.
Step 2: De-Flood the Engine:
- Procedure:
  1. Set the on/off switch to the “on” position.
  2. Move the choke lever to the “off” position.
  3. Hold the throttle lever wide open.
  4. Pull the starter cord repeatedly (10-15 times) to clear the excess fuel from the cylinder.
- My Experience: I’ve flooded my Stihl MS170 more times than I care to admit, especially when it’s cold outside. The de-flooding procedure usually works like a charm.
Step 3: Attempt to Start the Engine: After de-flooding the engine, try starting it again.
- Procedure:
  1. Set the choke lever to the “on” position (if the engine is cold).
  2. Pull the starter cord until the engine starts.
  3. Gradually move the choke lever to the “off” position as the engine warms up.
Strategic Advantage: Knowing how to de-flood your engine can save you a lot of time and frustration.

Additional Tips and Considerations

Compression Test (Advanced): If you’ve checked all of the above and your Stihl MS170 still won’t start, it’s possible that there’s a problem with the engine compression. A compression test can help you determine if the piston rings or cylinder are worn. This usually requires specialized tools and knowledge, so it’s best left to a professional.
Carburetor Adjustment (Advanced): The carburetor is responsible for regulating the air-fuel mixture. If the carburetor is not properly adjusted, it can cause starting problems and poor engine performance. Carburetor adjustment is a delicate process that requires specialized tools and knowledge.
Regular Maintenance: The best way to prevent starting problems is to perform regular maintenance on your Stihl MS170. This includes:
- Cleaning or replacing the air filter regularly.
- Replacing the spark plug annually.
- Using fresh, high-quality fuel.
- Storing the saw properly when not in use.
Safety First: Always wear appropriate safety gear when working on your chainsaw, including gloves, eye protection, and hearing protection.
Green Wood vs. Seasoned Wood: Understanding the difference between green and seasoned wood is crucial for firewood preparation. Green wood has a high moisture content (often over 50%), making it difficult to burn and producing a lot of smoke. Seasoned wood has been air-dried for several months, reducing the moisture content to around 20%. Seasoned wood burns more efficiently and produces less smoke.
- Drying Methods: The most common method for seasoning firewood is air-drying. Stack the wood in a single row, off the ground, and in a sunny, well-ventilated location. Allow the wood to dry for at least six months, or preferably a year.
Splitting Firewood: Splitting firewood can be a challenging and time-consuming task. Using a hydraulic log splitter can significantly increase efficiency and reduce the risk of injury.
- Hydraulic Log Splitters: Hydraulic log splitters use hydraulic power to split logs. They are available in a variety of sizes and models, ranging from small electric splitters to large gas-powered splitters.
- Felling Techniques: Proper felling techniques are essential for safe and efficient logging. Always assess the tree for hazards, such as dead limbs or power lines. Use a directional felling cut to control the direction of the fall.

Case Study: Reviving a Neglected MS170

I once acquired a Stihl MS170 from a friend who had left it sitting in his shed for several years. It was in rough shape – the fuel lines were cracked, the air filter was clogged, and the spark plug was corroded. It wouldn’t even sputter.

I started by completely disassembling the fuel system, replacing all of the fuel lines and the fuel filter. I then cleaned the carburetor and replaced the spark plug. After reassembling the saw, I gave it a few pulls, and it fired right up. It ran a little rough at first, but after a few minutes of running, it smoothed out and ran like new.

This experience reinforced the importance of regular maintenance and the effectiveness of these five pro fixes.

Next Steps

Now that you’ve learned these five pro fixes, you’re well-equipped to troubleshoot and repair your Stihl MS170. Remember to start with the simplest solutions first, and don’t be afraid to ask for help if you get stuck. With a little patience and perseverance, you can keep your chainsaw running smoothly for years to come. Get out there, tackle that woodpile, and enjoy the satisfaction of a job well done!

Stihl MS170 Won’t Start (5 Pro Fixes Every Arborist Must Try)