Stihl MS170 Carburetor Repair Tips (Troubleshooting for Woodcutters)

Why did the woodcutter break up with the tree? Because it was too sappy!

Alright, let’s talk carburetors, specifically the one in your Stihl MS170. As a seasoned woodcutter, I know that a finicky carburetor can turn a productive day into a frustrating one faster than you can say “timber!” I’ve wrestled with more MS170 carburetors than I care to admit, and I’m here to share my hard-earned wisdom, troubleshooting tips, and repair techniques to get your saw running like a champ.

Stihl MS170 Carburetor Repair: A Woodcutter’s Guide

The Stihl MS170 is a popular chainsaw for a reason: it’s lightweight, affordable, and generally reliable. But like any piece of machinery, it has its quirks, and the carburetor is often the culprit when things go wrong. This guide is designed to help you diagnose and fix common MS170 carburetor problems, even if you’re not a seasoned mechanic.

Understanding the MS170 Carburetor

Before we dive into repairs, let’s get acquainted with the basics. The carburetor’s job is to mix air and fuel in the correct proportions to create a combustible mixture that powers the engine. The MS170 uses a Walbro carburetor, typically either a WT-215 or similar variant. These carburetors are relatively simple in design, but precision is key.

Key Components of a Typical Walbro WT-215 Carburetor:

• Throttle Valve: Controls the amount of air entering the engine.
• Choke Valve: Restricts airflow to enrich the fuel mixture for starting a cold engine.
• Main Jet: Meters fuel at higher engine speeds.
• Idle Jet: Meters fuel at idle speed.
• Mixture Screws (H and L): Adjust the fuel-air mixture for high (H) and low (L) speed settings.
• Fuel Inlet Needle Valve: Controls fuel flow into the carburetor based on float level.
• Float: Regulates the fuel level in the carburetor bowl.
• Diaphragms: Flexible membranes that respond to pressure changes, controlling fuel flow.

Common MS170 Carburetor Problems and Troubleshooting

Here’s where my experience comes in handy. I’ve seen it all, from saws that won’t start to those that stall at idle. Let’s break down the most common issues and how to diagnose them.

1. Chainsaw Won’t Start:

• Possible Causes:
  • No Fuel: Obvious, but check the fuel tank. I’ve been there, trust me.
  • Stale Fuel: Fuel older than 30 days can degrade and clog the carburetor. Modern gasoline with ethanol is notorious for this. I recommend using fuel stabilizer, especially if you don’t use your saw frequently.
  • Clogged Fuel Filter: A dirty fuel filter restricts fuel flow. This is a very common issue.
  • Clogged Carburetor Jets: Small passages in the carburetor can become clogged with debris.
  • Faulty Spark Plug: A bad spark plug won’t ignite the fuel mixture.
  • Incorrect Choke Position: Using the choke incorrectly can flood the engine.
  • Low Compression: While less common, low compression can prevent starting.
• Troubleshooting Steps:
  1. Check Fuel: Ensure there’s fresh fuel in the tank.
  2. Inspect Fuel Filter: Remove the fuel filter from the tank and inspect it. If it’s dirty, replace it. I always keep a few spares on hand.
  3. Check Spark Plug: Remove the spark plug, clean it, and check the gap (0.5mm or 0.020 inches). If the spark plug is fouled or damaged, replace it.
  4. Try Starting with Choke: If the engine is cold, use the choke. If it’s warm, try starting without the choke.
  5. Check for Spark: With the spark plug removed, attach it to the spark plug wire and ground it against the engine. Pull the starter cord. You should see a strong spark. If not, the ignition coil may be faulty.
  6. Check Compression: Use a compression tester to check the engine’s compression. A healthy MS170 should have around 120-140 PSI. Lower than 100 PSI often indicates worn piston rings or a damaged cylinder. This is a more involved repair, often requiring a new cylinder and piston kit.

2. Chainsaw Starts but Stalls Immediately:

• Possible Causes:
  • Clogged Idle Jet: The idle jet is responsible for providing fuel at idle speed.
  • Incorrect Idle Speed Adjustment: The idle speed screw may be set too low.
  • Air Leak: An air leak can lean out the fuel mixture, causing the engine to stall. Check the carburetor mounting bolts, intake manifold, and crankshaft seals.
• Troubleshooting Steps:
  1. Adjust Idle Speed Screw: Turn the idle speed screw (usually marked with a “T”) clockwise to increase the idle speed. Do this in small increments.
  2. Inspect for Air Leaks: Check the carburetor mounting bolts to ensure they are tight. Spray carburetor cleaner around the intake manifold and crankshaft seals while the engine is running. If the engine speed changes, you’ve found an air leak.
  3. Clean Carburetor: If the problem persists, the carburetor likely needs cleaning.

3. Chainsaw Runs Poorly at High Speed:

• Possible Causes:
  • Clogged Main Jet: The main jet meters fuel at high speed.
  • Incorrect High-Speed Mixture Adjustment: The high-speed mixture screw may be set too lean.
  • Restricted Air Filter: A dirty air filter restricts airflow, causing the engine to run rich.
• Troubleshooting Steps:
  1. Clean Air Filter: Remove the air filter and clean it with soap and water. Allow it to dry completely before reinstalling. I prefer using compressed air to blow out any remaining debris.
  2. Adjust High-Speed Mixture Screw: The high-speed mixture screw (H) controls the fuel-air mixture at high speed. Turning it counter-clockwise enriches the mixture (more fuel), while turning it clockwise leans the mixture (less fuel).
  3. Clean Carburetor: If the problem persists, the carburetor likely needs cleaning.

4. Chainsaw Idles Poorly or Smokes Excessively:

• Possible Causes:
  • Incorrect Low-Speed Mixture Adjustment: The low-speed mixture screw may be set too rich.
  • Worn Carburetor Diaphragms: Over time, the carburetor diaphragms can become stiff or cracked, affecting fuel metering.
• Troubleshooting Steps:
  1. Adjust Low-Speed Mixture Screw: The low-speed mixture screw (L) controls the fuel-air mixture at idle speed. Turning it clockwise leans the mixture (less fuel), while turning it counter-clockwise enriches the mixture (more fuel).
  2. Check Carburetor Diaphragms: Inspect the carburetor diaphragms for signs of wear or damage. If they are stiff or cracked, replace them.

Cleaning the MS170 Carburetor: A Step-by-Step Guide

Cleaning the carburetor is often the first step in resolving many issues. Here’s how I do it:

Tools You’ll Need:

• Screwdrivers (Phillips and Flathead)
• Carburetor Cleaner
• Compressed Air
• Small Brush (like a toothbrush)
• Carburetor Cleaning Kit (optional, but recommended)
• Safety Glasses
• Gloves

Procedure:

1. Remove the Air Filter Cover and Air Filter: This exposes the carburetor.
2. Disconnect Fuel Lines: Carefully disconnect the fuel lines from the carburetor. Have a rag handy to catch any spilled fuel.
3. Remove the Carburetor: Unscrew the carburetor mounting bolts and carefully remove the carburetor from the engine.
4. Disassemble the Carburetor:
   • Remove the fuel pump cover and diaphragm.
   • Remove the metering lever cover and diaphragm.
   • Remove the fuel inlet needle valve and seat.
   • Remove the mixture screws (H and L) and the idle speed screw (T). Note their positions before removing them! Count the number of turns it takes to fully seat each screw. This will be your starting point for re-adjustment.
   • Remove the jets (main and idle) using a small screwdriver.
5. Clean the Carburetor Components:
   • Spray all carburetor components with carburetor cleaner.
   • Use a small brush to scrub away any deposits.
   • Use compressed air to blow out all the passages and jets. Pay close attention to the tiny holes in the jets. A clogged jet is a common culprit.
   • If using a carburetor cleaning kit, use the provided wire tools to clean the jets and passages.
6. Inspect the Components:
   • Check the diaphragms for cracks, tears, or stiffness. Replace them if necessary.
   • Check the fuel inlet needle valve for wear or damage. Replace it if necessary.
   • Check the jets for blockages.
7. Reassemble the Carburetor:
   • Reinstall the jets.
   • Reinstall the fuel inlet needle valve and seat.
   • Reinstall the metering lever cover and diaphragm.
   • Reinstall the fuel pump cover and diaphragm.
   • Reinstall the mixture screws (H and L) and the idle speed screw (T), turning them in until they are lightly seated, then back them out to the original positions you noted earlier.
8. Reinstall the Carburetor:
   • Carefully reinstall the carburetor onto the engine.
   • Reconnect the fuel lines.
   • Reinstall the air filter and air filter cover.

Adjusting the MS170 Carburetor

After cleaning or replacing the carburetor, you’ll need to adjust it for optimal performance. Here’s how I approach carburetor adjustment:

Important Note: Always adjust the carburetor with the engine warmed up and the air filter installed.

Procedure:

1. Start the Engine: Start the engine and let it warm up for a few minutes.
2. Adjust Idle Speed: Turn the idle speed screw (T) until the engine idles smoothly without stalling. The chain should not be moving at idle. The ideal idle speed is around 2,800 RPM. You might need a tachometer to measure this accurately.
3. Adjust Low-Speed Mixture: Turn the low-speed mixture screw (L) until the engine idles smoothly and responds quickly to throttle input. The goal is to find the setting where the engine transitions smoothly from idle to high speed without hesitation or bogging down.

4. Adjust High-Speed Mixture: This is the trickiest part. The high-speed mixture screw (H) controls the fuel-air mixture at high speed.

   • Too Lean (Clockwise): The engine will run hot, and you may hear a high-pitched whine. This can cause engine damage.
   • Too Rich (Counter-Clockwise): The engine will smoke excessively and may bog down at high speed.
   • Optimal Setting: The engine should run smoothly at high speed without hesitation or excessive smoking.

   My Method: I usually start by turning the high-speed mixture screw (H) clockwise until the engine starts to whine. Then, I slowly turn it counter-clockwise until the whine disappears and the engine runs smoothly. This is usually close to the optimal setting.

Fine-Tuning:

• After making adjustments, let the engine run for a few minutes and then check the spark plug.
  • Light Tan Color: Indicates a good mixture.
  • White Color: Indicates a lean mixture.
  • Black Color: Indicates a rich mixture.
• Adjust the mixture screws as needed to achieve a light tan color on the spark plug.

Important Considerations:

• Altitude: Carburetor settings may need to be adjusted for different altitudes. Higher altitudes require leaner settings (more air, less fuel).
• Temperature: Carburetor settings may also need to be adjusted for different temperatures. Colder temperatures require richer settings (more fuel).
• Fuel Type: Using different fuel types, such as ethanol-free gasoline, may require carburetor adjustments.

Replacing the Carburetor: When Cleaning Isn’t Enough

Sometimes, despite your best efforts, the carburetor is simply beyond repair. This is often the case with heavily corroded or damaged carburetors. Replacing the carburetor is a straightforward process.

Procedure:

1. Purchase a Replacement Carburetor: Make sure you get the correct carburetor for your MS170. Check the part number on the old carburetor or consult the Stihl parts catalog. Aftermarket carburetors are available, but I recommend using a genuine Stihl or Walbro carburetor for best performance and reliability.
2. Remove the Old Carburetor: Follow the steps outlined in the “Cleaning the MS170 Carburetor” section to remove the old carburetor.
3. Install the New Carburetor:
   • Install the new carburetor onto the engine.
   • Reconnect the fuel lines.
   • Reinstall the air filter and air filter cover.
4. Adjust the New Carburetor: Follow the steps outlined in the “Adjusting the MS170 Carburetor” section to adjust the new carburetor for optimal performance.

Data-Backed Content and Technical Specifications

Let’s get into some hard numbers and technical details.

Wood Moisture Content:

• Freshly Cut Wood: Typically contains 50-60% moisture content (MC).
• Air-Dried Wood: Achieves 15-20% MC in a well-ventilated environment over several months.
• Kiln-Dried Wood: Can reach as low as 6-8% MC.

• Ideal Firewood Moisture Content: 15-20% for efficient burning and minimal smoke.

  Why This Matters: Burning wood with high moisture content wastes energy, produces more smoke and creosote (a fire hazard), and can damage your stove or fireplace. I always recommend using a wood moisture meter to ensure your firewood is properly seasoned.

Log Dimensions and Cord Volumes:

• Standard Cord: 4 feet high x 4 feet wide x 8 feet long = 128 cubic feet.
• Face Cord (Rick, Stove Cord): 4 feet high x 8 feet wide x length of logs (typically 16 inches).

• Estimating Cord Volume: The actual wood volume in a cord is less than 128 cubic feet due to air gaps. Expect around 60-80 cubic feet of solid wood per cord, depending on log size and straightness.

  My Experience: I once underestimated the amount of wood I needed for a winter season and ended up scrambling to find more in the middle of January. Now, I always overestimate and order a bit extra. It’s better to have too much than not enough!

Chainsaw Calibration:

• Chain Tension: Should allow you to pull the chain away from the bar about 1/8 inch.
• Cutting Depth: The rakers (depth gauges) on the chain should be filed to the correct height to control the cutting depth. A depth gauge tool is essential for this.
• Chain Sharpness: A sharp chain is essential for efficient cutting and safety. I recommend sharpening the chain every time you refuel the saw.
• Carburetor Adjustment: As discussed above, proper carburetor adjustment is crucial for optimal performance and engine longevity.

Safety Codes and Standards:

• Personal Protective Equipment (PPE): Always wear a helmet, eye protection, hearing protection, gloves, chainsaw chaps, and sturdy boots when operating a chainsaw.
• Kickback: Kickback is a sudden, uncontrolled movement of the chainsaw that can cause serious injury. Be aware of the kickback zone (the upper quadrant of the bar tip) and avoid cutting with this area.
• Safe Cutting Techniques: Use proper cutting techniques to avoid pinching the bar, binding the chain, and losing control of the saw.
• OSHA Regulations: Familiarize yourself with OSHA regulations regarding chainsaw safety and operation.

Tool Performance Metrics:

• Cutting Speed: Measured in board feet per hour (BF/hr) or cubic feet per hour (CF/hr).
• Fuel Consumption: Measured in gallons per hour (gal/hr).
• Vibration Levels: Measured in meters per second squared (m/s²). High vibration levels can lead to hand-arm vibration syndrome (HAVS).
• Noise Levels: Measured in decibels (dB). Prolonged exposure to high noise levels can cause hearing loss.

Original Research and Case Studies

Let me share a specific project I worked on a few years back. I was tasked with clearing a heavily overgrown area on a friend’s property. The area was filled with dense brush, small trees, and a lot of downed wood.

Project Details:

• Location: Rural property in the Pacific Northwest.
• Vegetation: Mix of alder, maple, and Douglas fir.
• Objective: Clear the area for a future garden.
• Tools Used: Stihl MS170 chainsaw, brush cutter, loppers, hand saw.

Challenges:

• Dense Brush: Made it difficult to maneuver the chainsaw.
• Downed Wood: Required careful cutting to avoid pinching the bar.
• Wet Conditions: Increased the risk of slipping and falling.

Technical Details:

• Chainsaw Chain: I used a low-kickback chain with a 3/8″ pitch and .050″ gauge.
• Fuel Mixture: I used a 50:1 fuel-oil mixture with a high-quality synthetic oil.
• Cutting Techniques: I used a combination of bore cutting, plunge cutting, and conventional felling techniques.
• Wood Disposal: I bucked the larger logs into firewood and chipped the smaller branches and brush.

Carburetor Issues Encountered:

The MS170 I was using had been sitting idle for a few months, and the carburetor was gummed up. It would start, but it would stall immediately.

Solution:

I cleaned the carburetor using the steps outlined above. I also replaced the fuel filter and spark plug. After cleaning and adjusting the carburetor, the saw ran perfectly.

Lessons Learned:

• Regular Maintenance is Key: Regularly cleaning and maintaining your chainsaw can prevent many common problems.
• Use Fresh Fuel: Stale fuel is a common cause of carburetor problems.
• Proper Cutting Techniques are Essential: Using proper cutting techniques can prevent accidents and make the job easier.

Practical Tips and Best Practices

Here are some additional tips and best practices I’ve learned over the years:

• Always Use Fresh Fuel: As I’ve mentioned before, stale fuel is a major cause of carburetor problems. Use fuel stabilizer to extend the life of your fuel.
• Clean Your Chainsaw Regularly: Clean your chainsaw after each use to remove sawdust, debris, and oil.
• Sharpen Your Chain Regularly: A sharp chain is essential for efficient cutting and safety.
• Store Your Chainsaw Properly: Store your chainsaw in a dry place, away from direct sunlight and extreme temperatures.
• Use the Right Bar and Chain: Using the right bar and chain for the job can improve cutting performance and reduce wear and tear on your chainsaw.
• Consider Using Ethanol-Free Fuel: Ethanol can damage small engines. Consider using ethanol-free fuel, especially if you don’t use your saw frequently.
• Invest in Quality Tools: Investing in quality tools, such as a good carburetor cleaning kit and a wood moisture meter, can make your job easier and more efficient.

Conclusion

The Stihl MS170 is a reliable chainsaw, but like any machine, it requires proper maintenance and care. By understanding the carburetor and following the troubleshooting and repair tips outlined in this guide, you can keep your MS170 running smoothly for years to come. Remember to prioritize safety, use fresh fuel, and clean your chainsaw regularly. And if all else fails, don’t be afraid to take it to a qualified mechanic. Happy cutting!

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