Stihl MS170 Carburetor Adjustment Tips (5 Pro Fixes)

The Stihl MS170 is a workhorse. I’ve seen these chainsaws outlast seasons of tough use, proving their mettle time and again. A properly tuned carburetor is the heart of a well-functioning chainsaw, and the MS170 is no exception. In my years of felling trees and processing firewood, I’ve learned a thing or two about keeping these machines humming. Let’s dive into some pro fixes to get your MS170 back in top shape, ensuring it continues to be the reliable tool you depend on.

Understanding the Stihl MS170 Carburetor

Before we get our hands dirty, it’s crucial to understand what the carburetor does and how it affects your chainsaw’s performance. The carburetor’s job is to mix air and fuel in the correct proportions to create a combustible mixture that powers the engine. When the carburetor is out of adjustment, you might experience a range of problems, from difficulty starting to poor idling and reduced cutting power.

The Carburetor’s Role in Chainsaw Performance

The carburetor is the unsung hero responsible for creating the perfect air-fuel mixture that keeps your Stihl MS170 running smoothly. It’s a delicate balance. Too much fuel, and the engine floods; too little, and it starves. The right mixture means optimal combustion, resulting in efficient power, clean exhaust, and a long engine life.

Common Symptoms of Carburetor Problems

Recognizing the signs of a carburetor issue is half the battle. Here are some common symptoms I’ve encountered:

Hard Starting: The saw takes multiple pulls to start, or won’t start at all.
Poor Idling: The engine stalls when idling or runs erratically.
Reduced Power: The saw lacks the power it used to have, especially when cutting through tougher wood.
Excessive Smoke: The exhaust is unusually smoky, indicating an overly rich fuel mixture.
Hesitation: The engine hesitates or stumbles when you accelerate.

Basic Carburetor Components

Understanding the basic components of the carburetor will make the adjustment process much easier. The main parts to be aware of are:

Idle Speed Screw (LA): Controls the engine’s idling speed.
High-Speed Jet (H): Regulates the fuel flow at high engine speeds.
Low-Speed Jet (L): Controls the fuel flow at low engine speeds.

Note: Some MS170 models have limiter caps on the H and L screws, restricting the adjustment range for emissions compliance. We’ll address this later.

Pro Fix 1: Initial Inspection and Cleaning

Before you start fiddling with the adjustment screws, it’s crucial to ensure the carburetor is clean and free of debris. A small piece of dirt can wreak havoc on the delicate internal mechanisms.

Inspecting the Air Filter

A dirty air filter restricts airflow, causing the engine to run rich. This is one of the most common causes of carburetor-related issues. Here’s how I check and clean mine:

Remove the Air Filter Cover: Usually held in place by a clip or screw.
Inspect the Filter: Look for dirt, dust, and debris.
Clean the Filter: If it’s a foam filter, wash it with warm, soapy water, rinse thoroughly, and let it air dry completely. If it’s a felt or paper filter, gently tap it to remove loose debris, or use compressed air to blow it clean from the inside out. Replace the air filter if it’s excessively dirty or damaged.

Checking the Fuel Filter

A clogged fuel filter restricts fuel flow, causing the engine to run lean. Here’s how to check it:

Locate the Fuel Filter: It’s usually inside the fuel tank, attached to the end of the fuel line.
Remove the Fuel Filter: Use a small hook or wire to pull it out of the tank.
Inspect the Filter: Look for dirt, debris, and clogs.
Replace the Filter: If it’s dirty or clogged, replace it with a new one. Fuel filters are inexpensive and should be replaced regularly.

Cleaning the Carburetor Exterior

Use a carburetor cleaner or compressed air to clean the exterior of the carburetor, paying particular attention to the linkage and moving parts. This helps ensure smooth operation.

Data Point: Air Filter Maintenance

Study: A study by the US Forest Service found that regular air filter maintenance can extend the life of small engines by up to 30%.
Insight: A clean air filter ensures optimal airflow, preventing a rich fuel mixture and reducing carbon buildup.

Pro Fix 2: Adjusting the Idle Speed Screw (LA)

The idle speed screw controls the engine’s idling speed. If the engine stalls when idling or runs too fast, adjusting this screw can often solve the problem.

Identifying the Idle Speed Screw

The idle speed screw (LA) is usually located on the side of the carburetor, near the throttle linkage. It’s typically a small screw with a slot for a flathead screwdriver. Consult your owner’s manual if you’re unsure of its location.

Adjusting the Idle Speed

Start the Engine: Let the engine warm up for a few minutes.
Locate the Idle Speed Screw: As mentioned above.
Adjust the Screw:
- If the engine stalls: Turn the screw clockwise to increase the idle speed.
- If the engine runs too fast: Turn the screw counterclockwise to decrease the idle speed.
Fine-Tune: Adjust the screw until the engine idles smoothly without stalling or running too fast. The ideal idle speed for the MS170 is around 2,800 RPM. You can use a tachometer to measure the RPM accurately.

Common Mistakes and How to Avoid Them

Adjusting the screw too much: Make small adjustments and allow the engine to respond before making further changes.
Confusing the idle speed screw with other screws: Always consult your owner’s manual to identify the correct screw.

Story Time: The Case of the Stalling Saw

I once had a client who was ready to throw out his MS170 because it kept stalling. After a quick inspection, I realized the idle speed was set too low. A simple adjustment of the idle speed screw, and the saw was back in action. He was amazed that such a small adjustment could make such a big difference. This is a common issue, and often the easiest to resolve.

Pro Fix 3: Adjusting the Low-Speed Jet (L)

The low-speed jet controls the fuel flow at low engine speeds. Adjusting this screw can improve starting, idling, and low-end throttle response.

Identifying the Low-Speed Jet

The low-speed jet (L) is usually located on the carburetor body, often near the high-speed jet. It’s typically a small screw with a slot for a flathead screwdriver.

Adjusting the Low-Speed Jet

Note: As mentioned earlier, many MS170 models have limiter caps on the H and L screws. If your saw has these caps, you may need to remove them to access the full adjustment range. I’ll cover this in more detail later.

Start the Engine: Let the engine warm up for a few minutes.
Locate the Low-Speed Jet: As mentioned above.
Adjust the Screw:
- Start with the factory setting: This is usually 1 turn out from fully closed.
- Fine-Tune: Turn the screw slowly clockwise (leaner) or counterclockwise (richer) in small increments (1/8 of a turn at a time).
- Listen to the Engine: Listen for changes in the engine’s sound. The goal is to find the setting where the engine idles smoothly and responds quickly to the throttle.
The “Lean Best Idle” Method: This is a common technique for adjusting the low-speed jet. Turn the screw clockwise (leaner) until the engine starts to stumble or stall. Then, turn the screw counterclockwise (richer) until the engine runs smoothly again. This is the “lean best idle” setting.

Practical Example: Improving Throttle Response

I had an MS170 that was sluggish when I hit the throttle. It would hesitate before revving up. By carefully adjusting the low-speed jet, I was able to eliminate the hesitation and improve the saw’s throttle response. The key was making small adjustments and listening to the engine’s feedback.

Data Point: Carburetor Adjustment and Fuel Efficiency

Study: A study by the Society of Automotive Engineers (SAE) found that properly adjusted carburetors can improve fuel efficiency by up to 15%.
Insight: A well-tuned carburetor ensures optimal combustion, reducing fuel waste and emissions.

Pro Fix 4: Adjusting the High-Speed Jet (H)

The high-speed jet controls the fuel flow at high engine speeds. Adjusting this screw can improve the saw’s power and performance when cutting through wood.

Identifying the High-Speed Jet

The high-speed jet (H) is usually located on the carburetor body, often near the low-speed jet. It’s typically a small screw with a slot for a flathead screwdriver.

Adjusting the High-Speed Jet

Note: As with the low-speed jet, many MS170 models have limiter caps on the H screw. You may need to remove these caps to access the full adjustment range.

Start the Engine: Let the engine warm up for a few minutes.
Locate the High-Speed Jet: As mentioned above.
Adjust the Screw:
- Start with the factory setting: This is usually 1 turn out from fully closed.
- Make a Test Cut: Cut through a piece of wood and listen to the engine.
- Adjust Based on Performance:
  - If the engine bogs down or lacks power: Turn the screw counterclockwise (richer) in small increments.
  - If the engine screams or runs too lean: Turn the screw clockwise (leaner) in small increments.
The “Four-Stroking” Method: This is a common technique for adjusting the high-speed jet. When the engine is running at full throttle, it should “four-stroke” slightly, meaning it should sound like it’s missing a beat every few cycles. This indicates that the engine is getting enough fuel. If the engine is screaming or doesn’t four-stroke, it’s running too lean.

Understanding the Risks of a Lean Mixture

Running the engine too lean (not enough fuel) can cause serious damage, including overheating, piston seizure, and engine failure. Always err on the side of caution and run the engine slightly richer than lean.

Story Time: The Importance of Proper High-Speed Adjustment

I once worked on a logging crew where one of the guys kept burning up his chainsaws. He was constantly running them too lean, trying to get every last bit of power out of them. After a few costly repairs, he finally learned the importance of proper high-speed adjustment. It’s better to sacrifice a little bit of power than to destroy your engine.

Pro Fix 5: Removing Limiter Caps (If Necessary)

As mentioned earlier, many Stihl MS170 models have limiter caps on the H and L screws to restrict the adjustment range for emissions compliance. If you need to access the full adjustment range, you can remove these caps.

Identifying Limiter Caps

Limiter caps are small plastic or metal caps that are pressed onto the adjustment screws. They prevent you from turning the screws beyond a certain point.

Removing Limiter Caps

Locate the Caps: Identify the limiter caps on the H and L screws.
Use a Small Screwdriver or Pick: Gently pry the caps off the screws. Be careful not to damage the screws or the carburetor body.
Remove the Caps: Once the caps are loose, you can remove them with your fingers or pliers.

Considerations Before Removing Limiter Caps

Emissions Compliance: Removing the limiter caps may void the warranty and may not be legal in some areas.
Risk of Over-Adjustment: Without the limiter caps, you can easily over-adjust the carburetor, which can damage the engine.

Case Study: Restoring an Older MS170

I recently restored an older MS170 that had been sitting in a shed for years. The carburetor was completely gummed up, and the engine wouldn’t start. After cleaning the carburetor and removing the limiter caps, I was able to adjust the H and L screws to get the engine running smoothly again. The saw now runs like new, thanks to a little bit of elbow grease and some carburetor know-how.

Additional Tips and Tricks

Here are a few additional tips and tricks I’ve learned over the years:

Use Fresh Fuel: Old fuel can cause carburetor problems. Always use fresh fuel that is less than 30 days old.
Use the Correct Fuel Mixture: The MS170 requires a 50:1 fuel-to-oil mixture. Use a high-quality two-stroke oil and mix the fuel according to the manufacturer’s instructions.
Check the Spark Plug: A fouled or damaged spark plug can cause starting and performance problems. Replace the spark plug if it’s dirty or damaged.
Inspect the Fuel Lines: Cracked or damaged fuel lines can cause fuel leaks and performance problems. Replace the fuel lines if they’re cracked or damaged.
Consider a Carburetor Rebuild Kit: If you’ve tried all of the above fixes and the carburetor is still not working properly, you may need to rebuild it. Carburetor rebuild kits are available online and at most small engine repair shops.

Tool Selection for Carburetor Adjustment

Having the right tools makes the job much easier. Here are a few essential tools:

Small Flathead Screwdriver: For adjusting the H, L, and LA screws.
Carburetor Cleaner: For cleaning the carburetor.
Compressed Air: For blowing out debris.
Tachometer: For measuring the engine’s RPM.
Pliers: For removing fuel lines and other small parts.
Pick Set: For removing limiter caps and other small parts.

Safety Considerations

Work in a Well-Ventilated Area: Carburetor cleaner and gasoline fumes can be harmful.
Wear Safety Glasses: To protect your eyes from debris.
Disconnect the Spark Plug Wire: To prevent accidental starting.
Be Careful When Working with Fuel: Gasoline is flammable.

Conclusion

Adjusting the carburetor on your Stihl MS170 can seem daunting, but with a little patience and the right knowledge, it’s a manageable task. Remember to start with the basics, like cleaning the air filter and fuel filter, before diving into the adjustment screws. And always make small adjustments and listen to the engine’s feedback. With these pro fixes, you can keep your MS170 running smoothly for years to come, ensuring it remains the reliable workhorse you depend on. Now, go get that wood cut!