MS170 Stihl Chainsaw Troubleshooting (5 Expert Fixes)

“Aha!” It hit me like a rogue branch to the face – the MS170 Stihl chainsaw, my trusty workhorse, sputtered and died mid-cut. The familiar roar silenced, replaced by an irritating click-click-click as I tried to restart it. Been there? I bet you have. That’s why I’ve compiled this guide, drawing from years of experience felling trees and processing firewood, to help you troubleshoot your MS170 Stihl chainsaw. These five expert fixes will get you back to cutting in no time.

MS170 Stihl Chainsaw Troubleshooting: 5 Expert Fixes

The MS170 Stihl chainsaw is a popular choice for homeowners and light-duty users due to its affordability and ease of use. However, like any piece of machinery, it can experience problems. I’ve seen everything from simple fuel issues to more complex ignition problems in my time. Let’s dive into the most common issues and how to fix them.

1. Fuel System Issues: The Heart of the Matter

A chainsaw’s fuel system is its lifeline. If it’s not functioning correctly, your saw won’t run. This is often the first place I check when troubleshooting.

Dirty Air Filter:

A clogged air filter restricts airflow, leading to a lean fuel mixture and poor performance. This is the most common culprit I’ve encountered.

How to Check: Remove the air filter cover (usually secured by a knob or clip). Examine the filter element. If it’s visibly dirty, it needs cleaning or replacement.
How to Clean: Tap the filter gently to remove loose debris. For more stubborn dirt, wash the filter with warm, soapy water. Allow it to dry completely before reinstalling. I often use compressed air to speed up the drying process, but be careful not to damage the filter.
Replacement: Air filters are relatively inexpensive. If cleaning doesn’t restore proper airflow, replace the filter with a new one. Stihl recommends replacing the air filter annually, or more frequently if you use the saw in dusty conditions.
Pro Tip: I always carry a spare air filter in my toolkit. It’s a quick and easy fix that can save you a lot of downtime.

Stale Fuel:

Gasoline degrades over time, especially when mixed with oil for two-stroke engines. Stale fuel can cause starting problems and poor engine performance. I’ve learned the hard way that leaving fuel in the saw for extended periods is a recipe for trouble.

How to Check: If the fuel has been sitting in the saw for more than a month, it’s likely stale. The fuel may also have a varnish-like smell.
How to Fix: Drain the old fuel from the tank. Dispose of it properly (check your local regulations). Refill the tank with fresh, properly mixed fuel. I always use Stihl’s pre-mixed fuel or mix my own using a high-quality two-stroke oil at the recommended ratio (typically 50:1).
Prevention: To prevent fuel from going stale, use a fuel stabilizer. Add it to the fuel when you mix it or before storing the saw for an extended period. I prefer using a fuel stabilizer even with pre-mixed fuel for long-term storage.

Clogged Fuel Filter:

The fuel filter prevents debris from entering the carburetor. A clogged filter restricts fuel flow, causing starting problems or stalling.

How to Check: The fuel filter is located inside the fuel tank, attached to the fuel line. Use a piece of wire or a fuel filter removal tool to carefully pull the filter out of the tank. Examine the filter for dirt and debris.
How to Clean: If the filter is only slightly dirty, you can try cleaning it with carburetor cleaner.
Replacement: If the filter is heavily clogged or damaged, replace it with a new one. Fuel filters are inexpensive and readily available. I recommend replacing the fuel filter annually as part of routine maintenance.
Pro Tip: When replacing the fuel filter, make sure to use a new piece of fuel line if the old one is cracked or brittle.

Carburetor Issues:

The carburetor mixes air and fuel in the correct proportions for combustion. A dirty or malfunctioning carburetor can cause a variety of problems, including starting difficulties, poor idling, and stalling.

How to Check: If you’ve ruled out other fuel system issues, the carburetor is likely the culprit. Symptoms include the engine running rough, stalling at idle, or not reaching full RPM.
How to Fix:
- Carburetor Cleaning: The simplest fix is to try cleaning the carburetor with carburetor cleaner. Remove the air filter and spray carburetor cleaner into the carburetor throat while the engine is running (if possible).
- Carburetor Adjustment: The MS170 has adjustable carburetor screws (H and L) for high and low-speed settings. Use a small screwdriver to adjust these screws. The factory settings are a good starting point, but you may need to fine-tune them to achieve optimal performance. Consult your owner’s manual for the correct adjustment procedure.
- Carburetor Rebuild: If cleaning and adjustment don’t solve the problem, you may need to rebuild the carburetor. This involves disassembling the carburetor, cleaning all the parts, and replacing any worn or damaged components. Carburetor rebuild kits are available for the MS170. This is a more advanced repair, and I recommend having a professional do it if you’re not comfortable working on small engines.
Pro Tip: When adjusting the carburetor, make small adjustments (1/8 turn) at a time. Allow the engine to run for a few minutes after each adjustment to see how it responds.

Takeaway: Addressing fuel system issues promptly is crucial for maintaining your chainsaw’s performance. Start with the easiest fixes (air filter, fuel) and work your way to more complex problems (carburetor).

2. Ignition System Problems: Sparking the Solution

The ignition system provides the spark that ignites the fuel-air mixture in the engine. If the ignition system is faulty, the engine won’t start.

Faulty Spark Plug:

The spark plug ignites the fuel-air mixture. A fouled, cracked, or improperly gapped spark plug can prevent the engine from starting.

How to Check: Remove the spark plug using a spark plug wrench. Examine the spark plug for cracks, carbon buildup, or a worn electrode. The spark plug should be clean and dry.
How to Clean: If the spark plug is only slightly fouled, you can try cleaning it with a wire brush or spark plug cleaner.
How to Gap: Check the spark plug gap using a spark plug gapping tool. The correct gap for the MS170 is typically 0.5 mm (0.020 inches). Adjust the gap by carefully bending the electrode.
Replacement: If the spark plug is cracked or heavily fouled, replace it with a new one. Use the correct spark plug for the MS170 (typically a Bosch WSR6F or NGK BPMR7A). I always keep a spare spark plug on hand.
Pro Tip: When installing a new spark plug, tighten it to the correct torque specification (usually around 15-20 Nm). Overtightening can damage the cylinder head.

Ignition Coil Issues:

The ignition coil generates the high-voltage spark that ignites the fuel-air mixture. A faulty ignition coil can prevent the engine from starting.

How to Check: This requires a multimeter. Disconnect the spark plug wire from the spark plug. Connect the multimeter to the spark plug wire and ground. Pull the starter cord and check for a spark. If there is no spark, the ignition coil may be faulty.
How to Test with an Air Gap: Another test is to remove the spark plug and insert it into the spark plug boot. Hold the spark plug against the engine block (ground). Pull the starter cord and observe the spark plug gap. A strong, blue spark indicates a good ignition coil. A weak, yellow spark or no spark indicates a faulty ignition coil.
Replacement: Replacing the ignition coil is a more complex repair. It involves removing the flywheel and disconnecting the wiring. I recommend having a professional do this repair if you’re not comfortable working on small engines.
Pro Tip: Before replacing the ignition coil, check the wiring connections to make sure they are clean and secure.

Flywheel Key Shear:

The flywheel key is a small metal piece that connects the flywheel to the crankshaft. If the flywheel key shears, the timing will be off, and the engine won’t start. This can happen if the chainsaw experiences a sudden impact, like hitting a rock or a nail.

How to Check: Remove the spark plug and insert a piston stop tool into the cylinder. This will prevent the piston from moving. Use a socket wrench to remove the flywheel nut. Remove the flywheel. Examine the flywheel key for damage. If the key is sheared, it will be broken or bent.
Replacement: Replace the sheared flywheel key with a new one. Install the flywheel and tighten the flywheel nut to the correct torque specification.
Pro Tip: When installing the flywheel, make sure the keyway in the flywheel aligns with the key on the crankshaft.

Takeaway: A strong spark is essential for combustion. If you’re having starting problems, check the spark plug first. If the spark plug is good, investigate the ignition coil and flywheel key.

3. Compression Problems: The Power Within

Compression is the measure of how tightly the air-fuel mixture is compressed in the cylinder. Low compression can prevent the engine from starting or cause it to run poorly.

Worn Piston Rings:

Piston rings seal the combustion chamber and prevent compression from escaping. Worn piston rings can cause low compression.

How to Check: The most accurate way to check compression is with a compression tester. Remove the spark plug and screw the compression tester into the spark plug hole. Pull the starter cord several times and note the reading on the gauge. A healthy MS170 should have a compression reading of at least 120 PSI.
Alternative Check: If you don’t have a compression tester, you can perform a “finger test.” Remove the spark plug and place your finger over the spark plug hole. Pull the starter cord. You should feel a strong pressure pushing against your finger. If you feel little or no pressure, the compression is likely low.
How to Fix: Replacing the piston rings is a major repair that involves disassembling the engine. I recommend having a professional do this repair.
Pro Tip: Low compression can also be caused by a worn cylinder or piston. If the cylinder or piston is damaged, the engine may need to be replaced.

Cylinder Damage:

Scratches or scoring on the cylinder wall can also cause low compression.

How to Check: Remove the muffler and examine the cylinder wall through the exhaust port. Look for scratches, scoring, or other damage.
How to Fix: Cylinder damage often requires replacing the cylinder or the entire engine. This is a costly repair, and it may be more economical to purchase a new chainsaw.

Loose Cylinder Head:

A loose cylinder head can cause compression to leak.

How to Check: Inspect the cylinder head bolts to make sure they are tight.
How to Fix: Tighten the cylinder head bolts to the correct torque specification. Consult your owner’s manual for the correct torque specification.

Takeaway: Compression is critical for engine performance. If you suspect low compression, have the engine tested by a professional.

4. Bar and Chain Issues: Keeping the Cut Sharp

While not directly related to engine performance, a dull or improperly maintained bar and chain can make the chainsaw difficult to use and put extra strain on the engine.

Dull Chain:

A dull chain requires more force to cut, which can bog down the engine and make the saw difficult to control. I can always tell when my chain is dull – the saw produces fine sawdust instead of chips, and it takes much longer to make a cut.

How to Check: Examine the cutting teeth on the chain. If the teeth are rounded or damaged, the chain needs sharpening.
How to Sharpen: You can sharpen the chain using a chainsaw file or a chain grinder. I prefer using a chainsaw file because it’s more portable and allows me to sharpen the chain in the field. Make sure to use the correct file size and angle for your chain.
Pro Tip: I always keep a sharpening kit with me when I’m using my chainsaw. It includes a chainsaw file, a depth gauge tool, and a file guide.

Improper Chain Tension:

A chain that is too loose can come off the bar, while a chain that is too tight can bind and overheat.

How to Check: Lift the chain in the middle of the bar. The chain should lift about 1/8 inch.
How to Adjust: Loosen the bar nuts and use the chain tensioning screw to adjust the chain tension. Tighten the bar nuts securely.
Pro Tip: Adjust the chain tension frequently, especially when the chain is new.

Worn Bar:

A worn bar can cause the chain to bind and overheat.

How to Check: Examine the bar for wear, especially along the rails. If the rails are worn or damaged, the bar needs to be replaced.
How to Fix: Replace the bar with a new one. Make sure to use the correct bar length and pitch for your chainsaw.
Pro Tip: Rotate the bar periodically to even out the wear.

Takeaway: A sharp, properly tensioned chain and a well-maintained bar are essential for safe and efficient cutting.

5. Exhaust System Problems: Letting the Engine Breathe

The exhaust system removes exhaust gases from the engine. A clogged or damaged exhaust system can restrict airflow and cause the engine to run poorly.

Clogged Spark Arrestor:

The spark arrestor prevents sparks from escaping the exhaust system and potentially starting a fire. A clogged spark arrestor can restrict exhaust flow and cause the engine to overheat.

How to Check: The spark arrestor is located on the muffler. Remove the spark arrestor screen and examine it for carbon buildup.
How to Clean: Clean the spark arrestor screen with a wire brush. You can also soak it in carburetor cleaner to remove stubborn carbon deposits.
Replacement: If the spark arrestor screen is damaged, replace it with a new one. I clean mine every 25 hours of use, especially during dry seasons.
Pro Tip: Some areas require spark arrestors to be installed on chainsaws used in forests or near flammable materials. Check your local regulations.

Damaged Muffler:

A damaged muffler can leak exhaust gases and reduce engine performance.

How to Check: Inspect the muffler for cracks, holes, or other damage.
How to Fix: Replace the muffler with a new one. A damaged muffler can also be a fire hazard.

Takeaway: A properly functioning exhaust system is essential for engine performance and safety.

Preventative Maintenance: Avoiding Problems Before They Start

The best way to troubleshoot your MS170 Stihl chainsaw is to prevent problems from happening in the first place. Regular maintenance is key to keeping your saw running smoothly and extending its lifespan.

Clean the air filter regularly: As mentioned earlier, a clean air filter is crucial for proper engine performance.
Use fresh fuel: Avoid using stale fuel. Use a fuel stabilizer if you’re storing the saw for an extended period.
Sharpen the chain regularly: A sharp chain makes cutting easier and reduces strain on the engine.
Check the chain tension: Adjust the chain tension frequently.
Clean the bar and chain: Remove debris from the bar and chain after each use.
Lubricate the bar and chain: Use a high-quality bar and chain oil to keep the chain lubricated.
Inspect the spark plug: Check the spark plug periodically and replace it as needed.
Clean the spark arrestor: Clean the spark arrestor screen regularly.
Store the saw properly: Store the saw in a clean, dry place. Drain the fuel tank before storing the saw for an extended period.

Safety First: Always Prioritize Safety

Chainsaws are powerful tools that can be dangerous if used improperly. Always follow these safety precautions:

Wear appropriate safety gear: Always wear safety glasses, hearing protection, gloves, and sturdy boots when using a chainsaw. I also recommend wearing chaps or chainsaw-resistant pants.
Read the owner’s manual: Familiarize yourself with the chainsaw’s operating instructions and safety precautions.
Inspect the saw before each use: Check for loose parts, damaged components, and leaks.
Start the saw on the ground: Never start the saw in the air or on your leg.
Use a firm grip: Hold the saw firmly with both hands.
Keep a safe distance: Keep bystanders and pets at least 50 feet away from the cutting area.
Be aware of your surroundings: Watch out for obstacles, such as rocks, roots, and branches.
Never cut above your head: Cutting above your head is extremely dangerous.
Never cut with a dull chain: A dull chain is more likely to kickback.
Stop the saw before moving: Always stop the saw and engage the chain brake before moving from one cutting area to another.
Never use a chainsaw when you are tired or under the influence of drugs or alcohol: Chainsaw operation requires focus and coordination.

Conclusion: Back to Cutting with Confidence

Troubleshooting your MS170 Stihl chainsaw doesn’t have to be a daunting task. By following these five expert fixes and practicing preventative maintenance, you can keep your saw running smoothly and efficiently for years to come. Remember to prioritize safety and always consult a professional if you’re not comfortable performing a repair yourself. Now get back out there and cut some wood!

MS170 Stihl Chainsaw Troubleshooting (5 Expert Fixes)