Chainsaw Will Only Run on Choke (MS 261 CM Fuel & Carb Fix Tips)

Understanding the Choke’s Role

The choke is a crucial component in a chainsaw’s carburetor system. Its primary function is to restrict airflow, enriching the fuel-air mixture for cold starts. When the engine is cold, fuel doesn’t vaporize as readily, necessitating a richer mixture to ignite. Once the engine warms up, the choke should be disengaged, allowing the engine to run on the standard fuel-air ratio.

Why Your Chainsaw Only Runs on Choke

When a chainsaw only runs on choke, it indicates a lean fuel mixture. This means the engine isn’t receiving enough fuel relative to the amount of air it’s taking in. The choke compensates for this by restricting air, effectively increasing the fuel concentration. Several factors can contribute to a lean fuel mixture:

• Clogged Fuel Filter: A restricted fuel filter limits the amount of fuel reaching the carburetor.
• Dirty Carburetor: Deposits and debris within the carburetor can obstruct fuel flow.
• Fuel Line Issues: Cracks, leaks, or blockages in the fuel line can impede fuel delivery.
• Air Leaks: Air leaks in the intake system can dilute the fuel mixture.
• Incorrect Carburetor Adjustment: The carburetor’s high and low-speed jets might be improperly adjusted.

Safety First: Preparing for the Repair

Before diving into any chainsaw repair, prioritizing safety is paramount. Always wear appropriate personal protective equipment (PPE), including:

• Safety Glasses: To protect your eyes from debris.
• Gloves: To provide a secure grip and protect your hands.
• Hearing Protection: Chainsaws are loud; protect your hearing.
• Long Sleeves and Pants: To minimize the risk of cuts and abrasions.
• Steel-Toed Boots: To protect your feet from falling objects.

Additional Safety Precautions:

• Disconnect the Spark Plug: This prevents accidental starting during the repair process.
• Work in a Well-Ventilated Area: Gasoline fumes are hazardous.
• Have a Fire Extinguisher Nearby: In case of fuel spillage or accidental ignition.
• Refer to the Owner’s Manual: Consult your Stihl MS 261 CM owner’s manual for specific safety guidelines and procedures.

Step-by-Step Troubleshooting and Repair Guide

Now, let’s get our hands dirty and diagnose the root cause of the problem. I will share my personal experience, for example, I had a similar issue with my MS 261 after leaving it stored for the winter. I follow similar steps to troubleshoot and fix it.

1. Fuel System Inspection

The fuel system is the first place to investigate when dealing with a lean fuel mixture.

a. Checking the Fuel Tank and Vent

Procedure:

1. Inspect the Fuel Tank: Ensure the fuel tank is clean and free from debris. Look for any signs of contamination, such as water or sediment.
2. Check the Fuel Vent: The fuel vent allows air to enter the tank, preventing a vacuum from forming as fuel is consumed. A clogged vent can restrict fuel flow. Locate the fuel vent (usually on top of the fuel tank) and clean it with compressed air or a small wire.

My Experience: I once encountered a situation where a small insect had crawled into the fuel vent, completely blocking it. Cleaning the vent immediately resolved the issue.

Takeaway: A simple check of the fuel tank and vent can often reveal obvious problems.

b. Examining the Fuel Filter

Procedure:

1. Locate the Fuel Filter: The fuel filter is typically located inside the fuel tank, attached to the end of the fuel line.
2. Remove the Fuel Filter: Use a small hook or wire to carefully pull the fuel filter out of the tank.
3. Inspect the Fuel Filter: Examine the filter for clogs, debris, or damage.
4. Clean or Replace the Fuel Filter: If the filter is clogged, try cleaning it with carburetor cleaner. If it’s heavily contaminated or damaged, replace it with a new filter. I usually replace it because it’s inexpensive.

Measurements: The fuel filter for the Stihl MS 261 CM typically has a pore size of around 20-40 microns to effectively filter out contaminants.

Tool List:

• Small hook or wire
• Carburetor cleaner
• New fuel filter (Stihl part number: 0000 350 3500)

Takeaway: A clogged fuel filter is a common cause of fuel starvation. Regular filter replacement (every 3-6 months, depending on usage) is a good preventative measure.

c. Assessing the Fuel Line

Procedure:

1. Inspect the Fuel Line: Carefully examine the fuel line from the fuel tank to the carburetor for cracks, leaks, or kinks.
2. Check for Blockages: Disconnect the fuel line from the carburetor and the fuel tank. Use compressed air or a thin wire to check for any blockages.
3. Replace the Fuel Line: If you find any damage or blockages, replace the fuel line with a new one.

Real-World Example: I once had a fuel line that had become brittle and cracked due to age. The cracks were small enough to be difficult to see, but they were allowing air to enter the fuel system, causing a lean mixture. Replacing the fuel line solved the problem.

Takeaway: Fuel lines deteriorate over time. Regular inspection and replacement (every 1-2 years) are essential.

2. Carburetor Cleaning and Adjustment

The carburetor is a complex component responsible for mixing fuel and air in the correct proportions. A dirty or improperly adjusted carburetor can cause a variety of problems, including a lean fuel mixture.

a. Carburetor Cleaning

Procedure:

1. Remove the Air Filter: Access the carburetor by removing the air filter cover and the air filter itself.
2. Disconnect Fuel Lines and Linkages: Carefully disconnect the fuel lines and any linkages connected to the carburetor. Take pictures before disconnecting anything to ensure you can reassemble it correctly.
3. Remove the Carburetor: Unbolt the carburetor from the engine.
4. Disassemble the Carburetor (Optional): For a thorough cleaning, you can disassemble the carburetor. However, this requires some mechanical skill and a good understanding of carburetor components. If you’re not comfortable disassembling it, you can clean it without taking it apart.
5. Clean the Carburetor: Use carburetor cleaner to thoroughly clean all parts of the carburetor, paying particular attention to the jets and passages. Use compressed air to blow out any remaining debris.
6. Reassemble the Carburetor: If you disassembled the carburetor, carefully reassemble it, ensuring all parts are in their correct positions.
7. Reinstall the Carburetor: Bolt the carburetor back onto the engine, reconnect the fuel lines and linkages, and reinstall the air filter.

Tool List:

• Screwdrivers
• Wrenches
• Carburetor cleaner
• Compressed air
• Carburetor rebuild kit (optional)

Best Practice: When disassembling a carburetor, work on a clean surface and keep track of all the small parts. A carburetor rebuild kit can be useful, as it contains new gaskets and seals.

Case Study: A local logging company was experiencing frequent carburetor issues with their chainsaws. After implementing a regular carburetor cleaning schedule (every 50 hours of operation), they saw a significant reduction in chainsaw downtime.

Takeaway: Regular carburetor cleaning is crucial for maintaining optimal engine performance.

b. Carburetor Adjustment

After cleaning the carburetor, you may need to adjust the high (H) and low (L) speed jets to achieve the correct fuel-air mixture. The Stihl MS 261 CM carburetor typically has two adjustment screws labeled “H” and “L.”

Procedure:

1. Locate the Adjustment Screws: Find the “H” and “L” adjustment screws on the carburetor.
2. Initial Settings: Refer to your Stihl MS 261 CM owner’s manual for the recommended initial settings for the “H” and “L” screws. These settings are usually expressed as the number of turns out from the fully closed position. As a starting point, I usually start with 1 turn for both H and L.
3. Start the Engine: Start the engine and let it warm up to operating temperature.
4. Adjust the Low-Speed (L) Screw: Adjust the “L” screw to achieve a smooth and stable idle. Turn the screw clockwise to lean the mixture (reduce fuel) and counterclockwise to richen the mixture (increase fuel). Aim for the highest idle speed without the chain spinning.
5. Adjust the High-Speed (H) Screw: Adjust the “H” screw to achieve optimal high-speed performance. This adjustment is best done with the chainsaw under load (e.g., cutting a log). Turn the screw clockwise to lean the mixture and counterclockwise to richen the mixture. Listen for a smooth, powerful engine sound. Avoid running the engine too lean, as this can cause overheating and damage.
6. Fine-Tune the Idle Speed: After adjusting the “H” and “L” screws, you may need to fine-tune the idle speed using the idle speed adjustment screw (usually labeled “LA”).

Important Considerations:

• Lean vs. Rich: A lean mixture (too much air) can cause overheating and engine damage. A rich mixture (too much fuel) can cause poor performance and excessive smoke.
• Listen to the Engine: Pay close attention to the engine sound when adjusting the carburetor. A healthy engine should sound smooth and powerful.
• Refer to the Manual: Always refer to your Stihl MS 261 CM owner’s manual for specific carburetor adjustment instructions and recommendations.

My Insight: Carburetor adjustment is often a trial-and-error process. It takes practice to develop a feel for the correct settings. Don’t be afraid to experiment, but always start with the recommended initial settings and make small adjustments.

Takeaway: Proper carburetor adjustment is crucial for optimal engine performance and longevity.

3. Identifying and Addressing Air Leaks

Air leaks can disrupt the fuel-air mixture, leading to a lean condition and causing the chainsaw to only run on choke.

a. Inspecting the Intake Boot

Procedure:

1. Locate the Intake Boot: The intake boot is the rubber or plastic connector between the carburetor and the engine cylinder.
2. Inspect for Cracks and Damage: Carefully examine the intake boot for cracks, tears, or other damage.
3. Check for Loose Connections: Ensure the intake boot is securely connected to the carburetor and the engine cylinder.

My Experience: I once had an intake boot that had developed a small crack on the underside. The crack was difficult to see, but it was allowing air to enter the engine, causing a lean mixture. Replacing the intake boot resolved the issue.

Takeaway: The intake boot is a common source of air leaks. Regular inspection and replacement (every 2-3 years) are recommended.

b. Checking the Crankshaft Seals

Procedure:

1. Locate the Crankshaft Seals: The crankshaft seals are located on either side of the engine crankshaft.
2. Inspect for Leaks: Look for signs of oil leakage around the crankshaft seals.
3. Perform a Pressure Test: A pressure test can help identify leaky crankshaft seals. This requires specialized equipment and should be performed by a qualified technician.

Expert Advice: Crankshaft seal replacement is a more complex repair that may require disassembling the engine. If you’re not comfortable performing this repair yourself, it’s best to take your chainsaw to a qualified service technician.

Takeaway: Leaky crankshaft seals can cause a lean mixture and other engine problems.

c. Examining the Cylinder Base Gasket

Procedure:

1. Locate the Cylinder Base Gasket: The cylinder base gasket is located between the engine cylinder and the crankcase.
2. Inspect for Leaks: Look for signs of oil leakage around the cylinder base gasket.

Additional Note: Replacing the cylinder base gasket requires disassembling the engine. This is a more complex repair that should be performed by a qualified technician.

Takeaway: A damaged cylinder base gasket can cause air leaks and other engine problems.

4. Spark Plug Inspection and Replacement

While not directly related to a lean fuel mixture, a faulty spark plug can contribute to poor engine performance and make it difficult to start or run the chainsaw.

Procedure:

1. Locate the Spark Plug: The spark plug is located on the engine cylinder head.
2. Remove the Spark Plug: Use a spark plug wrench to remove the spark plug.
3. Inspect the Spark Plug: Examine the spark plug for signs of wear, damage, or fouling. The electrode should be clean and have a sharp edge. The insulator should be free from cracks or chips.
4. Clean or Replace the Spark Plug: If the spark plug is fouled, try cleaning it with a wire brush. If it’s worn or damaged, replace it with a new spark plug.

Measurements: The spark plug gap for the Stihl MS 261 CM is typically 0.5 mm (0.020 inches).

Tool List:

• Spark plug wrench
• Wire brush
• Spark plug gap tool
• New spark plug (Stihl part number: Bosch WSR 6 F or NGK BPMR6A)

Takeaway: A clean and properly gapped spark plug is essential for reliable engine performance. Regular spark plug replacement (every 100 hours of operation) is recommended.

5. Electronic Control System (M-Tronic) Considerations

The Stihl MS 261 CM features the M-Tronic engine management system, which automatically adjusts the fuel-air mixture for optimal performance. While this system is generally reliable, it can sometimes malfunction, leading to engine problems.

a. Diagnosing M-Tronic Issues

Diagnosing M-Tronic issues can be challenging, as it often requires specialized diagnostic tools and expertise.

Potential Causes:

• Sensor Malfunctions: The M-Tronic system relies on various sensors to monitor engine conditions. A faulty sensor can provide incorrect data, leading to improper fuel-air mixture adjustments.
• Software Glitches: The M-Tronic system is controlled by software. A software glitch can cause the system to malfunction.
• Wiring Problems: Damaged or corroded wiring can disrupt the M-Tronic system’s operation.

My Recommendation: If you suspect an M-Tronic issue, it’s best to take your chainsaw to a qualified Stihl service technician. They have the necessary diagnostic tools and expertise to identify and resolve M-Tronic problems.

Takeaway: The M-Tronic system is a complex component that requires specialized knowledge for diagnosis and repair.

6. Other Potential Issues

While the above steps cover the most common causes of a chainsaw only running on choke, there are a few other potential issues to consider.

a. Clogged Muffler

A clogged muffler can restrict exhaust flow, leading to poor engine performance.

Procedure:

1. Remove the Muffler: Remove the muffler from the engine.
2. Inspect the Muffler: Examine the muffler for carbon buildup or other obstructions.
3. Clean the Muffler: Clean the muffler with a wire brush or carburetor cleaner.

Takeaway: A clogged muffler can significantly impact engine performance.

b. Damaged Piston or Cylinder

In rare cases, a damaged piston or cylinder can cause a lean mixture.

Symptoms:

• Low compression
• Excessive engine noise
• Poor engine performance

Expert Advice: Piston and cylinder damage typically requires a complete engine rebuild, which is a complex and costly repair.

Takeaway: Piston and cylinder damage is a serious issue that requires professional attention.

Preventative Maintenance for Long-Term Reliability

Preventative maintenance is key to keeping your Stihl MS 261 CM chainsaw running smoothly and reliably for years to come.

a. Regular Cleaning

• Air Filter: Clean the air filter after each use.
• Carburetor: Clean the carburetor every 50 hours of operation.
• Cooling Fins: Clean the cooling fins on the engine cylinder regularly to prevent overheating.
• Sawdust Removal: Remove sawdust and debris from the chainsaw after each use.

b. Proper Fuel and Oil

• Use Fresh Fuel: Use fresh, high-quality gasoline with the correct octane rating.
• Use Two-Stroke Oil: Use a high-quality two-stroke oil specifically designed for chainsaws.
• Mix Fuel and Oil Correctly: Follow the manufacturer’s recommendations for the correct fuel-oil mixture ratio (typically 50:1).

c. Chain Maintenance

• Sharpen the Chain Regularly: A sharp chain cuts more efficiently and reduces strain on the engine.
• Lubricate the Chain: Use a high-quality chain oil to keep the chain lubricated.
• Check Chain Tension: Regularly check and adjust the chain tension.

d. Storage

• Drain the Fuel Tank: Before storing the chainsaw for an extended period, drain the fuel tank to prevent fuel degradation.
• Fog the Engine: Fog the engine with storage oil to protect it from corrosion.
• Store in a Dry Place: Store the chainsaw in a dry, protected place.

Actionable Metrics:

• Chain Sharpening Frequency: Sharpen the chain after every 2-3 hours of cutting.
• Air Filter Cleaning Frequency: Clean the air filter after each use.
• Carburetor Cleaning Frequency: Clean the carburetor every 50 hours of operation.
• Fuel Filter Replacement Frequency: Replace the fuel filter every 3-6 months.
• Spark Plug Replacement Frequency: Replace the spark plug every 100 hours of operation.

By following these preventative maintenance tips, you can significantly extend the life of your Stihl MS 261 CM chainsaw and avoid many common problems.

Final Thoughts

Troubleshooting a chainsaw that only runs on choke can be a challenging but rewarding experience. By systematically following the steps outlined in this guide, you can diagnose and fix the problem, saving yourself time and money. Remember to prioritize safety, refer to your owner’s manual, and don’t hesitate to seek professional help when needed. With a little patience and perseverance, you can get your Stihl MS 261 CM chainsaw back in top working condition and tackle those wood processing projects with confidence. Good luck, and happy cutting!

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