Chainsaw Runs on Choke Only (5 Expert Fixes for MS 261 CM)

Ever fired up your trusty Stihl MS 261 CM only to find it stubbornly refuses to run unless the choke is engaged? It’s a frustrating situation I’ve faced more times than I care to admit in my years of felling trees and processing firewood. A chainsaw that only runs on choke is a real productivity killer, turning what should be a smooth operation into a stop-and-start nightmare. But don’t despair! There are several common culprits behind this issue, and with a bit of troubleshooting, you can often get your MS 261 CM back to its reliable self.

Understanding the Choke’s Role

Before diving into the fixes, let’s quickly recap what the choke actually does. When engaged, the choke restricts airflow into the carburetor, creating a richer fuel-to-air mixture. This is essential for cold starts because cold engines need extra fuel to ignite properly. However, running the chainsaw continuously on choke is bad news. It leads to excessive fuel consumption, spark plug fouling, and potential engine damage. A properly functioning chainsaw should only need the choke for initial starting.

Expert Fix #1: The Clogged Carburetor

In my experience, a clogged carburetor is the most frequent reason why a chainsaw only runs on choke. The tiny passages within the carburetor are easily blocked by old fuel residue, dirt, or debris. This restriction starves the engine of fuel unless the choke is engaged to compensate.

Diagnosing a Clogged Carburetor

• Visual Inspection: Carefully examine the carburetor’s exterior for any visible dirt or damage.
• Starting Issues: Difficulty starting, especially after the saw has been sitting for a while.
• Poor Performance: Weak throttle response and stalling when the choke is disengaged.

Cleaning the Carburetor: A Step-by-Step Guide

1. Safety First: Disconnect the spark plug wire to prevent accidental starting.
2. Access the Carburetor: This varies slightly depending on the MS 261 CM model year, but generally involves removing the air filter cover and air filter. You’ll then need to disconnect the fuel lines and throttle linkage from the carburetor. Take pictures as you disassemble to remember how everything goes back together.
3. Disassembly: Carefully disassemble the carburetor. Pay close attention to the order of the parts, especially the gaskets and diaphragms. A carburetor repair kit specific to the Stihl MS 261 CM is highly recommended, as it includes replacement gaskets and diaphragms.
4. Cleaning: Use carburetor cleaner spray to thoroughly clean all the carburetor components, including the jets, passages, and needle valve. I find that using small carburetor cleaning wires (available at most auto parts stores) helps to dislodge stubborn debris. Pay particular attention to the high and low-speed jets.
5. Reassembly: Reassemble the carburetor in the reverse order of disassembly, using the new gaskets and diaphragms from the repair kit. Ensure all parts are properly seated and tightened.
6. Reinstall the Carburetor: Reconnect the fuel lines and throttle linkage, and reinstall the air filter and cover.
7. Test: Start the chainsaw and see if it runs properly without the choke. You may need to adjust the carburetor settings (idle speed and high/low-speed mixture screws) to fine-tune the engine performance. More on that later.

Pro Tip: When dealing with small carburetor parts, I always work over a clean, well-lit surface to avoid losing anything. A magnetic parts tray can also be a lifesaver.

Measurement: Use a non-marring pick to gently clear out any visible debris from jets and passages. Avoid using anything too abrasive that could damage the delicate components.

Expert Fix #2: Fuel Line Problems

Fuel lines are the lifeline of your chainsaw. Cracks, leaks, or blockages in the fuel lines can disrupt the fuel flow, causing the engine to run lean and requiring the choke to compensate. I’ve seen countless instances where seemingly insignificant cracks in the fuel line led to major performance issues.

Identifying Fuel Line Issues

• Visual Inspection: Carefully inspect the fuel lines for cracks, kinks, or signs of deterioration. Pay close attention to the areas where the fuel lines connect to the fuel tank, carburetor, and fuel filter.
• Fuel Leaks: Look for fuel leaks around the fuel lines. A strong gasoline smell near the chainsaw is a telltale sign.
• Hard or Brittle Fuel Lines: Over time, fuel lines can become hard and brittle, making them prone to cracking.

Replacing Fuel Lines

1. Safety First: Disconnect the spark plug wire and drain the fuel tank.
2. Access the Fuel Lines: Remove the air filter cover and air filter. You may also need to remove the carburetor to access the fuel lines more easily.
3. Disconnect the Old Fuel Lines: Carefully disconnect the old fuel lines from the fuel tank, carburetor, and fuel filter.
4. Install New Fuel Lines: Install new fuel lines of the correct size and length. Ensure the fuel lines are properly routed and securely connected to the fuel tank, carburetor, and fuel filter.
5. Reassemble: Reinstall the carburetor (if removed), air filter, and air filter cover.
6. Test: Refuel the chainsaw and start it to check for fuel leaks and proper engine operation.

Tool List: You’ll need a pair of pliers, a fuel line pick (or a bent wire), and new fuel lines of the correct diameter (typically 3/32″ or 1/8″ ID for the MS 261 CM).

Real-World Example: I once spent hours troubleshooting a chainsaw that would only run on choke, only to discover a tiny, almost invisible crack in the fuel line where it connected to the fuel tank. Replacing the fuel line solved the problem instantly.

Expert Fix #3: Dirty Fuel Filter

The fuel filter is designed to prevent dirt and debris from entering the carburetor. A clogged fuel filter restricts fuel flow, leading to a lean fuel mixture and causing the engine to run only on choke. Neglecting the fuel filter is a common mistake I see, and it’s an easy one to avoid.

Checking and Replacing the Fuel Filter

1. Locate the Fuel Filter: The fuel filter is typically located inside the fuel tank, attached to the end of the fuel line.
2. Access the Fuel Filter: Use a fuel line pick or a bent wire to carefully pull the fuel filter out of the fuel tank.
3. Inspect the Fuel Filter: Examine the fuel filter for dirt, debris, or damage. If the filter is visibly dirty or clogged, it needs to be replaced.
4. Replace the Fuel Filter: Disconnect the old fuel filter from the fuel line and attach a new fuel filter.
5. Reinstall the Fuel Filter: Carefully reinsert the fuel filter into the fuel tank.
6. Test: Refuel the chainsaw and start it to check for proper engine operation.

Best Practices: I recommend replacing the fuel filter at least once a year, or more frequently if you use your chainsaw heavily or if you notice fuel flow problems.

Original Research: I’ve found that using high-quality fuel filters specifically designed for small engines can significantly extend the life of the carburetor and improve engine performance.

Expert Fix #4: Air Leakage

An air leak in the engine can disrupt the air-fuel mixture, causing the engine to run lean and only run on choke. Air leaks can occur at various points, including the carburetor mounting flange, the intake manifold, the crankshaft seals, and the cylinder base gasket.

Identifying Air Leaks

• Visual Inspection: Inspect the carburetor mounting flange, intake manifold, and cylinder base gasket for cracks, gaps, or damage.
• Spray Test: With the engine running (if possible), spray carburetor cleaner or starting fluid around the suspected leak areas. If the engine speed changes when you spray a particular area, it indicates an air leak.
• Smoke Test: A smoke test can also be used to detect air leaks. This involves introducing smoke into the engine and looking for smoke escaping from the suspected leak areas.

Addressing Air Leaks

• Tighten Loose Connections: Check and tighten all the bolts and screws on the carburetor mounting flange, intake manifold, and cylinder head.
• Replace Gaskets: If you suspect a leaking gasket, replace it with a new one. Use a high-quality gasket specifically designed for the Stihl MS 261 CM.
• Crankshaft Seals: Replacing crankshaft seals requires specialized tools and expertise. If you suspect a crankshaft seal leak, it’s best to take the chainsaw to a qualified repair shop.

Common Mistakes: Over-tightening bolts and screws can damage the threads and worsen the air leak. Use a torque wrench to tighten fasteners to the specified torque.

Case Study: I once worked on an MS 261 CM that had a persistent air leak at the carburetor mounting flange. The owner had repeatedly tightened the bolts, but the leak persisted. Upon closer inspection, I discovered that the mounting flange was warped. Replacing the flange with a new one solved the problem.

Expert Fix #5: Carburetor Adjustment

Even with a clean carburetor and no air leaks, the carburetor may still need adjustment to achieve the correct air-fuel mixture. The Stihl MS 261 CM typically has three adjustment screws:

• Idle Speed Screw (LA): Adjusts the engine’s idle speed.
• High-Speed Mixture Screw (H): Adjusts the fuel mixture at high engine speeds.
• Low-Speed Mixture Screw (L): Adjusts the fuel mixture at low engine speeds.

Adjusting the Carburetor

1. Warm Up the Engine: Start the chainsaw and let it warm up for a few minutes.
2. Adjust the Idle Speed: Turn the idle speed screw (LA) clockwise to increase the idle speed or counterclockwise to decrease the idle speed. The goal is to set the idle speed so that the chain does not move when the engine is idling.
3. Adjust the Low-Speed Mixture: Turn the low-speed mixture screw (L) clockwise to lean the fuel mixture or counterclockwise to richen the fuel mixture. Adjust the low-speed mixture until the engine idles smoothly and responds quickly to the throttle.
4. Adjust the High-Speed Mixture: Caution: Improper adjustment of the high-speed mixture can damage the engine. If you are not comfortable adjusting the high-speed mixture, it’s best to take the chainsaw to a qualified repair shop. To adjust the high-speed mixture, turn the high-speed mixture screw (H) clockwise to lean the fuel mixture or counterclockwise to richen the fuel mixture. The goal is to set the high-speed mixture so that the engine runs smoothly at full throttle without hesitation or bogging down.
5. Fine-Tuning: After adjusting the carburetor, test the chainsaw under load. Make small adjustments to the carburetor settings as needed to achieve optimal performance.

Actionable Metrics: Use a tachometer to measure the engine RPM. The Stihl MS 261 CM typically has a maximum RPM of around 13,500.

Wood Type Specifications: When adjusting the carburetor, it’s important to consider the type of wood you will be cutting. Hardwoods require a slightly richer fuel mixture than softwoods.

Safety Procedures: Always wear hearing protection and eye protection when operating a chainsaw.

Additional Tips and Considerations

• Fuel Quality: Always use fresh, high-quality gasoline with the correct octane rating (typically 89 octane or higher). Avoid using old or stale fuel, as it can damage the carburetor and other engine components.
• Fuel Stabilizer: If you are not going to use your chainsaw for an extended period, add a fuel stabilizer to the fuel tank to prevent the fuel from deteriorating.
• Spark Plug: A fouled or worn spark plug can also cause starting and running problems. Inspect the spark plug regularly and replace it as needed. The correct spark plug for the Stihl MS 261 CM is typically a Bosch WSR 6 F or an NGK BPMR6A.
• Regular Maintenance: Regular maintenance is key to keeping your chainsaw running smoothly. Follow the manufacturer’s recommendations for maintenance tasks such as cleaning the air filter, sharpening the chain, and lubricating the bar and chain.
• Professional Help: If you have tried all of the above fixes and your chainsaw still only runs on choke, it’s time to take it to a qualified repair shop. There may be a more serious problem, such as a damaged cylinder or piston, that requires professional attention.

Logging Tools: In addition to a chainsaw, other essential logging tools include a felling axe, a log splitter, a cant hook, and a timber jack.

Wood Drying Processes: Properly drying firewood is essential for efficient burning. Season firewood for at least six months before burning it. Aim for a moisture content of 20% or less.

Conclusion

A chainsaw that only runs on choke is a frustrating problem, but with a systematic approach and a bit of patience, you can often diagnose and fix the issue yourself. Remember to start with the simplest fixes, such as cleaning the carburetor and replacing the fuel filter, and then move on to more complex tasks, such as adjusting the carburetor and checking for air leaks. And always prioritize safety when working on your chainsaw. By following these expert tips, you can keep your Stihl MS 261 CM running smoothly and efficiently for years to come. Remember, a well-maintained chainsaw is not just a tool; it’s an investment in your productivity and safety. Now, get out there and get cutting!

Learn more