Stihl MS250 Carburetor Fixes (3 Expert Tricks for Smooth Running)

Let’s dive into the heart of chainsaw maintenance – the carburetor. The Stihl MS250 is a workhorse of a chainsaw, known for its reliability and power. But like any precision machine, its carburetor can sometimes throw a wrench in the works. A poorly running MS250 is frustrating, especially when you’re in the thick of cutting firewood or clearing brush. That’s why I’m going to share my top three expert tricks for fixing a Stihl MS250 carburetor and getting your saw running smoothly again. These aren’t just quick fixes; they’re based on years of experience repairing and maintaining chainsaws, both for my own use and for others.

The Stihl MS250, in my experience, is a prime example of a robust and dependable machine. However, even the most durable equipment requires regular maintenance and occasional repairs to ensure optimal performance. The carburetor, being a critical component in the engine’s fuel delivery system, is often a common culprit when things go awry. By understanding the intricacies of the carburetor and implementing effective troubleshooting techniques, you can significantly extend the lifespan of your chainsaw and minimize downtime.

Understanding the Stihl MS250 Carburetor

Before we jump into the fixes, let’s get a solid understanding of what a carburetor does and how it works in the Stihl MS250.

• What is a Carburetor? Simply put, a carburetor mixes air and fuel in the right proportions to create a combustible mixture that the engine can burn. It’s like the heart of the engine’s fuel system.
• How it Works: The MS250 uses a diaphragm carburetor. As the engine runs, it creates a vacuum. This vacuum draws fuel from the fuel tank, through the carburetor, and into the engine’s cylinder. The carburetor regulates the amount of fuel based on the engine’s needs.
• Key Components:
  • Diaphragm: A flexible membrane that pumps fuel.
  • Needle Valve: Controls the flow of fuel into the carburetor.
  • Jets: Small openings that meter the fuel flow.
  • Adjustment Screws (H and L): Used to fine-tune the fuel-air mixture. “H” is for high-speed, and “L” is for low-speed.
  • Choke: Restricts airflow to create a richer fuel mixture for starting a cold engine.

Why Carburetors Go Wrong: Carburetors are sensitive to dirt, old fuel, and improper adjustments. Over time, fuel can break down and leave deposits that clog the jets and passages. Incorrect adjustments can lead to a lean (too much air) or rich (too much fuel) mixture, causing poor performance or even engine damage.

Personal Experience: I remember one time, I was helping a friend clear some land after a storm. His MS250 kept stalling, and he was about ready to give up. After a quick look, I realized the carburetor was gunked up with old fuel. A thorough cleaning and a few adjustments, and the saw was back in action. It saved the day and a lot of backache!

Expert Trick #1: The Master Carburetor Cleaning

This is the most common fix, and often the only one you’ll need. Over time, fuel residue builds up inside the carburetor, clogging the tiny jets and passages. A thorough cleaning is crucial.

Step-by-Step Guide:

1. Safety First: Disconnect the spark plug wire. This prevents accidental starting. Wear safety glasses and gloves. Work in a well-ventilated area.

2. Remove the Air Filter and Carburetor Cover: Typically, this involves removing a few screws. Refer to your MS250 owner’s manual for specific instructions.

3. Disconnect Fuel Lines and Throttle Linkage: Carefully detach the fuel lines from the carburetor. Use a small screwdriver or pliers if needed. Be mindful of any fuel spillage. Disconnect the throttle linkage.

4. Remove the Carburetor: Usually, the carburetor is held in place by a few bolts. Remove them and carefully pull the carburetor away from the engine.

5. Disassemble the Carburetor: This is where it gets a bit delicate.

   • Diaphragm Cover: Remove the screws holding the diaphragm cover in place. Carefully peel back the diaphragm. Note its orientation for reassembly.
   • Metering Lever: Remove the metering lever pin. The metering lever and needle valve will come loose.
   • Jets: Locate the high-speed (H) and low-speed (L) jets. These are usually accessible after removing the adjustment screws.
   • Other Small Parts: Pay close attention to the order and orientation of any other small parts as you disassemble the carburetor. Take pictures if needed.

6. Clean Each Part:

   • Carburetor Cleaner: Use a high-quality carburetor cleaner. I prefer aerosol cans with a small nozzle for precision cleaning.
   • Spray and Soak: Spray each part thoroughly with carburetor cleaner. Let them soak for at least 15-20 minutes to loosen deposits.
   • Compressed Air: Use compressed air to blow out all the passages and jets. Ensure no debris remains. Pay special attention to the tiny holes in the jets. You can use a very fine wire (like a guitar string) to gently clear any stubborn clogs.
   • Visual Inspection: Carefully inspect each part for damage or wear. Replace any worn or damaged parts.

7. Reassemble the Carburetor:

   • Follow your disassembly notes or photos: Reassemble the carburetor in the reverse order of disassembly.
   • Metering Lever Height: The metering lever height is crucial. It should be flush with the carburetor body. Use a straight edge to check. If it’s not, gently bend the lever until it’s flush. This affects fuel delivery.
   • New Diaphragm: Consider replacing the diaphragm and gaskets. They are relatively inexpensive and can make a big difference in performance.
   • Tighten Screws: Tighten all screws securely, but don’t overtighten.

8. Reinstall the Carburetor:

   • Reconnect Fuel Lines and Throttle Linkage: Reattach the fuel lines to the correct ports on the carburetor. Reconnect the throttle linkage.
   • Secure the Carburetor: Bolt the carburetor back onto the engine.
   • Reinstall the Air Filter and Carburetor Cover: Put the air filter back in place and reinstall the carburetor cover.

9. Initial Adjustment:

   • Turn Adjustment Screws: Turn both the high-speed (H) and low-speed (L) adjustment screws all the way in (gently) until they stop. Then, back them out 1 to 1.5 turns. This is a starting point.

Tools Needed:

• Screwdrivers (various sizes, including small flathead)
• Pliers
• Carburetor cleaner
• Compressed air
• Safety glasses
• Gloves
• Small container for soaking parts
• Clean rags
• Optional: Carburetor rebuild kit (includes new diaphragms and gaskets)

Cost: Carburetor cleaner costs around $10-15. A carburetor rebuild kit costs around $20-30.

Skill Level: Beginner to Intermediate. Disassembly and reassembly require patience and attention to detail.

Benefits: A clean carburetor ensures proper fuel delivery, resulting in smooth running, improved power, and better fuel efficiency.

Strategic Advantage: Regular carburetor cleaning (every 6-12 months, depending on usage) can prevent major problems and extend the life of your chainsaw.

Case Study: I once had a client who used his MS250 daily for firewood cutting. He complained of constant stalling and poor power. After a thorough carburetor cleaning, the saw ran like new. He was amazed at the difference. He now makes carburetor cleaning a regular part of his maintenance routine.

Expert Trick #2: Fine-Tuning the Carburetor Adjustments (H and L Screws)

Even after cleaning, the carburetor may need fine-tuning to achieve optimal performance. The high-speed (H) and low-speed (L) adjustment screws control the fuel-air mixture at different engine speeds.

Understanding the H and L Screws:

• L (Low-Speed): Affects the idle and low-end performance. Adjusting this screw affects starting, idling, and acceleration from idle.
• H (High-Speed): Affects the high-end performance. Adjusting this screw affects the maximum power and RPM.

Step-by-Step Adjustment Guide:

1. Warm-Up the Engine: Start the engine and let it warm up for a few minutes. This ensures accurate adjustments.

2. Idle Speed Adjustment:

   • Locate the Idle Speed Screw (LA): This screw is usually located near the throttle linkage.
   • Adjust Idle Speed: Turn the idle speed screw until the engine idles smoothly without stalling. The chain should not be moving at idle. If the chain is moving, reduce the idle speed.

3. Low-Speed (L) Adjustment:

   • Lean Best Idle: Slowly turn the L screw clockwise (leaner) until the engine starts to stumble or stall. Note the position.
   • Rich Best Idle: Slowly turn the L screw counter-clockwise (richer) until the engine starts to stumble or smoke. Note the position.
   • Sweet Spot: Turn the L screw to the midpoint between the lean and rich stumble points. This is usually the optimal setting.

4. High-Speed (H) Adjustment:

   • Full Throttle: With the engine running at full throttle, slowly turn the H screw.
   • Listen to the Engine: Listen for a smooth, consistent engine sound. The engine should not be screaming or sputtering.
   • Smoke Test: A slight amount of smoke is normal at full throttle. Excessive smoke indicates a rich mixture (too much fuel). No smoke indicates a lean mixture (too little fuel), which can damage the engine.
   • Tachometer (Optional): For precise adjustments, use a tachometer to measure the engine RPM. Refer to your MS250 owner’s manual for the recommended maximum RPM.
   • Adjust Accordingly: If the engine is screaming or sputtering, adjust the H screw accordingly until it runs smoothly. A slightly rich mixture is generally safer than a lean mixture.

Important Considerations:

• Small Adjustments: Make small adjustments (1/8 turn or less) at a time.
• Listen to the Engine: The engine sound is your best guide.
• Owner’s Manual: Refer to your MS250 owner’s manual for specific adjustment recommendations.
• Altitude: Carburetor adjustments may need to be tweaked based on altitude. Higher altitudes require leaner settings.

Tools Needed:

• Screwdriver (small flathead)
• Tachometer (optional)
• Ear protection

Cost: A tachometer costs around $30-50.

Skill Level: Intermediate. Requires a good understanding of engine performance and the ability to interpret engine sounds.

Benefits: Proper carburetor adjustments result in optimal engine performance, improved fuel efficiency, and reduced emissions.

Strategic Advantage: Fine-tuning the carburetor can maximize the power and longevity of your chainsaw.

Case Study: I had a customer who lived at a high altitude. His MS250 ran poorly, even after a thorough cleaning. After adjusting the carburetor for the altitude, the saw ran much better. He was able to cut firewood more efficiently and with less frustration.

Expert Trick #3: Checking and Replacing Fuel Lines and Filter

Sometimes, the problem isn’t the carburetor itself, but rather the fuel supply. Old, cracked, or clogged fuel lines and a dirty fuel filter can restrict fuel flow and cause carburetor problems.

Step-by-Step Guide:

1. Locate the Fuel Lines: The fuel lines run from the fuel tank to the carburetor and back.

2. Inspect the Fuel Lines:

   • Visual Inspection: Check the fuel lines for cracks, leaks, or kinks.
   • Flexibility: The fuel lines should be flexible. If they are stiff or brittle, they need to be replaced.

3. Replace Fuel Lines (If Necessary):

   • Disconnect the Old Lines: Carefully disconnect the old fuel lines from the fuel tank and carburetor.
   • Measure and Cut New Lines: Measure the length of the old fuel lines and cut new fuel lines to the same length. Use fuel-resistant tubing.
   • Install New Lines: Attach the new fuel lines to the fuel tank and carburetor. Ensure they are securely connected.

4. Check and Replace the Fuel Filter:

   • Locate the Fuel Filter: The fuel filter is usually located inside the fuel tank, attached to the end of the fuel line.
   • Remove the Fuel Filter: Use a small hook or pliers to pull the fuel filter out of the fuel tank.
   • Inspect the Fuel Filter: Check the fuel filter for dirt and debris.
   • Replace the Fuel Filter: If the fuel filter is dirty, replace it with a new one.

Tools Needed:

• Pliers
• Small hook or wire
• Fuel-resistant tubing
• New fuel filter

Cost: Fuel line tubing costs around $5-10 per foot. A new fuel filter costs around $5-10.

Skill Level: Beginner. This is a relatively easy task.

Benefits: Replacing old fuel lines and a dirty fuel filter ensures a clean and consistent fuel supply to the carburetor, preventing fuel starvation and improving engine performance.

Strategic Advantage: Regular fuel line and filter replacement (every 1-2 years) can prevent fuel-related problems and extend the life of your chainsaw.

Case Study: A local tree service company was experiencing frequent carburetor problems with their MS250s. After inspecting their saws, I discovered that the fuel lines were cracked and brittle. Replacing the fuel lines and filters solved the problem. They now have a regular fuel line and filter replacement schedule.

Other Potential Issues and Solutions

While the above tricks cover the most common carburetor-related problems, here are a few other things to consider:

• Spark Plug: A fouled or worn spark plug can cause starting and running problems. Check the spark plug and replace it if necessary. The gap for the MS250 should be 0.5mm (0.020 inches).
• Air Filter: A dirty air filter restricts airflow and can cause the engine to run rich. Clean or replace the air filter regularly.
• Fuel Quality: Use fresh, high-quality fuel with the correct oil mixture (usually 50:1). Old fuel can break down and leave deposits in the carburetor. I always recommend using fuel stabilizer, especially if the saw will be stored for an extended period.
• Crankshaft Seals: Worn crankshaft seals can cause air leaks and affect engine performance. This is a more complex repair that may require professional assistance.
• Impulse Line: The impulse line connects the carburetor to the engine crankcase and provides the vacuum pulse needed to operate the fuel pump. Check the impulse line for cracks or leaks.

Safety Considerations

Working on chainsaws can be dangerous. Always follow these safety precautions:

• Disconnect the spark plug wire before working on the saw.
• Wear safety glasses and gloves.
• Work in a well-ventilated area.
• Be careful when handling fuel.
• Refer to your MS250 owner’s manual for specific safety instructions.
• If you are not comfortable performing these repairs yourself, take the saw to a qualified technician.

Key Terms and Concepts

• Green Wood vs. Seasoned Wood: Green wood is freshly cut wood with high moisture content. Seasoned wood has been dried and has a lower moisture content, making it ideal for burning.
• Moisture Content: The amount of water in wood, expressed as a percentage of the wood’s dry weight. Ideal moisture content for firewood is below 20%.
• Lean Mixture: A fuel-air mixture with too much air and not enough fuel.
• Rich Mixture: A fuel-air mixture with too much fuel and not enough air.
• RPM: Revolutions per minute, a measure of engine speed.
• Carburetor Rebuild Kit: A kit containing new diaphragms, gaskets, and other small parts for rebuilding a carburetor.

Practical Next Steps

Now that you’ve learned these expert tricks, it’s time to put them into practice.

1. Gather your tools and supplies.
2. Choose a well-lit and ventilated workspace.
3. Start with the Master Carburetor Cleaning (Trick #1).
4. If cleaning doesn’t solve the problem, move on to Fine-Tuning the Carburetor Adjustments (Trick #2).
5. Check and Replace Fuel Lines and Filter (Trick #3) if necessary.
6. Refer to your MS250 owner’s manual for specific instructions and recommendations.
7. Be patient and methodical.
8. Don’t be afraid to ask for help if you get stuck.

Conclusion

A well-maintained carburetor is essential for the smooth running and long life of your Stihl MS250 chainsaw. By following these expert tricks, you can keep your saw in top condition and tackle any wood processing or firewood preparation project with confidence. Remember to prioritize safety and refer to your owner’s manual for specific instructions. With a little knowledge and effort, you can become a carburetor master and keep your MS250 running like a champ for years to come.

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