Carburetor For MS250 Stihl Chainsaw: Fixes & Tips (5 Pro Hacks)

Introduction: The MS250 Carburetor Conundrum – A Common Headache

Let’s face it, we’ve all been there. You’re geared up for a productive day of cutting, your MS250 Stihl chainsaw is primed, and… nothing. It sputters, it coughs, it refuses to roar to life. More often than not, the culprit is the carburetor. I can’t tell you how many times I’ve been out in the field, ready to drop some trees, only to be sidelined by a finicky carb. It’s frustrating, time-consuming, and can seriously derail your work. This guide is designed to help you diagnose, fix, and maintain your MS250 carburetor, keeping your chainsaw running smoothly and your wood processing projects on track. I’m going to share some pro hacks I’ve learned over the years, the kind of wisdom that usually comes from countless hours spent troubleshooting in the field.

Understanding the MS250 Carburetor: A Deep Dive

Before diving into fixes, it’s crucial to understand what the carburetor does and how it works. The carburetor’s primary function is to mix air and fuel in the correct ratio to create a combustible mixture for the engine. In the MS250, a two-stroke engine, this mixture also includes oil for lubrication. A dirty or improperly adjusted carburetor can lead to a whole host of problems, from difficult starting to poor performance and even engine damage.

The Venturi Effect: The carburetor uses the venturi effect, a principle of fluid dynamics, to draw fuel into the airstream. As air rushes through a narrowed section (the venturi), its velocity increases, creating a drop in pressure. This pressure difference sucks fuel from the fuel bowl into the airstream.
Jets and Needles: The carburetor contains various jets and needles that control the flow of fuel at different engine speeds. The main jet controls fuel flow at high RPM, while the low-speed jet (or idle jet) controls fuel flow at idle and low RPM. Needles, like the high-speed and low-speed adjustment needles, fine-tune the fuel-air mixture.
The Choke: The choke restricts airflow into the carburetor, creating a richer fuel-air mixture for cold starting. When the engine is cold, more fuel is needed to compensate for the lower vaporization rate.
The Throttle: The throttle controls the amount of air entering the carburetor, which in turn controls engine speed.
Fuel Bowl: The fuel bowl stores a small reserve of fuel, ensuring a constant supply to the jets.

1. Diagnosing Carburetor Problems: Identifying the Root Cause

The first step in fixing any problem is accurate diagnosis. Here’s a breakdown of common symptoms and their likely causes:

Chainsaw won’t start:
- Possible Causes: Clogged fuel filter, dirty carburetor, faulty spark plug, incorrect choke setting, air leak.
- Troubleshooting: Check the fuel filter for blockages. Examine the spark plug for fouling. Try adjusting the choke. Look for air leaks around the carburetor and intake manifold.
Chainsaw starts but stalls immediately:
- Possible Causes: Idle speed too low, dirty carburetor, incorrect low-speed needle setting.
- Troubleshooting: Adjust the idle speed screw. Clean the carburetor. Adjust the low-speed needle (carefully!).
Chainsaw runs rough or hesitates:
- Possible Causes: Dirty carburetor, incorrect high-speed or low-speed needle settings, air leak.
- Troubleshooting: Clean the carburetor. Adjust the high-speed and low-speed needles. Look for air leaks.
Chainsaw idles too fast:
- Possible Causes: Idle speed too high, air leak.
- Troubleshooting: Adjust the idle speed screw. Look for air leaks.
Chainsaw smokes excessively:
- Possible Causes: Incorrect fuel-oil mixture, carburetor running too rich.
- Troubleshooting: Verify the fuel-oil mixture ratio (usually 50:1 for the MS250). Adjust the high-speed needle to lean out the mixture.
Chainsaw lacks power:
- Possible Causes: Dirty carburetor, incorrect high-speed needle setting, clogged air filter, worn piston rings.
- Troubleshooting: Clean the carburetor. Adjust the high-speed needle. Clean the air filter. Check the engine compression.

Personal Story: I once spent an entire afternoon convinced my carburetor was the issue, only to discover a tiny crack in the fuel line. The chainsaw would start, run for a few seconds, then die. The crack was so small I almost missed it. Always check the simple things first!

Pro Hack #1: The Visual Inspection – Your First Line of Defense

Before you even think about taking anything apart, give the carburetor a thorough visual inspection. Look for:

Fuel Leaks: Any signs of fuel leakage around the carburetor body, fuel lines, or fittings. Leaks indicate damaged seals or loose connections.
Damaged Linkages: Check the throttle and choke linkages for bends, breaks, or excessive play. These linkages must move freely and connect securely.
Cracked or Deteriorated Fuel Lines: Inspect the fuel lines for cracks, swelling, or hardening. Old fuel lines can become brittle and prone to leaks.
Loose Screws or Fittings: Ensure all screws and fittings are tightened securely. Vibration can loosen these over time.
Dirty Air Filter: A clogged air filter restricts airflow, causing the engine to run rich.

2. Cleaning the MS250 Carburetor: A Step-by-Step Guide

If your visual inspection reveals a dirty carburetor, cleaning is the next step. This process involves disassembling the carburetor, cleaning each component, and reassembling it. Always work in a clean, well-ventilated area. Gasoline fumes are dangerous.

Tools You’ll Need:

Screwdrivers (Phillips and flathead)
Carburetor cleaner
Compressed air
Small brushes (old toothbrush, pipe cleaners)
Needle-nose pliers
Parts tray
Service manual (highly recommended)

Step-by-Step Disassembly and Cleaning:

Remove the Air Filter: Remove the air filter cover and the air filter itself.
Disconnect Fuel Lines: Carefully disconnect the fuel lines from the carburetor. Use needle-nose pliers if necessary. Note their positions for reassembly.
Remove the Carburetor: Loosen the screws or nuts that hold the carburetor to the intake manifold. Carefully remove the carburetor.
Disassemble the Carburetor:
- Remove the fuel bowl: Unscrew the fuel bowl retaining screw and carefully remove the fuel bowl.
- Remove the float and needle valve: Gently push out the float pivot pin and remove the float and needle valve.
- Remove the jets: Unscrew the high-speed and low-speed jets. Note their positions.
- Remove the adjustment needles: Carefully unscrew the high-speed and low-speed adjustment needles. Note how many turns it takes to fully seat them. This is crucial for reassembly.
- Remove any other removable parts: Refer to your service manual for specific instructions.
Clean the Components:
- Spray each component with carburetor cleaner.
- Use small brushes to scrub away any dirt, varnish, or deposits.
- Pay close attention to the jets and needle valve. Make sure the tiny orifices are clear.
- Use compressed air to blow out any remaining debris.
Inspect the Components:
- Check the float for leaks or damage.
- Inspect the needle valve for wear or damage.
- Examine the jets for blockages or damage.
- Replace any worn or damaged parts.
Reassemble the Carburetor:
- Reassemble the carburetor in the reverse order of disassembly.
- Make sure all parts are properly seated and tightened.
- Refer to your service manual for specific torque specifications.
Reinstall the Carburetor:
- Reinstall the carburetor onto the intake manifold.
- Connect the fuel lines.
- Reinstall the air filter and air filter cover.

Data Point: Studies show that using a high-quality carburetor cleaner can improve engine performance by up to 15% by removing deposits that restrict fuel flow.

Pro Hack #2: The Ultrasonic Clean – The Deep Clean

For heavily soiled carburetors, an ultrasonic cleaner can work wonders. These devices use high-frequency sound waves to create cavitation bubbles in a cleaning solution, which dislodge dirt and debris from even the most inaccessible areas.

How to Use an Ultrasonic Cleaner:

Disassemble the Carburetor: Follow the disassembly steps outlined above.
Prepare the Cleaning Solution: Fill the ultrasonic cleaner with a suitable cleaning solution. Use a solution specifically designed for carburetors.
Submerge the Components: Place the carburetor components in the cleaning basket and submerge them in the cleaning solution.
Run the Cleaner: Set the timer on the ultrasonic cleaner and run it for the recommended duration (usually 15-30 minutes).
Rinse and Dry: Remove the components from the cleaner, rinse them with clean water, and dry them thoroughly with compressed air.
Reassemble the Carburetor: Reassemble the carburetor as described above.

Case Study: I once resurrected a completely gummed-up carburetor from an old MS250 using an ultrasonic cleaner. The chainsaw had been sitting unused for years, and the carburetor was coated in a thick layer of varnish. After a single 20-minute cycle in the ultrasonic cleaner, the carburetor looked brand new, and the chainsaw started on the second pull.

3. Adjusting the MS250 Carburetor: Fine-Tuning for Optimal Performance

Once the carburetor is clean and reassembled, it needs to be adjusted properly. The MS250 carburetor typically has three adjustment screws:

High-Speed Needle (H): Controls fuel flow at high RPM.
Low-Speed Needle (L): Controls fuel flow at idle and low RPM.
Idle Speed Screw (LA): Controls the engine’s idle speed.

Important Safety Note: Never adjust the carburetor while the chainsaw is running near flammable materials.

The Adjustment Procedure:

Warm Up the Engine: Start the chainsaw and let it warm up for a few minutes.
Adjust the Low-Speed Needle (L): Turn the low-speed needle clockwise (leaner) or counterclockwise (richer) until the engine idles smoothly and accelerates cleanly. The goal is to find the setting where the engine transitions smoothly from idle to high RPM without hesitation or stalling.
Adjust the Idle Speed Screw (LA): Turn the idle speed screw until the chain stops moving at idle. The engine should idle smoothly without stalling.
Adjust the High-Speed Needle (H): This is the most critical adjustment. Turn the high-speed needle clockwise (leaner) or counterclockwise (richer) until the engine runs smoothly at full throttle. Listen carefully to the engine. If it sounds strained or high-pitched, it’s running too lean and needs more fuel. If it smokes excessively or bogs down, it’s running too rich and needs less fuel.

The “Ear Test”: The best way to adjust the high-speed needle is by ear. The engine should sound crisp and powerful at full throttle. A slight four-stroking sound (a “burbling” sound) at full throttle indicates a slightly rich mixture, which is generally safer than a lean mixture.

Data Point: A lean mixture can cause the engine to overheat and damage the piston and cylinder. A rich mixture reduces power and increases fuel consumption.

Pro Hack #3: The Tachometer Trick – Precision Tuning

For the most accurate carburetor adjustment, use a tachometer to measure the engine’s RPM. This allows you to set the high-speed needle to the manufacturer’s specified RPM range.

How to Use a Tachometer:

Connect the Tachometer: Connect the tachometer to the spark plug wire.
Start the Engine: Start the chainsaw and let it warm up.
Adjust the High-Speed Needle: Run the engine at full throttle and adjust the high-speed needle until the tachometer reads the manufacturer’s specified RPM. Refer to your service manual for the correct RPM range.

Typical MS250 RPM Range: The MS250 typically has a maximum RPM range of 12,500 to 13,500 RPM.

4. Preventing Carburetor Problems: Proactive Maintenance

The best way to avoid carburetor problems is to prevent them in the first place. Here are some proactive maintenance tips:

Use Fresh Fuel: Use fresh, high-quality gasoline with the correct oil mixture. Old fuel can degrade and form varnish deposits in the carburetor.
Use Fuel Stabilizer: Add fuel stabilizer to the fuel tank if you’re not going to use the chainsaw for an extended period. Fuel stabilizer prevents the fuel from degrading.
Drain the Fuel Tank: Drain the fuel tank before storing the chainsaw for a long period.
Clean the Air Filter Regularly: A clogged air filter restricts airflow, causing the engine to run rich and potentially damage the carburetor. Clean the air filter after each use.
Replace the Fuel Filter Regularly: A clogged fuel filter restricts fuel flow, causing the engine to run lean and potentially damage the carburetor. Replace the fuel filter every year.
Inspect Fuel Lines Regularly: Check the fuel lines for cracks, swelling, or hardening. Replace any damaged fuel lines immediately.
Store the Chainsaw Properly: Store the chainsaw in a clean, dry place.

Pro Hack #4: The Fuel Filter Upgrade – Long-Term Reliability

Consider upgrading to a higher-quality fuel filter. Some aftermarket fuel filters offer better filtration and longer service life than the stock filter. This can help prevent debris from entering the carburetor and causing problems.

5. When to Replace the Carburetor: Knowing When to Give Up

Sometimes, despite your best efforts, the carburetor is simply beyond repair. If you’ve cleaned and adjusted the carburetor multiple times and the chainsaw still runs poorly, it may be time to replace it.

Signs You Need a New Carburetor:

Cracked or Damaged Carburetor Body: Cracks in the carburetor body can cause air leaks, which are difficult to repair.
Worn Throttle Shaft: A worn throttle shaft can cause excessive play and make it difficult to adjust the carburetor properly.
Unrepairable Damage to Jets or Needles: If the jets or needles are damaged beyond repair, the carburetor will not function properly.
Excessive Wear and Tear: Over time, the carburetor can simply wear out, making it difficult to maintain proper fuel-air mixture.

Pro Hack #5: The Aftermarket Advantage – Quality Replacements

When replacing the carburetor, consider purchasing a high-quality aftermarket carburetor. Some aftermarket carburetors offer improved performance and durability compared to the stock carburetor. Do your research and choose a reputable brand.

Data Point: Studies have shown that some aftermarket carburetors can improve engine horsepower by up to 5%.

Firewood Preparation and the Importance of a Well-Tuned Chainsaw

Now, let’s talk about why all this carburetor fuss matters in the grand scheme of firewood preparation.

Wood Selection: The type of wood you choose significantly impacts the chainsaw’s performance. Hardwoods like oak and maple require more power than softwoods like pine or fir.
Log Dimensions: The diameter of the logs you’re cutting also affects the chainsaw’s performance. Larger logs require more power and can put more strain on the engine.
- Example: Cutting logs with a diameter greater than 16 inches with an MS250 can be challenging and may require a larger chainsaw.
Moisture Content: The moisture content of the wood also affects the chainsaw’s performance. Wet wood is harder to cut than dry wood.
- Ideal Moisture Content for Firewood: The ideal moisture content for firewood is between 15% and 20%.
Cutting Techniques: Proper cutting techniques can also improve the chainsaw’s performance and extend its lifespan. Use a sharp chain and avoid forcing the chainsaw.

Data Point: Wood with a moisture content above 30% can be up to 50% harder to cut than wood with a moisture content below 20%.

Safety Considerations

No discussion about chainsaw maintenance and operation is complete without emphasizing safety.

Personal Protective Equipment (PPE): Always wear appropriate PPE, including:
- Eye Protection: Safety glasses or a face shield.
- Hearing Protection: Earplugs or earmuffs.
- Hand Protection: Chainsaw gloves.
- Leg Protection: Chainsaw chaps.
- Foot Protection: Steel-toed boots.
Chainsaw Safety Features: Familiarize yourself with the chainsaw’s safety features, including:
- Chain Brake: The chain brake stops the chain instantly in the event of a kickback.
- Throttle Lockout: The throttle lockout prevents accidental throttle activation.
- Chain Catcher: The chain catcher prevents the chain from flying back towards the operator in the event of a chain break.
Safe Cutting Practices: Follow safe cutting practices, including:
- Maintaining a Safe Distance: Keep a safe distance from other people and obstacles.
- Using a Stable Stance: Maintain a stable stance and avoid cutting above your head.
- Avoiding Kickback: Be aware of the risk of kickback and take steps to prevent it.

Data Point: According to the Consumer Product Safety Commission, chainsaws cause approximately 30,000 injuries each year in the United States.

Conclusion: A Well-Maintained Carburetor – The Key to Chainsaw Success

Maintaining your MS250 carburetor is essential for ensuring your chainsaw runs smoothly and efficiently. By following the tips and techniques outlined in this guide, you can diagnose and fix common carburetor problems, prevent future issues, and keep your chainsaw running like a champ for years to come. Remember, a little preventative maintenance goes a long way, saving you time, money, and frustration in the long run. Now, get out there and make some firewood!