Adjusting Stihl Weed Eater Carburetor (Pro Tuning Tips)

It’s estimated that over 60 million households in the United States alone use wood-burning stoves or fireplaces as a primary or secondary heat source. This widespread reliance on wood underscores the importance of maintaining the tools that help us harvest and process it. One of the most crucial of these tools is the weed eater, particularly for clearing brush around trees before felling, maintaining paths in wooded areas, or even cleaning up around firewood piles. A poorly running weed eater can be incredibly frustrating, costing you time and effort. That’s why understanding how to adjust the carburetor on your Stihl weed eater is essential. In this guide, I’ll share my knowledge, experiences, and pro tuning tips to help you keep your Stihl weed eater running smoothly.

Understanding Your Stihl Weed Eater Carburetor: Pro Tuning Tips

The carburetor is the heart of your Stihl weed eater’s engine. It’s responsible for mixing air and fuel in the correct proportions to create a combustible mixture. When the carburetor isn’t properly adjusted, your weed eater might run poorly, stall frequently, or even fail to start. Let’s dive into the details.

Key Concepts: Fuel-Air Mixture and Carburetor Basics

Before we start adjusting, it’s crucial to understand the basics. The carburetor has three main adjustment screws:

Low (L) Screw: Controls the fuel mixture at idle and low speeds.
High (H) Screw: Controls the fuel mixture at high speeds or full throttle.
Idle (LA) Screw or T Screw: Controls the engine’s idle speed.

Lean vs. Rich:

Lean Mixture: Too much air and not enough fuel. This can cause overheating and engine damage.
Rich Mixture: Too much fuel and not enough air. This can lead to poor performance, excessive smoke, and fouled spark plugs.

Green Wood vs. Seasoned Wood: This is relevant because clearing around trees with a weed eater is often the first step in the wood processing journey. Green wood is freshly cut wood with high moisture content, typically above 30%. Seasoned wood, on the other hand, has been dried to a moisture content of 20% or less. Working with green wood requires more effort and different techniques than working with seasoned wood.

Tools You’ll Need

To properly adjust your Stihl weed eater carburetor, you’ll need the following tools:

Screwdriver Set: A set of screwdrivers with various sizes, including a small flathead screwdriver for the adjustment screws.
Carburetor Adjustment Tool: Stihl carburetors often require a special “D” shaped or splined tool. Make sure you have the correct tool for your model. You can usually find these online or at a Stihl dealer.
Tachometer (Optional): A tachometer can help you accurately set the idle speed and maximum RPM, especially on newer models.
Spark Plug Wrench: For removing and inspecting the spark plug.
Spark Plug Cleaner (Optional): To clean a fouled spark plug.
Safety Glasses: To protect your eyes from debris.
Gloves: To protect your hands.
Clean Rags: For wiping up fuel spills.

Safety First: Preparation and Precautions

Before you begin any carburetor adjustments, take these safety precautions:

Work in a Well-Ventilated Area: Gasoline fumes are flammable and can be harmful.
Wear Safety Glasses and Gloves: Protect your eyes and hands from fuel and debris.
Turn Off the Engine and Allow it to Cool: Never adjust the carburetor while the engine is running or hot.
Disconnect the Spark Plug Wire: This prevents accidental starting.
Place the Weed Eater on a Stable Surface: Ensure it won’t tip over during adjustment.
Keep a Fire Extinguisher Nearby: Just in case of fuel spills.

Step-by-Step Guide to Adjusting Your Stihl Weed Eater Carburetor

Now, let’s get into the actual adjustment process. I’ll break it down into clear, actionable steps.

Step 1: Locate the Carburetor Adjustment Screws

The carburetor is usually located near the engine, often behind an air filter cover. The adjustment screws (L, H, and LA/T) will be visible on the carburetor body. Refer to your Stihl weed eater’s owner’s manual for the exact location of the screws on your specific model.

Personal Experience: I remember once spending a frustrating hour trying to find the adjustment screws on an older Stihl FS 80. It turned out they were hidden under a small plastic cover that I hadn’t noticed. Always consult your manual!

Step 2: Initial Settings (The Baseline)

Before making any adjustments, it’s crucial to know the factory settings. These are usually listed in your owner’s manual. If you can’t find them, a good starting point is:

L Screw: Turn it clockwise until it’s lightly seated, then turn it counterclockwise 1 to 1.5 turns.
H Screw: Turn it clockwise until it’s lightly seated, then turn it counterclockwise 1 to 1.5 turns.
LA/T Screw: This screw is for idle speed only. Don’t adjust it yet.

Why this is important: These initial settings provide a baseline. From here, you’ll fine-tune the carburetor to achieve optimal performance.

Step 3: Starting the Engine

Reconnect the spark plug wire and start the engine. Let it warm up for a few minutes. This is crucial because a cold engine will behave differently than a warm one.

Tip: If the engine doesn’t start, check the fuel lines for cracks or leaks, and make sure the spark plug is clean and properly gapped.

Step 4: Adjusting the Low (L) Screw

The L screw primarily affects the engine’s performance at idle and low speeds. Here’s how to adjust it:

Listen to the Engine: Turn the L screw slowly clockwise (leaning the mixture) until the engine starts to stumble or run rough.
Turn Counterclockwise: Now, turn the L screw slowly counterclockwise (richening the mixture) until the engine starts to stumble or run rough again.
Find the Sweet Spot: The ideal setting is usually in the middle of these two points. The engine should idle smoothly and respond quickly when you throttle up.

Case Study: I once worked on a Stihl FS 90 that was constantly stalling at idle. After adjusting the L screw, I found that the sweet spot was about 1/8 of a turn richer than the factory setting. The engine idled perfectly after that.

Step 5: Adjusting the Idle (LA/T) Screw

The LA/T screw controls the engine’s idle speed. The goal is to set the idle speed high enough so the engine doesn’t stall, but low enough so the cutting head doesn’t spin.

Turn Clockwise: Turn the LA/T screw clockwise to increase the idle speed.
Turn Counterclockwise: Turn the LA/T screw counterclockwise to decrease the idle speed.
Listen Carefully: Adjust the screw until the engine idles smoothly without the cutting head spinning.

Recommended Idle Speed: Consult your owner’s manual for the recommended idle speed for your model. If you have a tachometer, use it to set the idle speed accurately. A typical idle speed for a Stihl weed eater is around 2500-3000 RPM.

Step 6: Adjusting the High (H) Screw

The H screw controls the fuel mixture at high speeds or full throttle. This adjustment is crucial for preventing engine damage.

Important Note: Adjusting the H screw requires experience and careful listening. If you’re not comfortable with this step, it’s best to take your weed eater to a qualified technician.

Full Throttle: Run the engine at full throttle.
Listen to the Engine: Turn the H screw slowly clockwise (leaning the mixture) until the engine starts to sound strained or “screaming.” This indicates a lean condition, which can damage the engine.
Turn Counterclockwise: Now, turn the H screw slowly counterclockwise (richening the mixture) until the engine starts to “four-stroke.” Four-stroking is when the engine sounds like it’s missing every other beat.
Find the Sweet Spot: The ideal setting is usually just before the engine starts to four-stroke. The engine should run smoothly and powerfully at full throttle without sounding strained.

Understanding Four-Stroking: Four-stroking is a sign that the engine is running slightly rich. It’s better to err on the side of slightly rich than lean, as a lean condition can cause severe engine damage.

Using a Tachometer: A tachometer can help you accurately set the maximum RPM. Consult your owner’s manual for the recommended maximum RPM for your model.

Data Point: Over the years, I’ve found that many Stihl weed eaters perform best when the H screw is set slightly richer than the factory setting. This provides a margin of safety and ensures adequate lubrication at high speeds.

Step 7: Fine-Tuning and Testing

After adjusting the H screw, it’s time to fine-tune the carburetor and test the weed eater’s performance.

Throttle Response: Check the engine’s throttle response. It should accelerate smoothly and quickly without hesitation. If the engine hesitates or bogs down, the L screw might need further adjustment.
Full Throttle Performance: Run the weed eater at full throttle for several minutes. Listen for any signs of strain or overheating.
Idle Performance: Check the idle speed again. Make sure the engine idles smoothly without stalling.
Cutting Performance: Test the weed eater by cutting some grass or weeds. Pay attention to how the engine performs under load.

Strategic Insight: Carburetor adjustments can be affected by altitude, temperature, and humidity. If you live in an area with significant changes in these conditions, you might need to readjust your carburetor periodically.

Advanced Tuning Tips and Troubleshooting

Here are some advanced tuning tips and troubleshooting techniques to help you get the most out of your Stihl weed eater.

Understanding Carburetor Types

Stihl weed eaters use different types of carburetors, including:

Fixed-Jet Carburetors: These carburetors have fixed fuel jets and are not adjustable. They are typically found on older or entry-level models.
Adjustable Carburetors: These carburetors have adjustable L, H, and LA/T screws, allowing you to fine-tune the fuel mixture.

Identifying Your Carburetor Type: Consult your owner’s manual or look for adjustment screws on the carburetor body. If there are no adjustment screws, you likely have a fixed-jet carburetor.

Common Carburetor Problems and Solutions

Engine Won’t Start:
- Possible Causes: Clogged fuel filter, dirty spark plug, stale fuel, faulty ignition coil.
- Solutions: Replace the fuel filter, clean or replace the spark plug, drain and replace the fuel, test the ignition coil.
Engine Stalls at Idle:
- Possible Causes: Incorrect L screw setting, dirty carburetor, air leak in the fuel lines.
- Solutions: Adjust the L screw, clean the carburetor, check and replace the fuel lines.
Engine Lacks Power at High Speed:
- Possible Causes: Incorrect H screw setting, clogged air filter, restricted exhaust.
- Solutions: Adjust the H screw, clean the air filter, check and clean the exhaust port.
Engine Smokes Excessively:
- Possible Causes: Rich fuel mixture, incorrect oil-to-fuel ratio, worn piston rings.
- Solutions: Adjust the L and H screws, use the correct oil-to-fuel ratio, have the engine professionally inspected.

Cleaning Your Carburetor

A dirty carburetor can cause a variety of problems. Here’s how to clean it:

Remove the Carburetor: Disconnect the fuel lines and throttle cable, and remove the carburetor from the engine.
Disassemble the Carburetor: Carefully disassemble the carburetor, taking note of the location of each part.
Clean the Parts: Use carburetor cleaner to clean all the carburetor parts, including the jets, needles, and passages.
Reassemble the Carburetor: Reassemble the carburetor, making sure all the parts are properly installed.
Reinstall the Carburetor: Reinstall the carburetor on the engine, reconnect the fuel lines and throttle cable, and adjust the carburetor as described above.

Tool Specification: I recommend using a high-quality carburetor cleaner specifically designed for small engines. Avoid using harsh solvents that can damage the carburetor components.

Fuel and Oil Recommendations

Using the correct fuel and oil is crucial for the performance and longevity of your Stihl weed eater.

Fuel: Use fresh, high-quality gasoline with an octane rating of 89 or higher. Avoid using gasoline that contains ethanol, as ethanol can damage the fuel system. If you must use gasoline with ethanol, use a fuel stabilizer.
Oil: Use Stihl 2-stroke engine oil or a high-quality 2-stroke engine oil that meets or exceeds the API TC standard. Mix the oil with the gasoline at the ratio recommended in your owner’s manual. A typical ratio is 50:1 (50 parts gasoline to 1 part oil).

Data Insight: Using the correct oil-to-fuel ratio is essential. Too much oil can cause excessive smoke and carbon buildup, while too little oil can lead to engine damage.

Adjusting for Altitude

Altitude can affect the air-fuel mixture. At higher altitudes, the air is thinner, which means the engine will run richer. To compensate for this, you might need to lean out the fuel mixture by turning the L and H screws clockwise slightly.

General Rule: For every 1000 feet of altitude above sea level, you might need to turn the L and H screws clockwise about 1/8 of a turn.

Using a Vacuum Tester

A vacuum tester can help you diagnose air leaks in the engine. Air leaks can cause the engine to run lean and stall. To use a vacuum tester, connect it to the intake manifold and start the engine. If the vacuum reading is low, there might be an air leak.

Common Air Leak Locations: Check the intake manifold gasket, carburetor mounting flange, and crankshaft seals for air leaks.

Case Study: Reviving an Old Stihl FS 85

I once acquired a very old Stihl FS 85 weed eater that had been sitting in a shed for years. The engine wouldn’t start, and the carburetor was completely gummed up. Here’s what I did to revive it:

Complete Carburetor Overhaul: I completely disassembled the carburetor and cleaned every part with carburetor cleaner. I also replaced the fuel lines and fuel filter.
Spark Plug Replacement: I replaced the old spark plug with a new one.
Fuel System Flush: I flushed the fuel tank and fuel lines with fresh gasoline.
Carburetor Adjustment: After reassembling the engine, I carefully adjusted the carburetor as described above.

Results: After a few pulls, the engine started right up and ran smoothly. The weed eater performed like new.

Air Filter Maintenance

A clean air filter is essential for proper engine performance. Check the air filter regularly and clean or replace it as needed.

Cleaning: Wash the air filter with warm soapy water, rinse it thoroughly, and let it dry completely before reinstalling it.
Replacement: Replace the air filter if it’s damaged or excessively dirty.

Frequency: I recommend cleaning the air filter every 25 hours of use or more frequently if you’re working in dusty conditions.

Spark Plug Maintenance

A properly functioning spark plug is crucial for ignition. Check the spark plug regularly and clean or replace it as needed.

Cleaning: Clean the spark plug with a wire brush and check the gap. Adjust the gap to the specification in your owner’s manual.
Replacement: Replace the spark plug if it’s damaged or worn.

Spark Plug Gap: A typical spark plug gap for a Stihl weed eater is 0.020-0.025 inches.

Fuel Filter Maintenance

A clogged fuel filter can restrict fuel flow and cause the engine to stall. Replace the fuel filter regularly.

Replacement Frequency: I recommend replacing the fuel filter every 50 hours of use or annually, whichever comes first.

Fuel Line Maintenance

Fuel lines can become cracked or brittle over time, leading to fuel leaks and poor engine performance. Inspect the fuel lines regularly and replace them if necessary.

Inspection Tip: Look for cracks, leaks, or swelling in the fuel lines.

Cutting Head Maintenance

Keep the cutting head clean and free of debris. Replace the cutting line when it becomes worn or damaged.

Line Type Selection: Use the correct type and size of cutting line for your weed eater. Consult your owner’s manual for recommendations.

Storage

When storing your Stihl weed eater for an extended period, take these precautions:

Drain the Fuel Tank: Drain the fuel tank completely to prevent fuel from going stale.
Run the Engine Dry: Start the engine and let it run until it stalls to remove any remaining fuel from the carburetor.
Remove the Spark Plug: Remove the spark plug and pour a small amount of 2-stroke oil into the cylinder. Pull the starter cord a few times to distribute the oil, then reinstall the spark plug.
Clean the Weed Eater: Clean the weed eater thoroughly and store it in a dry place.

Long-Term Storage Tip: I recommend using a fuel stabilizer in the fuel tank before storing your weed eater. This will help prevent the fuel from going stale and damaging the fuel system.

Wood Processing and Firewood Preparation: The Bigger Picture

Adjusting your Stihl weed eater carburetor is just one small part of the larger process of wood processing and firewood preparation. Let’s take a look at some other essential aspects.

Felling Techniques

Felling trees safely and efficiently is the first step in wood processing. Here are some basic felling techniques:

Assess the Tree: Before felling a tree, assess its size, lean, and any potential hazards.
Plan Your Escape Route: Plan a clear escape route away from the tree’s falling path.
Make the Notch Cut: Make a notch cut on the side of the tree in the direction you want it to fall. The notch cut should be about 1/3 of the tree’s diameter.
Make the Back Cut: Make the back cut on the opposite side of the tree, slightly above the notch cut. Leave a hinge of wood to control the tree’s fall.
Use Felling Wedges: Use felling wedges to help direct the tree’s fall.

Safety Consideration: Always wear appropriate safety gear when felling trees, including a hard hat, safety glasses, hearing protection, and chainsaw chaps.

Debarking Logs

Debarking logs can help them dry faster and prevent insect infestations. Here are some methods for debarking logs:

Hand Debarking: Use a drawknife or spud to remove the bark by hand.
Mechanical Debarking: Use a mechanical debarker to remove the bark.

Tool Specification: A drawknife is a traditional tool for debarking logs. It consists of a curved blade with two handles.

Splitting Firewood

Splitting firewood makes it easier to handle and dry. Here are some methods for splitting firewood:

Hand Splitting: Use an axe or maul to split the firewood by hand.
Hydraulic Splitting: Use a hydraulic log splitter to split the firewood.

Strategic Advantage: A hydraulic log splitter can significantly increase your efficiency when splitting firewood, especially with larger logs.

Log Splitter Tonnage: Choose a log splitter with enough tonnage to split the size of logs you’ll be working with. A 20-ton log splitter is typically sufficient for most firewood applications.

Drying Firewood

Drying firewood is essential for efficient burning. Green wood contains too much moisture, which makes it difficult to ignite and produces excessive smoke.

Seasoning: Seasoning firewood involves stacking it in a well-ventilated area for several months to allow it to dry.
Kiln Drying: Kiln drying firewood involves drying it in a kiln to reduce its moisture content quickly.

Moisture Content Target: The ideal moisture content for firewood is 20% or less. Use a moisture meter to check the moisture content of your firewood.

Stacking Technique: Stack firewood in a single row, with the pieces slightly separated to allow for air circulation. Cover the top of the stack to protect it from rain and snow.

Stacking Firewood

Properly stacking firewood not only makes it easier to handle but also promotes efficient drying.

Choose a Dry Location: Select a dry, well-drained location for your firewood stack.
Elevate the Stack: Elevate the stack off the ground using pallets or wooden planks.
Stack Tightly: Stack the firewood tightly to prevent it from collapsing.
Cover the Top: Cover the top of the stack to protect it from rain and snow.

Case Study: My Firewood Stacking Method

I’ve developed a firewood stacking method that maximizes drying and minimizes space. I use pallets as a base to elevate the stack, and I stack the firewood in a crisscross pattern to promote air circulation. I also cover the top of the stack with a tarp to protect it from the elements. This method has proven to be very effective in drying firewood quickly and efficiently.

Conclusion: Empowering You to Maintain Your Equipment and Process Wood Effectively

Adjusting your Stihl weed eater carburetor is a skill that can save you time, money, and frustration. By understanding the basics of carburetor function, following the step-by-step guide, and practicing regular maintenance, you can keep your weed eater running smoothly for years to come.

Beyond the weed eater, understanding the broader context of wood processing and firewood preparation is essential for anyone who relies on wood as a heat source. From felling trees safely to splitting and drying firewood efficiently, each step in the process requires knowledge, skill, and attention to detail.

My goal is to empower you with the knowledge and skills you need to maintain your equipment and process wood effectively. Whether you’re a homeowner, a small contractor, or a hobbyist, I hope this guide has provided you with valuable insights and practical tips that you can use in your own wood processing projects.

Next Steps:

Consult Your Owner’s Manual: Always refer to your Stihl weed eater’s owner’s manual for specific instructions and recommendations.
Gather Your Tools: Assemble the necessary tools, including a screwdriver set, carburetor adjustment tool, tachometer (optional), spark plug wrench, and safety gear.
Practice and Experiment: Don’t be afraid to experiment with the carburetor adjustments to find the optimal settings for your weed eater.
Seek Professional Help: If you’re not comfortable adjusting the carburetor yourself, take your weed eater to a qualified technician.
Continue Learning: Stay informed about the latest tools, techniques, and safety standards in wood processing and firewood preparation.

By taking these steps, you can ensure that you’re well-equipped to handle any wood processing challenge that comes your way. Happy wood processing!