Stihl 640 Chainsaw Maintenance Tips (5 Expert Engine Fixes)

Did you know that improper chainsaw maintenance accounts for over 50% of chainsaw-related injuries? As someone who’s spent over 20 years felling trees, processing timber, and preparing firewood, I can tell you firsthand that a well-maintained chainsaw isn’t just about performance; it’s about safety and longevity. In this guide, I’ll walk you through five expert engine fixes specifically for the Stihl 640 chainsaw, a workhorse favored by professionals and serious hobbyists alike. We’ll cover everything from basic troubleshooting to more advanced repairs, ensuring your saw runs smoothly and safely for years to come.

Stihl 640 Chainsaw Maintenance Tips (5 Expert Engine Fixes)

The Stihl 640 is a powerful machine, but like any tool, it needs regular maintenance to perform optimally. Neglecting your chainsaw can lead to frustrating starting problems, reduced cutting efficiency, and, most importantly, increased risk of accidents. I’ve seen it happen countless times: a buddy pushes his saw too hard, skips routine checks, and ends up with a costly repair bill – or worse, an injury. This guide is designed to prevent those situations.

Understanding Key Terms

Before we dive into the fixes, let’s clarify some essential terms:

Green Wood vs. Seasoned Wood: Green wood is freshly cut and has a high moisture content (often 50% or more). Seasoned wood, on the other hand, has been dried, typically to a moisture content of 20% or less. Cutting green wood puts more strain on your chainsaw’s engine due to the increased resistance.
Two-Stroke Engine: The Stihl 640 uses a two-stroke engine, meaning it completes a power cycle in two strokes of the piston instead of four. This requires a specific fuel mixture of gasoline and two-stroke oil.
Carburetor: The carburetor mixes air and fuel to create a combustible mixture for the engine. It’s a crucial component for proper engine function.
Spark Plug: The spark plug ignites the fuel-air mixture in the cylinder, initiating the combustion process.
Air Filter: The air filter prevents dirt and debris from entering the engine, ensuring clean air intake.

Fix #1: Diagnosing and Resolving Starting Problems

One of the most common frustrations with any chainsaw is difficulty starting. Here’s a systematic approach to diagnosing and fixing starting issues on your Stihl 640.

Step 1: Fuel Check and Mixture Inspection

Action: Ensure the fuel tank has fresh fuel. Two-stroke fuel degrades over time, especially if mixed with ethanol. Old fuel can clog the carburetor.
My Experience: I once spent an hour troubleshooting a saw that wouldn’t start, only to realize the fuel was over six months old. Now, I always date my fuel containers.
Fuel Mixture: Stihl typically recommends a 50:1 fuel-to-oil ratio. Using the wrong ratio can cause engine damage. I use a pre-mixed fuel like Stihl MotoMix to eliminate any guesswork.
Data: A study by the Outdoor Power Equipment Institute (OPEI) found that 70% of small engine problems are fuel-related.
Tool: A fuel stabilizer can extend the life of your fuel. I recommend adding it to any fuel I plan to store for more than a month.
Example: If you’re using a 1-gallon (3.78 liters) container, you’ll need 2.6 ounces (77 ml) of two-stroke oil for a 50:1 ratio.

Step 2: Spark Plug Inspection and Replacement

Action: Remove the spark plug and inspect it for fouling (carbon buildup), damage, or excessive wear.
Procedure: Use a spark plug wrench to remove the plug. Clean the electrode with a wire brush if it’s just slightly fouled.
Replacement: If the spark plug is heavily fouled, cracked, or the electrode is worn, replace it with a new one. Use the correct spark plug specified for your Stihl 640 (typically a Bosch WSR6F or NGK BPMR7A).
Gap: Ensure the spark plug gap is correct (usually around 0.020 inches or 0.5 mm). Use a spark plug gap tool to adjust it.
Case Study: I had a Stihl 640 that kept stalling. After replacing the spark plug, it ran like new. Sometimes, it’s the simplest fix.
Tool: Spark plug wrench, spark plug gap tool, wire brush.

Step 3: Air Filter Examination and Cleaning

Action: A clogged air filter restricts airflow, leading to a rich fuel mixture and difficulty starting.
Procedure: Remove the air filter cover and inspect the filter. Clean a foam filter with warm, soapy water, rinse thoroughly, and allow it to dry completely before re-oiling it lightly with air filter oil. Replace a paper filter if it’s excessively dirty or damaged.
Frequency: I clean my air filter after every 10 hours of use, or more frequently in dusty conditions.
Insight: A clean air filter not only improves starting but also extends the engine’s life.
Tool: Warm water, soap, air filter oil (Stihl air filter oil is recommended).
Example: I was cutting firewood in a dry, dusty area. The air filter was completely clogged after just a few hours. Cleaning it made a significant difference in the saw’s performance.

Step 4: Checking the Ignition System

Action: If the fuel, spark plug, and air filter are all good, the problem might be with the ignition system.
Procedure: Remove the spark plug and reattach it to the spark plug wire. Hold the spark plug against the engine cylinder (grounding it) and pull the starter cord. You should see a strong, blue spark. If there’s no spark or a weak, yellow spark, the ignition coil might be faulty.
Caution: Be careful not to get shocked when testing the spark plug.
Repair: Replacing the ignition coil is a more complex repair and might require professional assistance.
Insight: I’ve found that ignition coil failures are more common in older saws.
Tool: Insulated gloves, spark plug wrench.

Step 5: Inspecting the Carburetor

Action: A clogged carburetor can prevent fuel from reaching the engine.
Procedure: This requires some disassembly. Carefully remove the air filter housing and inspect the carburetor. Look for any visible dirt or debris.
Cleaning: You can try cleaning the carburetor with carburetor cleaner. Follow the instructions on the cleaner carefully.
Advanced Repair: If the carburetor is heavily clogged or damaged, it might need to be rebuilt or replaced. This is often best left to a professional.
My Experience: I once had a Stihl 640 that wouldn’t idle properly. After cleaning the carburetor jets with a fine wire and carburetor cleaner, it idled perfectly.
Tool: Carburetor cleaner, small screwdrivers, fine wire.

Fix #2: Addressing Engine Stalling Issues

Engine stalling, especially after running for a short period, is another common problem. Here’s how to troubleshoot it:

Step 1: Idle Speed Adjustment

Action: The idle speed might be set too low, causing the engine to stall.
Procedure: Locate the idle speed adjustment screw on the carburetor. Turn it clockwise to increase the idle speed.
Caution: Don’t set the idle speed too high, as this can cause the chain to rotate when the saw is idling.
Adjustment: Adjust the idle speed until the engine runs smoothly without stalling.
Insight: I usually adjust the idle speed after cleaning the carburetor or replacing the spark plug.
Tool: Screwdriver.

Step 2: Fuel Line Inspection

Action: A cracked or damaged fuel line can cause air leaks, leading to stalling.
Procedure: Inspect the fuel lines for cracks, leaks, or kinks. Replace any damaged fuel lines.
Replacement: Fuel lines are relatively inexpensive and easy to replace.
My Experience: I had a saw that would run fine for a few minutes and then stall. It turned out the fuel line had a small crack that was letting air in.
Tool: Screwdriver, pliers.

Step 3: Fuel Filter Check

Action: A clogged fuel filter can restrict fuel flow, causing the engine to stall.
Procedure: Locate the fuel filter inside the fuel tank. Remove it and inspect it for dirt and debris. Clean or replace the fuel filter as needed.
Frequency: I replace the fuel filter annually, or more frequently if I’m cutting in dirty conditions.
Insight: A clean fuel filter ensures a consistent fuel supply to the engine.
Tool: Pliers, small screwdriver.

Step 4: Venting Issues

Action: A blocked fuel tank vent can create a vacuum in the tank, preventing fuel from flowing to the carburetor.
Procedure: Locate the fuel tank vent and ensure it’s not blocked. You can try blowing air through it to clear any obstructions.
Insight: Sometimes, the vent can become clogged with sawdust or debris.
Tool: Compressed air (optional).

Step 5: Overheating

Action: Overheating can cause the engine to stall.
Procedure: Check the cooling fins on the cylinder for dirt and debris. Clean them with a brush or compressed air. Ensure the chainsaw is properly lubricated.
Insight: I’ve seen overheating issues occur more frequently in hot weather or when cutting dense wood.
Tool: Brush, compressed air.

Fix #3: Enhancing Cutting Performance

Even with a properly running engine, cutting performance can suffer if the chain is dull, the bar is worn, or the lubrication system isn’t working correctly.

Step 1: Chain Sharpening

Action: A sharp chain is essential for efficient and safe cutting.
Procedure: Use a chainsaw file and guide to sharpen the chain’s cutters. Maintain the correct filing angle and depth.
Technique: I prefer to use a chainsaw file guide to ensure consistent sharpening.
Frequency: Sharpen the chain after every few hours of use, or more frequently if you’re cutting dirty or abrasive wood.
Alternative: You can also use a chainsaw chain grinder for faster and more precise sharpening.
Tool: Chainsaw file, file guide, chainsaw chain grinder (optional).
Data: A sharp chain can increase cutting speed by up to 50% and reduce the strain on the engine.

Step 2: Bar Maintenance

Action: A worn or damaged bar can reduce cutting efficiency and increase the risk of chain breakage.
Procedure: Inspect the bar for wear, damage, and burrs. Remove any burrs with a file. Clean the bar groove with a bar groove cleaner. Ensure the bar oil holes are clear.
Flipping: Rotate the bar periodically to distribute wear evenly.
Replacement: Replace the bar if it’s excessively worn or damaged.
Insight: I always check the bar after sharpening the chain.
Tool: File, bar groove cleaner.

Step 3: Chain Lubrication System

Action: Proper chain lubrication is crucial for reducing friction and extending the life of the chain and bar.
Procedure: Ensure the chain oil tank is full. Check the oiler output by running the chainsaw at full throttle and observing the oil spray. Adjust the oiler output if necessary.
Oil: Use a high-quality chain oil specifically designed for chainsaws.
Troubleshooting: If the chain isn’t being lubricated properly, check the oiler pump and oil lines for clogs or damage.
Case Study: I once had a Stihl 640 that kept throwing its chain. It turned out the oiler was clogged, and the chain was overheating and stretching.
Tool: Screwdriver, chain oil.

Step 4: Chain Tension Adjustment

Action: Correct chain tension is essential for safe and efficient cutting.
Procedure: Loosen the bar nuts and adjust the chain tension using the chain tensioning screw. The chain should be snug against the bar but still able to be pulled around by hand. Tighten the bar nuts securely.
Insight: I check the chain tension before each use.
Tool: Bar wrench.

Step 5: Wood Selection and Technique

Action: The type of wood you’re cutting and your cutting technique can significantly impact cutting performance.
Wood Type: Green wood is harder to cut than seasoned wood.
Technique: Use proper felling and bucking techniques to avoid pinching the chain. Avoid cutting in dirt or sand, as this will dull the chain quickly.
Insight: I always try to plan my cuts to minimize the risk of pinching the chain.
Data: Cutting dirty wood can reduce chain life by up to 50%.

Fix #4: Resolving Engine Overheating Issues

Overheating can cause significant damage to your Stihl 640. Here’s how to address it:

Step 1: Cooling Fin Cleaning

Action: Dirty or blocked cooling fins prevent proper heat dissipation.
Procedure: Clean the cooling fins on the cylinder and cylinder head with a brush or compressed air.
Frequency: I clean the cooling fins after every few uses, especially in dusty conditions.
Insight: Overheating is more common in hot weather.
Tool: Brush, compressed air.

Step 2: Airflow Obstruction Removal

Action: Ensure there are no obstructions blocking airflow around the engine.
Procedure: Remove any debris from around the carburetor and exhaust.
Insight: Sawdust and other debris can accumulate around the engine and restrict airflow.
Tool: Screwdriver, brush.

Step 3: Fuel Mixture Adjustment

Action: A lean fuel mixture (too much air, not enough fuel) can cause the engine to overheat.
Procedure: Adjust the carburetor settings to ensure a proper fuel mixture. This might require professional assistance if you’re not familiar with carburetor adjustments.
Caution: Incorrect carburetor adjustments can damage the engine.
Insight: I usually adjust the carburetor in small increments and monitor the engine temperature.
Tool: Screwdriver, tachometer (optional).

Step 4: Spark Arrestor Cleaning

Action: A clogged spark arrestor can restrict exhaust flow, causing the engine to overheat.
Procedure: Remove the spark arrestor screen from the muffler and clean it with a wire brush.
Frequency: I clean the spark arrestor screen annually.
Insight: A clogged spark arrestor can also reduce engine power.
Tool: Wire brush, screwdriver.

Step 5: Cylinder and Piston Inspection

Action: If the engine continues to overheat, there might be internal damage to the cylinder or piston.
Procedure: This requires removing the cylinder head and inspecting the cylinder and piston for scoring or damage. This is best left to a professional.
Repair: Damaged cylinders or pistons might need to be replaced.
Insight: Overheating can cause significant internal engine damage.

Fix #5: Maintaining Optimal Idling and Throttle Response

A smooth idling engine and responsive throttle are indicators of a healthy chainsaw. Here’s how to maintain them:

Step 1: Carburetor Adjustment (Idle and High Speed)

Action: The carburetor controls the air-fuel mixture at idle and high speeds.
Procedure: Locate the idle speed screw (T), the low-speed mixture screw (L), and the high-speed mixture screw (H) on the carburetor. Adjust these screws according to the manufacturer’s instructions.
Idle: Adjust the idle speed screw (T) until the engine idles smoothly without stalling or the chain rotating.
Low Speed: Adjust the low-speed mixture screw (L) for smooth acceleration from idle.
High Speed: Adjust the high-speed mixture screw (H) for optimal power at full throttle.
Caution: Incorrect carburetor adjustments can damage the engine.
Tool: Screwdriver, tachometer (optional).

Step 2: Throttle Cable Inspection

Action: A damaged or misadjusted throttle cable can affect throttle response.
Procedure: Inspect the throttle cable for damage or kinks. Ensure the throttle cable is properly connected to the carburetor and the throttle trigger.
Adjustment: Adjust the throttle cable tension if necessary.
Insight: I always check the throttle cable after adjusting the carburetor.
Tool: Screwdriver, pliers.

Step 3: Clutch Inspection

Action: A worn or damaged clutch can affect throttle response and cause the chain to rotate at idle.
Procedure: Inspect the clutch shoes for wear. Replace the clutch if the shoes are excessively worn or damaged.
Insight: I’ve found that clutch failures are more common in older saws.
Tool: Clutch removal tool (optional), screwdriver.

Step 4: Flywheel Key Check

Action: A sheared flywheel key can cause timing problems and affect throttle response.
Procedure: Remove the flywheel and inspect the flywheel key for damage. Replace the flywheel key if it’s sheared.
Insight: A sheared flywheel key can also cause starting problems.
Tool: Flywheel puller, screwdriver.

Step 5: Regular Maintenance

Action: Consistent maintenance is key to preventing problems and ensuring optimal performance.
Procedure: Follow the manufacturer’s recommended maintenance schedule. This includes regularly cleaning the air filter, replacing the spark plug, and inspecting the fuel lines.
Insight: I always keep a log of my chainsaw maintenance.

Strategic Insights and Benefits

Beyond the tactical fixes, consider these strategic advantages of proper Stihl 640 maintenance:

Increased Lifespan: Regular maintenance significantly extends the life of your chainsaw, saving you money in the long run.
Improved Safety: A well-maintained chainsaw is safer to operate. Sharp chains and properly functioning components reduce the risk of kickback and other accidents.
Enhanced Efficiency: A properly tuned chainsaw cuts faster and more efficiently, saving you time and effort.
Reduced Downtime: Regular maintenance prevents unexpected breakdowns, minimizing downtime and keeping your projects on schedule.
Higher Resale Value: A well-maintained chainsaw will fetch a higher price if you ever decide to sell it.

Safety Considerations

Chainsaw operation and maintenance can be dangerous. Always follow these safety precautions:

Wear Personal Protective Equipment (PPE): This includes a helmet, eye protection, hearing protection, gloves, chainsaw chaps, and sturdy boots.
Read the Owner’s Manual: Familiarize yourself with the chainsaw’s operating instructions and safety precautions.
Work in a Safe Environment: Clear the area of obstacles and ensure you have a stable footing.
Use Proper Cutting Techniques: Avoid kickback by using proper felling and bucking techniques.
Never Modify the Chainsaw: Modifying the chainsaw can compromise its safety and performance.
Disconnect the Spark Plug: Before performing any maintenance, disconnect the spark plug wire to prevent accidental starting.
Handle Fuel Safely: Store fuel in approved containers and avoid spilling fuel.
Seek Professional Help: If you’re not comfortable performing a repair, seek professional assistance from a qualified chainsaw technician.

Next Steps and Implementation Guidance

Now that you’ve learned these expert engine fixes, here’s how to put them into practice:

Create a Maintenance Schedule: Develop a regular maintenance schedule for your Stihl 640. Include tasks such as cleaning the air filter, sharpening the chain, and inspecting the fuel lines.
Gather the Necessary Tools: Assemble a toolkit with the tools you’ll need for chainsaw maintenance, such as a chainsaw file, file guide, spark plug wrench, screwdriver, and carburetor cleaner.
Start with the Basics: Begin by performing the simple maintenance tasks, such as cleaning the air filter and sharpening the chain.
Tackle More Complex Repairs: As you become more comfortable, you can tackle more complex repairs, such as adjusting the carburetor or replacing the fuel lines.
Document Your Work: Keep a log of all the maintenance and repairs you perform on your chainsaw. This will help you track its performance and identify potential problems.
Seek Professional Assistance: If you encounter a problem you can’t solve, don’t hesitate to seek professional assistance from a qualified chainsaw technician.

By following these steps and implementing the expert engine fixes outlined in this guide, you can keep your Stihl 640 chainsaw running smoothly and safely for years to come. Remember, a well-maintained chainsaw is not only more efficient but also safer to operate. So, take the time to care for your saw, and it will reward you with years of reliable service.

Stihl 640 Chainsaw Maintenance Tips (5 Expert Engine Fixes)