7 Most Common Stihl MS 661 Problems and How to Fix Them

Stihl MS 661 is the go-to chainsaw for homeowners and professionals. It handles the most demanding tasks around your home and suits commercial applications.

The chainsaw is powerful, fuel-efficient, and will last longer than most models from other brands. But like any other chainsaw, the Stihl MS 661 has its shortcomings. The most common Stihl MS 661 problems are chainsaw failing to start, erratic idling, overheating, losing power, and excessively vibrating.

Understanding these problems can help you diagnose and resolve them efficiently. Herein is a detailed Stihl MS 661 troubleshooting guide.

Stihl MS 661 Starting Problems

Issues with the spark plug or the air filter can cause difficulties when starting your chainsaw. Generally, a dirty or damaged spark plug or air filter might be to blame.

If these components are okay, the starting problem might result from insufficient fuel. Remember that the engine requires sufficient fuel flow to start and keep running.

How to Fix:

Before checking for damaged components, ensure the tank has enough fuel. If the chainsaw’s fuel level is low, fill the tank with fresh fuel. 

Avoid leaving the fuel in the tank unused for over 30 days, as it can degrade and damage the fuel lines.

If your MS 661 chainsaw has enough fuel but won’t start, locate the spark plug and pull it out. If it’s dirty, clean it. If it appears burnt or corroded, replace it. Next, check the air filter as it accumulates dirt and debris over time. Clean the dirty filter or replace it if it appears worn.

Engine Losing Power

Does your Stihl MS 661 engine start, run for a while, and begin to lose power? Sometimes, the engine will run for a few minutes, become weak and stop.

Insufficient engine power indicates issues with the fuel system, air filter, or spark plug. If the problem results from the fuel system, check for inadequate fuel, old fuel, or a clogged fuel filter.

How to Fix:

A low fuel level in the tank causes power loss and stalling problems. So, check the tank and fill it with fresh fuel as necessary. If there’s fuel in the tank, check if it’s old or contaminated. Old or contaminated fuel can release sediment that might block the lines and filter.

Drain the old/contaminated fuel and clean the tank to remove the clogs. Then add a fuel additive cleaner and fresh fuel into the tank and start your chainsaw. The cleaner will clear out any clogs in the fuel lines. Then, replace the clogged fuel filter and clean the carburetor.

If the chainsaw loses power again, inspect and clean the air filter to remove buildup. You’ll also need to examine the spark plug and replace it if it’s defective.

Poor Chain Acceleration

Poor chain acceleration is one of the most common Stihl MS 661 problems. The chain needs to accelerate smoothly and quickly to keep up with your cutting needs.

A few reasons might cause poor chain acceleration in your Stihl chainsaw. The first one might be a chain that’s too tight. If not, the problem might result from:

  • Insufficient lubricant
  • Incorrectly adjusted oiler
  • Clogged oiler

How to Fix:

If the problem occurs due to an overtightened chain, locate the chainsaw’s tension screw and turn it counterclockwise. Adjust the screw accordingly by turning it counterclockwise and clockwise until you have a perfectly loose chain.

As you adjust the chain, check if it’s too dry. If it’s not adequately lubricated, check the oiler and adjust it accordingly. You can add fresh oil if you find the oiler empty. Clean the oiler to remove any debris blocking oil flow.

Erratic Idling

Erratic idling of your 661 Stihl chainsaw indicates engine performance issues. You might notice that your chainsaw has inconsistent idle speed that rises and falls erratically.

Several factors can lead to erratic engine idles. The most common ones include the following:

  • Improper idle adjustment
  • Faulty spark plug
  • Clogged air filter
  • An iced-up carburetor

How to Fix:

Check the engine idle speed to see if an improper adjustment is the cause. You can adjust the low-speed screw by moving it counterclockwise. Move the screw slightly until the engine runs properly.

If incorrect engine idle speed is not the cause, inspect the spark plug and the air filter. Replace the plug if it’s defective and clean a clogged filter.

Then check for an iced-up carburetor that might prevent sufficient airflow into the engine. Open the throttle to allow airflow. Then start the chainsaw’s engine and allow it to warm up to resolve an iced-up carburetor problem.

Malfunctioning Chain Brake

The chain brake serves as a safety feature on your 661 Stihl chainsaw. It stops the rotation of the chain if an engine kickback occurs. You should ensure the brake engages and disengages correctly before using the chainsaw.

Unfortunately, the chainsaw might get stuck over time and fail to engage or disengage. 

The chain brake malfunctions or gets stuck due to the following:

  • Accumulated debris
  • A worn or damaged brake band
  • A broken spring
  • Insufficient lubricant

How to Fix:

Clean the brake system to remove any accumulated dirt and wood chips. You can use a wire brush to clear out the accumulated debris. Then apply lubricating oil to the brake band and pivot points.

If the chain brake problem persists, check for a worn or broken brake band or spring. Replace these components if they’re defective.

Engine Overheating

The Stihl MS 661 chainsaw requires regular maintenance to run smoothly. Failure to stick to the scheduled maintenance can lead to most Stihl MS 661 problems. For example, the engine might overheat and start smoking heavily.

An overheating problem in this chainsaw occurs due to a dirty/clogged air filter or cooling fins. The chainsaw might also run hot if it has a malfunctioning carburetor or clogged muffler.

How to Fix:

If the chainsaw feels too hot to the touch, the air filter or cooling fins might be clogged. Sawdust and other debris can accumulate on the filter or fins, preventing cool air from reaching the carburetor. So clean the filter and cooling fins to remove any stuck debris.

Check for a misadjusted carburetor if the chainsaw overheats after fixing the filter and fins. An incorrectly adjusted carburetor will cause the engine to run too lean and overheat. Fix this issue by modifying the carburetor to allow more air into the engine.

A blocked muffler could also be the culprit. If the engine runs too rich for an extended period, ash clogs the muffler and causes overheating problems. Dirt can also block the muffler. Remove the clog with a wire brush and clean the stubborn clogs with a degreaser.

Excessive Vibration

Stihl MS 661 is a robust and durable chainsaw designed with low vibration levels. However, excessive use of the chainsaw over time can cause vibration issues. The chainsaw handles heavy-duty tasks, and parts can become loose and vibrate excessively when the engine is running.

Vibration problems in your 661 Stihl chainsaw can result from loose or damaged parts.

How to Fix:

Look for the loose parts and tighten any loose bolts and screws. If you find any worn or damaged screws and bolts, replace them. Then, look for broken or damaged components, such as the shroud or bar, and replace them.

Stihl MS 661 Recall

Stihl recalled their MS 661 in the US, UK, and Germany due to performance issues. However, the recall for chainsaws sold in the US and UK was voluntary. Users could return their chainsaws or keep them.

In Germany, the recall required users to return the MS 661 in exchange for the MS 660.

The Stihl MS 661 chainsaw recall was due to poor performance and stalling problems. These problems result from issues with the ignition module and the cylinders.

Stihl MS 661 problems

Are Stihl MS 661 Problems a Deal Breaker?

The good news is that despite having several issues, the 661 Stihl chainsaw is worth the investment. It’s a reliable chainsaw that will last for years if properly maintained. If you face the above Stihl chainsaw issues, diagnose and fix them or seek help from your local mechanic.