Why Does My Mower Stop?

If your lawn mower starts and then suddenly stops, it's a common issue often pointing to a few key areas that need attention. The primary culprits typically involve old fuel, a clogged air filter, or a carburetor that needs cleaning. Other possibilities include problems with the spark plug or the ignition system.

Understanding these common issues can help you quickly diagnose and resolve why your mower is stalling.

Common Reasons Your Mower Stops Suddenly

A lawn mower relies on a precise mix of fuel and air, a strong spark, and a clean pathway for both to operate smoothly. When any of these components are compromised, your mower can sputter and die.

1. Old or Stale Fuel

• Why it happens: Gasoline can degrade over time, especially when left in the tank for extended periods (e.g., over winter). Old fuel loses its combustibility, attracts moisture, and can leave sticky residues or varnish that clog fuel lines and carburetor passages. This "varnish" prevents fuel from flowing correctly, starving the engine.
• Practical Insight: Modern gasoline, especially with ethanol, can become stale in as little as 30 days.
• Solutions:
  • Drain and Refill: Siphon out any old fuel from the tank.
  • Add Fresh Fuel: Refill with fresh, high-quality gasoline.
  • Fuel Stabilizer: For storage, always add a fuel stabilizer to prevent degradation. This is crucial for preventative maintenance Learn More about Mower Maintenance.

2. Clogged Air Filter

• Why it happens: The air filter's job is to prevent dirt, dust, and debris from entering the engine. Over time, it can become saturated with grime, restricting the vital flow of air to the carburetor. An engine needs a specific fuel-to-air ratio; if it doesn't get enough air, it runs "rich" (too much fuel, not enough air) and can effectively suffocate, causing it to stop.
• Practical Insight: A dirty filter puts extra strain on the engine and reduces fuel efficiency.
• Solutions:
  • Inspect and Clean: Remove the air filter and check its condition. Paper filters can be gently tapped to dislodge debris, while foam filters can often be washed with soap and water, dried thoroughly, and lightly oiled.
  • Replace: If the filter is heavily soiled, torn, or damaged, replace it with a new one.

3. Dirty Carburetor

• Why it happens: As mentioned, old or dirty fuel can leave deposits and varnish inside the carburetor. The carburetor is a complex component responsible for mixing fuel and air in precise proportions before sending it to the engine. If its tiny jets and passages are blocked, the engine won't receive the correct mixture, leading to erratic running or sudden stops.
• Practical Insight: This is a very common reason for stalling, especially after a period of disuse.
• Solutions:
  • Carburetor Cleaner: For minor clogs, a specialized carburetor cleaner spray can sometimes clear blockages without disassembly.
  • Manual Cleaning: For more stubborn issues, the carburetor may need to be disassembled and thoroughly cleaned with carburetor cleaner and compressed air.
  • Professional Service: If you're uncomfortable with disassembling the carburetor, consider professional servicing.

4. Spark Plug Issues

• Why it happens: The spark plug is crucial for igniting the fuel-air mixture in the engine's combustion chamber. A fouled (coated with carbon or oil), worn-out, cracked, or improperly gapped spark plug will produce a weak or inconsistent spark, preventing proper ignition and causing the mower to stop.
• Practical Insight: A visual inspection can often reveal a problem with the spark plug.
• Solutions:
  • Inspect: Remove the spark plug and check for signs of damage or heavy deposits.
  • Clean and Gap: If only lightly fouled, clean it with a wire brush and ensure the gap between the electrodes is correct using a feeler gauge (refer to your mower's manual for the correct gap).
  • Replace: If it's heavily fouled, worn, or cracked, replacing it with a new, correctly gapped spark plug is the best course of action.

5. Problems with the Ignition System

• Why it happens: Beyond the spark plug, other components of the ignition system could be at fault. This includes the ignition coil (which generates the high voltage for the spark plug), the kill switch (safety feature that can sometimes malfunction), or associated wiring. If any of these components fail, the spark plug won't receive power, and the engine won't be able to ignite the fuel.
• Practical Insight: These issues are less common but can be trickier to diagnose without specific tools.
• Solutions:
  • Check Connections: Ensure all wires and connections in the ignition system are secure and free of corrosion.
  • Test Components: Testing the ignition coil or kill switch often requires a multimeter or specialized diagnostic tools. If suspected, professional help may be needed.

Quick Troubleshooting Guide

Here's a summary of common issues and their typical solutions:

Proactive Maintenance Tips

To prevent your mower from stopping unexpectedly, consider these regular maintenance practices:

• Use Fresh Fuel: Always start with fresh gasoline, especially at the beginning of the mowing season.
• Fuel Stabilizer: If your mower will sit for more than a few weeks, use a fuel stabilizer.
• Clean/Replace Air Filter: Check your air filter regularly (e.g., every 25 operating hours or annually) and clean or replace it as needed.
• Inspect Spark Plug: Check and clean your spark plug annually, replacing it if it shows signs of wear or damage.
• Run It Dry: Before long-term storage, consider running the mower until the fuel tank is empty to prevent fuel-related issues.