That familiar sound of your lawnmower starting strong, only to choke and lose RPM when it hits a patch of thick grass, is a clear signal: your engine is struggling under load.
A loss of power under load means the engine isn’t getting enough fuel/air or is facing too much resistance to maintain maximum speed (RPM). The good news is that the fix is usually one of the Big Five easy maintenance checks.
1. Air Starvation: The Clogged Filter
The number one cause of power loss is a choked engine. When the mower is idling, it only needs a tiny bit of air. When you hit heavy grass, it demands a rush of air to mix with the fuel and create power.
- The Problem: A dirty, oil-soaked, or debris-covered air filter restricts this airflow. The fuel-to-air mixture becomes too rich (too much fuel), leading to weak combustion and instant power loss.
- The Fix:
- Inspect and Replace: Pull the air filter. If it’s a paper filter that’s dark or won’t pass the “Sunlight Test” (can’t see light through it), replace it immediately.
- Clean Pre-Filter: If you have a foam pre-filter, clean it with warm, soapy water, dry it completely, and re-oil it if recommended by the manufacturer.
2. Fuel Starvation: Old Gas and Clogged Filters
If the air is fine, the next thing to check is the fuel supply. The engine needs a steady, clean flow of gas to handle the increased load.
- The Problem:
- Stale Fuel: Gas (especially with ethanol) goes bad in as little as 30 days, losing its ability to combust efficiently.
- Clogged Fuel Filter: The filter (either inline or inside the tank—the clunk) can be choked with sediment, restricting the fuel flow necessary for high-power operation.
- The Fix:
- Use Fresh Gas: Drain any old fuel and refill with fresh, clean gasoline (use a fuel stabilizer if you plan to keep the gas longer than a month).
- Replace the Fuel Filter: It’s a cheap preventative measure. If you have a translucent inline filter, check for visible debris.
3. Physical Resistance: The Deck and Blades
Sometimes the problem isn’t the engine itself, but the excessive demand placed on it by the cutting system.
- The Problem:
- Caked Grass/Debris: A thick, heavy layer of old, wet grass packed under the deck dramatically increases drag on the blade, forcing the engine to work much harder.
- Dull Blades: Dull blades don’t cut; they tear the grass, requiring significantly more force and bogging down the engine.
- The Fix:
- Clean the Deck: Safely disconnect the spark plug wire and tip the mower back. Scrape and hose down the underside of the deck to remove all built-up grass.
- Sharpen or Replace Blades: Ensure your blades have a sharp edge. Sharp blades cut cleanly and require less power.
4. Ignition Issues: The Spark Plug
The spark needs to be strong and perfectly timed to ignite the larger charge of fuel and air that the engine pulls under load.
- The Problem: A fouled, oily, or carbon-caked spark plug creates a weak spark. While this might be enough to maintain a fast idle, it often fails under the increased pressure and load of heavy cutting.
- The Fix:
- Inspect and Clean: Remove the spark plug. If the tip is coated with black, dry carbon or wet oil, clean it gently with a wire brush or replace it entirely.
- Check the Gap: Use a feeler gauge (or gapper tool) to ensure the spark plug gap is set exactly to the manufacturer’s specification.
5. Carburetor Trouble: The Main Jet
If you’ve checked the top four and the problem persists, the issue is likely restricted fuel flow inside the carburetor.
- The Problem: When the engine transitions to full throttle, it relies on the main jet to supply the bulk of the fuel. This tiny passage is highly susceptible to clogging from fuel varnish and debris, starving the engine just when it needs power most.
- The Fix:
- Fuel Additive Soak: Try running the mower for 10 minutes with a high concentration of a quality fuel system cleaner (like Sea Foam or Stabil) in the gas. Then, shut it off and let it hot-soak for an hour before running it again.
- Clean the Carburetor: If the soak fails, you will need to disassemble the carburetor to clean the main jet and bowl.
The next time your mower bogs down, remember the five main culprits. A few minutes of checking these points can be the difference between a stalled machine and a clean cut!