There’s nothing more frustrating than when your lawn mower loses power when cutting. You’re moving along just fine, and then you hit a thicker patch of grass and the engine bogs down, struggling under heavy load. It’s a common problem that can turn a simple chore into a real headache. But don’t worry, this issue is almost always fixable with some basic troubleshooting. Let’s walk through the most common causes and their solutions, so you can get your mower running smoothly again.
A mower engine needs three things to run properly: fuel, air, and spark. When it struggles under load, it’s usually because one of these elements is being restricted or isn’t up to par. The good news is that most fixes are simple and don’t require expensive tools or a trip to the repair shop. We’ll start with the easiest checks and work our way to the less common problems.
Lawn Mower Loses Power When Cutting
This specific symptom points to a problem that only shows up when the engine is asked to do its hardest work. At idle or even moving on flat, thin grass, it might seem okay. But when the blade engages thick grass, the engine needs maximum power. If it can’t get it, it bogs down. Here are the primary systems to investigate.
Fuel System Problems: The Most Common Culprits
A starved engine is a weak engine. Fuel issues are the number one reason for power loss under load. Modern gasoline can start to go bad in as little as 30 days, leaving behind gummy residues that clog everything.
Old or Contaminated Gasoline
Gasoline that has been sitting in your mower’s tank over the winter is the prime suspect. It breaks down, loses volatility, and leaves varnish.
* Drain the old fuel from the tank and the carburetor bowl completely.
* Use fresh, clean gasoline with a maximum 10% ethanol content, if possible.
* Consider adding a fuel stabilizer to every can to prevent future issues.
Dirty or Clogged Fuel Filter
The fuel filter’s job is to catch debris before it reaches the carburetor. Over time, it can become so clogged it restricts fuel flow.
* Locate the fuel filter in the fuel line between the tank and the carburetor.
* Check if it looks dark or has visible debris inside.
* Replace it if it’s dirty—it’s an inexpensive part and a quick swap.
Faulty or Clogged Carburetor
This is where the real magic happens, and where most fuel problems end up. Tiny jets and passages inside the carburetor meter the fuel. Even a small piece of grit can block them.
* The main jet, which supplies fuel under high load, is often the culprit.
* You can try using a carburetor cleaner spray, but a partial or full clog often requires removing and cleaning the carburetor thoroughly.
* Sometimes, a rebuild kit or complete replacement is the most reliable fix.
Air Flow Restrictions: Choking the Engine
Your mower’s engine needs to breathe. For every part of fuel, it needs about 15 parts of air. If the air supply is restricted, the fuel mixture becomes too “rich” (too much fuel, not enough air) and power plummets.
Dirty Air Filter
This is the easiest check of all. A filter clogged with grass clippings, dust, and dirt is like trying to run a marathon while breathing through a straw.
* Remove the air filter cover and take out the filter.
* For a paper filter, hold it up to the light. If you can’t see light through it, replace it.
* For a foam filter, wash it in warm, soapy water, rinse, let it dry completely, and then lightly re-oil it if required.
Faulty or Stuck Choke
If the choke mechanism is stuck in the “on” or partially on position, it’s artificially restricting air flow all the time.
* Check that the choke lever or cable moves freely and returns to the “off” position when the engine is warm.
* A bent linkage or a broken spring can keep the choke plate closed.
Ignition System Issues: A Weak Spark
When the engine is under load, the combustion chamber pressure is higher. A weak spark system might fire just fine at idle but can fail under this increased pressure.
Worn or Fouled Spark Plug
A spark plug with a wide, eroded gap or one coated in carbon or oil can’t produce a strong, hot spark.
* Remove the spark plug using a proper socket wrench.
* Inspect the electrode. The gap should match your mower’s specification (check the manual).
* Look for heavy deposits or a cracked porcelain insulator.
* It’s often cheapest and easiest to just install a brand new, correctly gapped plug.
Failing Ignition Coil
The coil generates the high voltage for the spark. As it ages or gets hot, it can break down and produce a weak spark.
* Testing an ignition coil properly requires a special tool (an ohmmeter).
* A common symptom is a mower that runs poorly when hot but seems okay after cooling down.
* If you’ve ruled out fuel and air, and the spark plug is new, the coil is a likely suspect.
Mechanical and Physical Problems
Sometimes the problem isn’t with creating power, but with applying it or with the engine itself being restricted.
Dull Mower Blade
This is a hugely overlooked cause! A dull blade doesn’t cut grass—it tears it. This creates massive drag on the engine, making it work much harder than it should.
* Disconnect the spark plug wire for safety.
* Tip the mower on its side (check your manual for which side—usually with the air filter up).
* Inspect the blade. If it’s rounded, nicked, or bent, it needs sharpening or replacement.
* A sharp, balanced blade is essential for clean cutting and engine health.
Debris Under the Deck
A buildup of wet grass clippings under the mower deck acts like a heavy, unbalanced wheel and restricts the proper discharge of grass.
* Clean the underside of the deck thoroughly with a scraper and hose.
* Ensure all discharge chutes and openings are clear.
Faulty or Slipping Drive System (On Self-Propelled Mowers)
If your self-propelled mower seems to slow down in thick grass, the issue might not be the engine—it could be the drive belt or transmission.
* Listen for a squealing sound, which often indicates a worn or slipping drive belt.
* Check the drive belt for cracks, glazing, or looseness.
* On some models, the drive system itself can wear out, making it struggle to turn the wheels under load.
Engine Compression Loss
This is a less common but more serious issue. Over many years, an engine can lose compression due to worn piston rings, valves, or a blown head gasket.
* Symptoms include easy starting when cold but significant power loss, and sometimes blue or white smoke from the exhaust.
* A compression test requires a gauge, but it gives a definitive answer on engine health.
* Low compression often means a major engine repair or replacement.
Step-by-Step Diagnostic Guide
Follow this logical order to find the cause of your mower’s power loss.
1. Safety First: Always disconnect the spark plug wire before working near the blade or doing any significant work.
2. Check the Air Filter: Remove and inspect it. Clean or replace it if it’s dirty.
3. Inspect the Spark Plug: Remove it, check its condition and gap. Replace it if in doubt—it’s a cheap part.
4. Examine the Fuel: Smell the gas in the tank. If it’s old or smells sour, drain it completely.
5. Look at the Blade and Deck: Tip the mower safely and check for a dull blade and a clogged deck. Sharpen and clean as needed.
6. Investigate the Fuel Filter: If your mower has one in the line, replace it if it looks old or dirty.
7. Clean the Carburetor: If problems persist after fresh fuel and a new filter, the carburetor likely needs cleaning.
8. Consider the Ignition Coil: If the engine runs but is weak, especially when hot, test or replace the coil.
9. Check Drive Components: For self-propelled models, inspect the drive belt and mechanism for wear.
Prevention is Better Than Cure
A little regular maintenance can prevent most of these problems from ever starting.
* Use Fresh Fuel: Add stabilizer to your gas can, and try to use up fuel in your mower within 30 days.
* Annual Tune-Up: Each season, change the air filter, spark plug, and oil. Sharpen or balance the blade.
* Clean After Use: Let the mower run for a minute after finishing to help dry the deck, and occasionally scrape off clippings.
* Proper Storage: At the end of the season, either run the carburetor dry or use fuel stabilizer in a full tank.
FAQ Section
Why does my lawn mower run fine until I put it under load?
This is the classic sign of a fuel delivery issue (like a clogged main jet in the carburetor) or a weak ignition component (like a failing coil) that can’t cope with the higher demand.
What would cause a mower to bog down in thick grass?
The two most likely causes are a dull mower blade, which creates excessive drag, or a dirty air filter, which chokes the engine of air just when it needs it most.
How do I fix a lawn mower that lacks power?
Start with the simple checks: fresh gas, a clean air filter, and a new spark plug. If that doesn’t work, move on to cleaning the carburetor and checking the blade sharpness.
Is it worth repairing a mower that loses power?
In most cases, yes. The repairs are often inexpensive DIY projects. If the engine has major mechanical failure (low compression), then you’ll need to weigh the repair cost against the price of a new mower.
Can bad gas cause a mower to sputter and lose power?
Absolutely. Bad gasoline is one of the top reasons for poor performance. It can cause sputtering, difficulty starting, and a complete lack of power under load.
Getting your mower back to full strength is usually straightforward. By methodically checking the fuel, air, and spark systems, you’ll almost certainly find the culprit. Remember, regular maintenance is the key to avoiding these hassles in the future. With a sharp blade, fresh gas, and clean filters, your mower should handle even the toughest lawn without breaking a sweat.