Lawn Mower Loses Power When Cutting

It’s frustrating when your lawn mower loses power when cutting. That sudden bogging down in thick grass can really slow your progress. This common problem usually means the engine is struggling under heavy load. But don’t worry, the fix is often simple and something you can handle yourself.

We’ll walk through the most likely causes, from simple fixes to more involved checks. You’ll be back to a smooth, even cut in no time.

This specific symptom points to a few key areas. When the engine runs fine at idle or on flat, thin grass but then falters, it’s a sign of a fuel, air, or spark issue under demand. Think of it like you trying to run while breathing through a straw. The engine needs the right mix of all three elements to make power.

Common Culprits Behind the Power Loss

Let’s start with the easiest and most frequent problems. These are the things you should check first before moving on to more complex possibilities.

1. A Dirty or Clogged Air Filter

This is the number one cause. A dirty air filter chokes the engine, preventing it from getting the oxygen it needs to burn fuel efficiently, especially when it needs more power. It’s a simple and cheap fix.

Check: Remove the air filter cover and take out the filter.
Paper Filter: Hold it up to the light. If you can’t see light through it, it needs replacing.
Foam Filter: Wash it in warm, soapy water, rinse, and let it dry completely. Then lightly coat it with fresh filter oil.
Never run the mower without the air filter, as this can cause serious engine damage.

2. Old or Contaminated Fuel

Gasoline starts to degrade in as little as 30 days. Old fuel or fuel with water in it can cause sputtering and power loss. Ethanol in gas can also attract moisture and leave gummy deposits.

Drain the old fuel from the tank and the carburetor bowl.
Use fresh, clean gasoline, ideally with an ethanol treatment stabilizer.
For long-term storage, either run the engine dry or use a fuel stabilizer.

3. A Clogged Fuel System

If bad fuel has been sitting, it can clog the tiny passages in the carburetor or the fuel filter. This restricts the flow of gas just when the engine needs it most.

Locate the fuel filter (usually in the fuel line) and replace it if it looks dark or dirty.
For carburetor issues, you may need to clean it. This involves removing it and spraying carburetor cleaner through all the jets and passages.

4. A Fouled or Worn Spark Plug

A weak spark can’t reliably ignite the fuel mixture. Under heavy load, a faulty plug will often cause the engine to miss or stall. It’s a $5 part that makes a huge difference.

Disconnect the spark plug wire.
Use a socket wrench to remove the plug.
Inspect the electrode. It should be a light tan or gray color. If it’s black, oily, or has heavy deposits, replace it.
Set the gap on the new plug to your mower’s specification (check the manual) before installing.

Less Common But Important Checks

If the basics didn’t solve it, these next areas are worth investigating. They require a bit more effort but are still very manageable.

5. The Cooling System: Overheating Engine

Engines need to stay cool. If the cooling fins on the engine block are packed with grass clippings and debris, the engine will overheat and lose power. This is a very common oversight.

After the engine is completely cool, use a brush or compressed air to clean all the grass and gunk from the fins around the cylinder head.
Also, make sure the engine shroud or cover is in place, as it directs the cooling airflow.

6. The Carburetor Adjustment

Sometimes the mixture screws on the carburetor can vibrate out of adjustment. An engine running too “lean” (not enough fuel) will overheat and lack power. One running too “rich” (too much fuel) will smoke and bog down.

Note: Some newer mowers have sealed carburetors that are not adjustable.

Find your engine manual for the base settings.
With the engine warm and running, slowly turn the adjustment screws (usually marked ‘H’ for high speed and ‘L’ for low) in small increments.
Listen for the smoothest running speed. If you’re unsure, it’s best to consult a professional for this adjustment.

7. The Blade: Dull or Unbalanced

A dull blade tears grass instead of cutting it cleanly. This puts a massive strain on the engine. An unbalanced blade causes vibration, which can also affect performance and is hard on the mower’s bearings.

Disconnect the spark plug wire for safety.
Tip the mower on its side (check your manual for which side to avoid oil leaks).
Remove the blade. Sharpen it evenly with a file or grinder, maintaining the original cutting angle.
Balance it by hanging it on a nail through the center hole. If one side dips, file a little metal off the end of the heavy side until it hangs level.

8. Mechanical Drag: Stuck Brakes or Bad Bearings

Something might be physically holding the mower back, making the engine work harder than it should. This is less common but worth a look.

Check that the drive system (if self-propelled) disengages fully and isn’t dragging.
Spin the wheels and the blade spindle by hand (with spark plug disconnected!). They should turn freely without grinding or stiffness.
A bent blade can also cause drag by hitting the deck.

When to Call a Professional

If you’ve checked all of the above and your mower still loses power when cutting, the issue may be internal. This includes low engine compression from worn piston rings or valves, or a failing governor inside the engine. At this point, the cost and skill required often make a professional repair the best choice, especially for an older mower.

FAQ: Quick Answers to Common Questions

Q: Why does my mower run fine until I engage the blade?
A: This directly points to a load-related issue. Immediately check the blade (dull, bent, unbalanced) and the air filter. The engine can’t handle the extra demand due to a restriction in air, fuel, or spark.

Q: My mower bogs down in tall grass only. What gives?
A: You’re asking it to do to much at once. The engine is struggling under heavy load. Try cutting tall grass in two passes: first at a higher setting, then lower for the final cut. Also ensure your blade is sharp and you’re not cutting when the grass is wet.

Q: Could it be the oil level?
A: Yes, absolutely. Both low oil and overfilled oil can cause problems. Low oil leads to overheating. Overfilled oil can cause foaming and reduced lubrication. Always check the dipstick on a level surface and fill to the marked line.

Q: I hear a “pinging” sound under load. Is that bad?
A: That sound is often “detonation,” where fuel ignites too early. It’s usually caused by using gasoline with a lower octane rating than recommended, or by carbon buildup in the combustion chamber. Try using a higher octane fuel and a fuel system cleaner.

By following this checklist, you’ll systematically eliminate the reasons why your lawn mower loses power. Start with the simple, free checks like the air filter and cleaning the deck, then move to fuel and spark. Most of the time, the solution is quick and gets you right back to a perfectly manicured lawn without the struggle.