Lawn Mower Slows Down When Cutting

When you buy via links on our site, we may earn an affiliate commission at no cost to you. Learn more.

It can be quite frustrating if your lawn mower slows down when cutting, especially when you have a lot of mowing to do. 

That doesn’t have to happen to you anymore starting from today.

In this article, I share the common reasons your lawn mower slows down when mowing, as well as troubleshooting tips to help you diagnose and fix the problem.

Reasons Why Your Lawn Mower Slows Down When Cutting

Your lawn mower may slow down when cutting for the following reasons:

  • Fuel restrictions
  • Carburetor problems
  • Dirty air filter
  • Worn spark plug
  • Insufficient/stale oil
  • Worn belts
  • Danger idler pulley
  • Clogged mower deck 
  • Work Overload 

Tools You Need For This Job

The tools you may need to diagnose and fix this problem include:

  • Work gloves
  • Screwdrivers
  • Socket wrench
  • Pliers
  • Empty container
  • Carburetor cleaner
  • Clean rag
  • Scraper tool
  • Compressed air

Lawn Mower Slows Down When Cutting: Causes And Fixes

1. Fuel Restrictions

The primary reason your lawn mower slows down when cutting is insufficient fuel getting into the carburetor.

Fuel restrictions could be caused by a clogged fuel filter, clogged fuel lines, or a malfunctioning fuel pump.

A problem with any of these parts will significantly reduce fuel flow and cause the engine to lose power and slow down when cutting.

Diagnosis:

Check the fuel lines and fuel filter for clogging and obstructions. If fuel isn’t flowing freely through the lines and filter, they are clogged.

If the lines and filter aren’t clogged, check the fuel pump to see if it is damaged or malfunctioning.

Fix:

Replace clogged fuel lines and blocked filters. If the pump is malfunctioning, repair it or replace it with a new one.

2. Carburetor Problems

The carburetor controls the air and fuel ratio which are essential for running the engine. If the carburetor is dirty, this ratio will be affected, making the fuel mixture too lean or too rich.

If this happens, the engine can bog down or begin to lose power when cutting grass.

Also, the carburetor adjustment screws may be set too lean, allowing only a small amount of fuel to go into the combustion chamber. 

Diagnosis:

If the carburetor is dirty, the jets and bowl may be clogged and will need to be cleaned.

Fix:

Clean the carburetor with a carburetor cleaner and regulate the carburetor adjustment screw to allow more fuel into the engine.

2. Dirty Air Filter

As the engine runs and cuts grass, the temperature rises, and airflow is needed to cool the engine and formulate the fuel mixture.

The air filter screens the air entering the mower. If it gets clogged with dirt and debris, the engine will lose power when cutting.

Diagnosis:

Remove the air filter housing and inspect the air filter. If it is dirty or oil stained, it is clogged.

Fix:

Clean the air filter or replace it with a new one.

3. Worn Spark Plug

A worn spark plug can cause the mower to slow down when cutting. The plug ignites the air-fuel mixture required to run the engine. 

If it is worn and produces an insufficient spark, the engine may lose power and begin to slow down as it cuts.

Diagnosis:

Remove the spark plug wire and check the spark plug ends for carbon build-up, oil stains, or burnt edges. These indicate that the spark plug is worn.

Fix:

Replace the spark plug.

4. Insufficient/Stale Oil

As the mower runs, the engine oil cools and lubricates the moving part to prevent excessive friction and engine overload.

Insufficient oil or stale oil can cause the mower to slow down and lose power when cutting.

Diagnosis:

If the oil is dark and thin, it is stale. If it is below the ‘Add’ Mark of the dipstick it is insufficient.

Fix:

Drain out old oil and refill the tank with new oil. Top up the oil if it is low/insufficient. Always change the oil after every 50 hours of use. 

5. Worn Belts

If your lawnmower has been used for a long time and begins to slow down when cutting, the drive belts or deck belts may be worn or slipping.

The belts control the movement of the mower and its blades. The mower slowing down could be a sign that the belts are glazed, slipping, shredded, or completely damaged.

Diagnosis:

Open the mower deck and inspect the deck belts and transmission drive belts. If it is worn, shiny/glazed, torn, shredded, or slack, the belt is worn and may be slipping.

If the belt is shredded at the edges, a pulley might be seized. Inspect the pulleys carefully and replace the seized pulley.

Fix:

Replace the deck belt and align it carefully around the pulley so that it is not loose. 

6. Damaged Idler Pulley

The idler tension pulley provides tension and guides the engine drive belt. If the belt tension is too low, the belt will continue to slip causing the mower to slow down.

If the idler pulley gets cracked, broken, or damaged, the tension it provides will be too low to spin the belt efficiently. This will cause a significant loss of power when cutting grass.

Diagnosis:

Inspect the idler pulley. If it is cracked, broken, free spinning, or seized, it is damaged.

The idler pulley spring is worn if it is flat and not popping up.

Fix:

Lubricate/replace damaged pulleys and replace the idler pulley spring if it is worn out.

7. Work Overload

Overworking your lawn mower can cause it to slow down when cutting. When pressure that is greater than the power supplied to the blades is applied, it will cause the mower to slow down.

Cutting thick, long, or wet grasses at very low cutting heights or with dull blades will put excess pressure on the mower belts, blades, and cutting mechanism.

Try to avoid cutting wet grasses with your mower as much as possible as they can bend the blades and overwork the mower.

Diagnosis:

If the mower is losing power when cutting tall or thick grass, the cutting height may be set too low.

Also, if the mower blades are dull, bent, or out of balance, they should be fixed.

Fix:

Raise the cutting height to the highest and gradually reduce it as the grass decreases.

Sharpen the mower blades if they are blunt/bent. Replace them if they are too bent or severely damaged.

8. Clogged Mower Deck

A clogged mower deck will not only overwork the mower but will also obstruct the mower belt and pulley system causing the mower to slow down when cutting down.

Diagnosis:

Tilt the mower slightly and inspect the deck. If it has a build-up of grass, soil, or debris. It is clogged and could be reducing the speed of the belt and blades.

Fix:

Using a water hose, small scraping tool, or putty knife, remove grass and debris blocking the mower deck.

Frequently Asked Questions

1. Why does my lawn mower lose power when the blades are engaged?

Your lawn mower losing power when the blades are engaged could be a result of a worn belt, stuck pulley, fuel delivery issues, carburetor issues, faulty safety switch, or a clogged air filter.

2. Why is my lawn mower running slow and rough?

Your lawn mower could be running slow and rough due to a clogged carburetor, air filter, fuel filter, or a bad spark plug. Check the various parts and clear the clogged items to stop your mower from running slow and rough.

3. Why is my riding mower not cutting?

A dull, bent, or unbalanced blade could be the reason your mower fails to cut grass. Sharpen or replace the blades to get them sharp enough to cut.

Conclusion 

The reason your lawn mower slows down when cutting includes fuel restrictions, worn spark plug, clogged air filter, insufficient/stale oil, engine overload, slipping belts, or a clogged mower deck.

You can easily fix these problems once you identify the cause and follow the right repair steps.

Follow this guide to diagnose the cause of the problem and fix it. Troubleshoot and apply the recommended fixes to get your mower cutting properly again.