Lawn Mower Doesn’t Run At Full Speed – 7 Causes

Person in casual attire tuning a lawn mower on a sunny day, preparing for garden maintenance.

Lawnmowers have small engines that do not produce a lot of power. However, they are highly efficient for cutting grass. Proper care and regular maintenance will help sustain the performance of any type of lawnmower.

If your lawn mower engine doesn’t run at full speed, here are 7 causes that might be affecting it and the solutions to get it back up and running. They operate in outdoor areas where they are exposed to dirt, debris, and moisture. All of which can significantly damage their engines and cause a multitude of operational issues. A slow run is one such outcome. It is a common problem that can be caused by various things.

Fortunately, most lawnmower issues can be easily fixed at home, at a fraction of the price of professional services. It is essential to switch off machines and ground spark plug wires before carrying out any lawnmower maintenance.

Common Problems That Make Lawn Mowers Run Slowly

1. Disengaged Or Damaged Drive Belt

In lawnmowers, drive belts transfer power from the engine shaft to the transmission. The transmission then propels the wheels forward (or backward) and also spins the blade. When lawnmowers move slowly, the issue is usually caused by a dislodged or damaged drive belt.


The following details are explained in the owner’s manual. Turn off the lawnmower and access the drive belt. Reattach it, if it is dislocated, or replace it, if it has damage.

2. Clogged Spark Plug, Air Filter, Or Carburetor

Clogging is a common occurrence in most engine parts. When blockages occur in the spark plug, air filter, and carburetor, mainly, the speed of a lawnmower can be greatly reduced. They are essential units that affect performance.


Dirty elements must be cleaned, and damaged ones must be replaced. Spark plugs and air filters are easy to clean by adhering to instructions laid out in owners’ manuals. Remove the spark plug from the lawnmower using a socket twist and clean it using a brake cleaner. If the top of the spark plug is black, replace the spark plug.

Additionally, they are readily available and cheap to replace when blockages cause damage. Carburetors are more expensive to replace, but they can be cleaned and rebuilt when clogging has not caused extensive damage and corrosion.

3. Petrol Blend

Different fuel providers sell different types of petrol blends. Fuels mixed with ethanol are some of the most popular. They are cost-effective and environmentally sustainable. The different varieties of fuel cause minimal damage to car engines.

However, they can cause a lot of destruction to the smaller and less robust engines of lawnmowers. This can lead to poor performance and a slow engine pull.


Contact the lawnmower supplier or check the manual for details on the best petrol blend to use. Only use fuel suppliers that offer the particular blend required.

Additionally, if using the wrong fuel damages any engine parts, they must be replaced before switching to the correct blend.

4. Stale Petrol

After a month, petrol begins to break down due to exposure to air and other environmental factors. Some of its compounds evaporate, and it becomes thicker. This makes it lose a lot of its combustibility so that petrol lawnmowers will run slower.


Old petrol must be drained from a lawnmower and replaced with fresh fuel. A fuel stabilizer can also be mixed in with petrol to extend its stability. It is essential to know that petrol left within a storage container also gets bad after a month. It must not be used to refill lawnmowers.

5. Old Battery

Cordless lawnmowers often become slower as their batteries age. Old batteries will not have enough power to drive a machine at high speed, even after a full throttle night’s charge.


If a lawnmower has reduced power after a battery has been charged all night, it needs to be replaced.

Individual using a string trimmer in overgrown grass, complementing a lawn mower for landscape upkeep.

6. Cutting Tall Grass

If your grass is too tall for your lawnmower’s cutting blade, the grass can get tangled up in the blades and slow down movement.


Most lawnmowers have multiple cutting heights. Often between 2.5 to 4 inches The cutting height can be adjusted to the height of the grass. If the grass is too long for the highest blade setting, use a weed whacker first to trim it down. You can then use your lawnmower on the trimmed grass.

7. Dirty Blade

Blades that are coated with mud, grass, and other gunk move slowly, and engines work harder to spin them.


With the engine off and wearing protective gloves, clean the blades to remove all blockages. Use a brush first, and then a wet cloth to clean the blade of the lawnmower and the lawnmower unit too. You may use some warm and soapy water to remove persistent dirt and debris from the lawnmower.

Do not forget to clean the deck of the weed eater and the handles. After cleaning the lawnmower and its parts with water, let it stand for 15-25 minutes to dry off. Storing the lawnmower while it is wet causes rust and damage to the lawnmower.

Maintenance Is Key

The best way to prevent any type of lawnmower problem is by performing frequent maintenance checks. If left unresolved, minor issues will lead to extensive damage that will cause your machine to operate at a subpar level. All lawnmower parts must be cleaned regularly, the correct replacement parts must be used, and they must be stored in cool, dry places.

Lastly, lubricate all the moving parts of the lawnmower. Consult the owner’s manual of your lawnmower for directions on how to carry out lubrication. The owner’s manual also gives you guidance about the parts that you should lubricate.

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