Lawn mower engine speed. Lawn Mower Doesn’t Run At Full Speed – 7 Causes

Why Is My Lawnmower Revs Up And Down? How Do You Fix An Idle Surge?

Keeping your lawnmower in tip-top condition is essential for smooth operation. Over time, it will show different unwanted problems. You may see your lawnmower is running either too fast or too slow.

lawn, mower, engine, speed, doesn

Why is my lawnmower revs up and down?

When your lawnmower revs up and down, the governor gives its best effort to ensure a sustained speed but fails to do it. A lack of fuel or an insufficient amount of air could cause the problem.

Not cleaning each part of the lawnmower separately and regularly without removing it may also cause the issue.

  • Why Is My Lawnmower Revs Up And Down?
  • Insufficient Amount Of Air
  • Lack Of Fuel
  • Old Or Contaminated Fuel
  • Spark Plug Issue
  • Incorrectly Adjusted Carburetor
  • Vacuum Leaks
  • Fuel System Problems
  • Dirty Carburetor
  • Clogged Air Filter
  • Step 1 – Disconnect The Cables
  • Step 2 – Take Out The Air Filter
  • Step 3 – Wash The Air Filter
  • Step 4 – Replace If Required
  • Parameter Hunting
  • Idle Speed Control Issues
  • The Ignition System
  • The Fuel Pressure Regulator
  • Bad Valve
  • Dirty Spark Plugs
  • Bad Spark Plugs

In this article, you will learn about “Why Is My Lawnmower Revs Up And Down?” in detail and how to fix this issue. Let’s jump in!

Why Is My Lawnmower Revs Up And Down?

Is your lawnmower revs unsteadily? Why is it happening?

Whatever the causes, you need to solve the issue as early as possible. If you delay, you may end up damaging the mower engine heavily. Let’s learn all the probable reasons about “Why Is My Lawnmower Revs Up And Down?”

Insufficient Amount Of Air

The air filter of the lawnmower engine should have a free airflow system. If any insufficiency occurs for the air, the air filter will clog up.

An insufficient amount of air may decrease the normal speed of the engine. Similarly, the engine may rev too fast if the blockage shifts or clears suddenly. Dirty air filters may cause this.

Initially, you need to clean the air filter to resolve this issue. Also, don’t forget to inspect the vented gas cap. Cleaning both parts may eliminate the problem.

Lack Of Fuel

Have you checked the fuel level?

Lack of fuel may also cause this issue as the engine needs more effort to run the mower. Simply fill the fuel tank with an accurate amount of fuel.

Old Or Contaminated Fuel

Is your lawnmower fuel sit for several months during the off-season?

Then, the oil may become contaminated. Fresh oil is necessary to run the mower smoothly. Take out the old oil from the tank and add new, high-quality fuel.

Spark Plug Issue

The lawnmower may rev up and down if the spark plug is damaged or having an unbalanced adjustment. You want to make sure the cables connected to the spark plug don’t become loose and move unsteadily in a particular direction.

Apart from that, check inside the spark plug if there are any corrosion build-up issues. If yes, rust can be removed from the surface of the object by using a soft brush.

Correct adjustment is also important as you want to make sure there is an accurate gap. Changing the spark plug is essential if it is damaged.

Incorrectly Adjusted Carburetor

If you fail to adjust the carburetor correctly, your lawnmower may rev up and down after a certain time. You can change the carburetor setting on most lawnmowers by adjusting the two screws.

One screw adapts the fuel-air mixture while the other screw controls the perfect level of speed for a lawnmower. If you are not much knowledgeable about the system, you want to check out the instruction manual.

Follow it carefully. First, tighten the screw normally and run the machine for a few minutes. After that, adjust the screw position as per the engine requirement.

So, these are all the possible reasons for this question: why is my lawnmower revs up and down?

Why Does My Lawn Mower Keeps Surging?

If your lawnmower surges occasionally, you have no reason to panic. But if it is continually surging while using it, then something is wrong.

Though you are not facing any issue in mowing grasses, the annoying sound makes you wonder, “Why does my lawnmower keep surging?”

This could happen for various reasons, such as

Vacuum Leaks

A loose carburetor lets the crevices between the items will draw in air and creates blockage issues. Gas and air ratios will be unbalanced due to the excess air.

The fuel and air will fail to move freely and adequately due to this issue. It will ultimately affect the engine performance negatively.

To eliminate the issue, make sure you tighten the bolts and air intake manifold properly.

Fuel System Problems

There is a small hole in the fuel tank cap, which permits aeration from the lid to the tank. This increases fuel delivery to the carburetor by creating backpressure.

If dust or dirt gets into this hole, it can block it. Therefore, insufficient fuel will cause the carburetor to surge.

What’s more, it is also possible to experience a surge if water is present in your fuel. On a red-hot summer day or torrential rain, gasoline can accumulate moisture as a result of condensation.

Here are the steps you want to follow in dealing with this issue.

  • Your gas tank cap has a vent. Check it out and do a proper cleaning if it has become dirty.
  • Remove the gas cap and drain the tank. Also, do a clean-up for the gas bowl and include new fuel. Alternatively, cleaning the fuel tank is also okay by injecting a fuel cleaner.
  • Check out your lawnmower performance after replacing the fuel or cleaning the fuel tank.

Noted: Under no circumstances should the fuel gauge be above maximum.

Dirty Carburetor

Try this carburator cleaning kit.

Among many causes that promote surging issues, a dirty carburetor is one of the most common issues. A carburetor’s internal components can become clogged with grime and dirt. Fuel cannot flow correctly because it is blocked.

Taking to a nearby garage shop is a perfect solution to clean your dirty carburetor. However, if you’re experienced with repairs, it’s possible for you to do it yourself. Follow these steps:

  • Take out the carburetor from the engine. Disassemble all the detachable parts of the carburetor carefully. If you can, remove other closed parts of the carburetor to clean them.
  • Use an effective carburetor cleaner for deep cleaning. Make sure you give more FOCUS on needle valves, orifices, and ports.
  • If the condition of the carburetor is too bad, replacing it is the best solution.
  • After cleaning the carburetor deeply, reassemble each part correctly and tighten the bolts and nuts properly.

Clogged Air Filter

Often a lawnmower keeps surging if the air filter is not clear. Mower engines need oxygen for combustion to run, so dirty hay depletes it of oxygen.

If the air filter is heavily damaged, you want to replace it. But a normal clogged air filter can run in a smooth manner if you clean it properly.

Here are the steps:

Step 1 – Disconnect The Cables

First of all, the cable of the spark plug should be disconnected. It is essential to go through all the necessary degrees of safety whenever you perform any maintenance or repair task. By doing so, injuries are prevented from occurring.

Step 2 – Take Out The Air Filter

Take out the air filter and its cover from the engine.

Step 3 – Wash The Air Filter

First, give your hands on the foam filter and wash it with soapy water. Let it dry after washing. After drying it completely, make a light engine oil coating over on the foam filter surface. It will prevent dirt or grease from coming into the carburetor.

Step 4 – Replace If Required

However, replacing your lawnmower with a paper filter will be a better option when it is excessively dirty, which prevents you from seeing the light. Before installing a new filter, remove all dirt from the housing.

These four reasons could be the main culprit about “Why does my lawnmower keep surging?”

What Causes An Engine To Hunt And Surge?

There are many reasons why engines perform poorly, including maintenance problems. And differents engine surge for various reasons. If you notice the issue pops out on your lawnmower machine, you want to test some important parts or systems of the engine.

What causes an engine to hunt and surge?

It is imperative to get the right mixture of air and fuel, perfect spark timing, and effective exhaust management that results in good engine performance. The system will surge if any of these parameters are out of range.

Here we are mentioning the primary causes of “What causes an engine to hunt and surge?”

Parameter Hunting

“Parameter hunting” is essentially what fuel-injected engines do when a surging condition arises. Remember, an engine’s ECM will not monitor an engine parameter that is outside its expected range. As a result, all these factors will be adjusted automatically to ensure that everything is in line again

“Parameter hunting” is an experimental method. Whenever the engine loses power, the system tends to overcompensate with excessive fuel or an insufficient timing advance. Almost all surging issues stem from the falling/extreme compensation cycle.

lawn, mower, engine, speed, doesn

Idle Speed Control Issues

Some lawnmowers may have idle speed control problems. The machine may surge at idle or run at low speeds if idle speed control becomes damaged or faulty.

This issue mainly occurs due to the development of carbon. A throttle plate is stopped from bypassing air because of it.

The Ignition System

Have you checked out if your mower ignition system is okay? Surging may encounter a foul pickup coil, which is located inside the distributor.

It may fail to function properly at idle or anytime you run the mower. Consult with an expert and replace any important parts if necessary.

The Fuel Pressure Regulator

If the fuel pressure regulator is damaged or fouled, it will fail to supply the right pressure and fuel to the engine. In the case of prolonged surges, the issue will become worse.

Don’t forget to change the fuel filter regularly when the schedule comes. Plus, make sure your lawnmower engine is running on with the correct fuel pressure.

You can check out the instruction manual where it is instructed how much your lawnmower needs in a particular situation.

Apart from that, a damaged fuel injector can also cause the engine to surge. In that case, you want to repair or replace the fuel injector as early as possible. It is always a rule of thumb to check the internal lawnmower component to know if any visual changes occur.

Bad Valve

Have you checked out whether your

If your lawnmower has a bad valve, you can learn it in various ways, including oil leakage, struggle to start, rough running, inefficient fuel consumption, etc. Make sure you address the issue soonest.

Hopefully, you have learned all the important causes about “What causes an engine to hunt and surge?”

How Do You Fix An Idle Surge?

It is possible that a lawnmower is idling erratically because of the accumulation of fuel varnish or carbon deposits. Under this circumstance, you want to clean the valve.

Cleaning the dirty valve requires a deep cleaner for effective cleaning. You can use an engine top cleaner or aerosol throttle cleaner for it. After cleaning it, hopefully, you will stop wondering, “How do you fix an idle surge?”

Here is the perfect way to do it:

  • First of all, separate the connection between the throttle body and the air injector.
  • Next, you want to run on your mower and increase the idle speed gradually.
  • Put a small amount of throttle cleaner on the choke body throat
  • Boost the cleanliness of this area by spraying the solution for a few seconds.
  • Let the cleaner soak into the passageway by turning the engine off.
  • Let three minutes pass.
  • Start the mower machine again and continue doing the cleaning several times.
  • If you don’t see any improvement, consider removing the valve from the machine and clean it properly.
  • If you still see an idle surge, take your mower to a nearby store to fix the issue.

So, this is the perfect way to know about “How do you fix an idle surge?”

Can Spark Plugs Cause Surging?

If your lawnmower is surging, it may happen due to combustion issues. Both spark plugs and ignition systems play a crucial role in effective and smooth lawnmower performance.

Can spark plugs cause surging?

If your lawnmower spark plug has become damaged, faulty, or dirty, it may cause surging issues. An individual cylinder’s combustion chamber receives current from its spark plug wires via the ignition coil. A compressed air/fuel mixture is ignited with an electrical impulse transmitted via the plug.

Dirty Spark Plugs

If your lawnmower spark plugs become dirty, it will cause surging problems. You will notice a fluctuation in speed while running the mower.

Spark plugs are not firing at the right rate, so these surges are caused by unfired fuel in your engine. Because of it, there is an uneven pace.

Safety is at stake in this problem because your controlling ability on your lawnmower is reduced due to this issue.

If you know how to clean a lawn mower spark plug, do it after inspecting its conditions. If you don’t know, a mechanic must be consulted to address surging and lagging immediately.

Bad Spark Plugs

Spark plugs and pistons are part of each cylinder in a lawnmower. With different spark plug styles, different voltages are required for combustion.

Electrons travel through the ignition process, and when they cross the gap between the plugs, they ignite the fuel mixture.

The amount of voltage required for the smooth operation depends on several factors, including engine compression and electrode condition.

Every piston must hit the same spot in the cylinder before sparking occurs. Spark plugs do not produce any heat, yet they play a major role in eliminating unnecessary heat from the combustion area.

But if the lawnmower spark plug becomes bad, it may misfire and can also cause surging. You want to note down these two important points when you want to know about “Can spark plugs cause surging?”

How To Prevent Lawnmower Surging?

No matter how careful you are, you can’t keep your lawnmower problem-free in a lifetime. After a particular time, it will show unwanted issues. A little bit of caution will prevent the problem from occurring quickly. The same goes for surging problems too. Here we are mentioning some important tips to avoid lawnmower surging.

  • Regularly inspect your lawnmower air filter and clean it properly when it becomes dirty.
  • It is important to check various parts of the lawnmower periodically for proper maintenance.
  • Never use low-quality, bad, incompatible, or too old fuel for your lawnmower.
  • Ensure there is no issue with the gas cap and free air entering the gas tank.
  • Make a schedule for cleaning the spark plug and carburetor for each season. If you use the mower heavily each season, you want to do for 2-3 times.

Final Verdict

Are you still thinking about “Why is my lawnmower revs up and down?” We got you covered everything about this topic, and hopefully, you have figured the root causes why your lawnmower revs up and down. Thanks for reading!

Last update on 2023-01-28 / Affiliate links / Images from Amazon Product Advertising API

Lee Safin was born near Sacramento, California on a prune growing farm. His parents were immigrants from Russia who had fled the Bolshevik Revolution. They were determined to give their children a better life than they had known. Education was the key for Lee and his siblings, so they could make their own way in the world. Lee attended five universities, where he studied plant sciences and soil technologies. He also has many years of experience in the U.S. Department of Agriculture as a commercial fertilizer formulator.

Thoughts on Why Is My Lawnmower Revs Up And Down? How Do You Fix An Idle Surge?

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

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 lawnmower 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

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.

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.

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.

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.

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 night’s charge.


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

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.

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.

Lawn Mower Horsepower: How to compare lawn mower engine power

Lawn mower horsepower – the power output of a gas powered lawn mower engine – is an important consideration when you are trying to choose the right lawn mower for your needs.

Understanding lawn mower engine power is not straightforward because manufacturers state engine power using different measures. These are usually horsepower, kilowatts, torque, or cubic capacity. They also publish power outputs based on different conditions. For example, power may be stated with the engine turning at various revolutions per minute, or ‘net’ or ‘gross’ according to whether with accessory parts are attached. When comparing lawn mower engine power, it is therefore vital to make sure you compare like with like. This article will show you how to do that.

  • Lawn mower engine power for different conditions
  • Lawn mower engine power: how to decipher what the manufacturers tell us
  • Horsepower
  • Gross Horsepower vs Net Horsepower
  • How much lawn mower horsepower do you need for your conditions?
  • Cubic capacity
  • What size lawn mower engine in cc do you need for your conditions?
  • Kilowatts
  • How much lawn mower power in KW do you need for your conditions?
  • Torque
  • How much lawn mower torque do you need for your conditions?
  • Rpms
  • Comparison of lawn mower engine power
  • Honda lawn mower engine power
  • Briggs and Stratton lawn mower engine power
  • If you want to know what I think …

Power output matters because it directly affects the performance, fuel efficiency, and the ability of the mower to handle various lawn conditions.

Put simply, a more powerful engine delivers more power to the critical moving parts – the drive wheels, and the cutting blades (or just the blades on a push mower).

What this means in practice is that a lawn mower with more power capacity can work more effectively than a lower powered mower on tougher terrain, e.g., significantly sloping ground, or uneven, bumpy surfaces. A higher-powered mower can also cut through rough, overgrown lawns and longer grass more effectively than a lower powered mower.

Why is my engine running wide open at full speed

You should bear in mind that the amount of power delivered to the wheels and blades is not only determined by the engine output. It also depends on how efficiently power is delivered to these parts, and that depends upon the quality and efficiency of the mower’s overall design and construction.

However, engine power is the most important factor you need to understand if you want to know if a mower will have the ‘muscle’ for the kinds of conditions you need it for.

As we noted in this post about how to choose a gas powered lawn mower, it is critical think about the size of the areas you need to mow and the kind of ground you’ll be mowing on when you’re determining how powerful your lawn mower needs to be. Consider the size of the lawn, whether you have slopes to deal with, rough or uneven ground, long or lush grass or grass that is mown infrequently. If you have any of those conditions, you’ll need more power.

Lawn mower engine power for different conditions

I’ve set out below under each measure of power some suggested levels of power that you will generally need for the following different kinds of conditions:

  • Small gardens: less than 1/4 acre, flat ground, no major obstacles, regularly mown;
  • Medium gardens: 1/4 to 1/2 acre, some slopes, some rougher terrain, less frequent mowing;
  • Large gardens: than half an acre, obstacles, slopes, less frequent mowing

Note, these are broad guidelines, not hard and fast rules. Your conditions will likely be a bit different from the ‘typical’ conditions that I have used for these definitions, and more likely will fall somewhere on the scale between definitions.

You may also just want a machine with more or less power based on budget or personal preference. So you will need to adjust your thinking accordingly.

Lawn mower engine power: how to decipher what the manufacturers tell us

I’ll start this section by saying that I’m no mechanic or engineer. But I am long standing gardener and lawn mower user, and in this post I feature images from two of the lawn mowers I have owned:

  • a John Deere self propelled mower powered by a 190cc 700 series Briggs and Stratton engine and
  • a Honda HRU mower powered by a 163cc Honda GXC160 engine.

I also have a background as a practicing lawyer and a Master of Science degree. So, although I’m not technically qualified in mechanics or engineering, I’ve owned mowers powered by engines from the leading engine manufacturers, and I know how to research and sift information and data and how to present it in a way that is comprehensible to non-technicians.

And, when it comes to lawn mower engine power, that is pretty important.

This is because there are several different ways that power can be measure and expressed and it is hard to make comparisons across these different measures. It is especially so, because the lawn mower engine manufactures tend to present the data in different ways.

So with that said, let’s look at how you might see the power of lawn mower engines described when you are looking for the best gas powered lawn mower for your needs.


Horsepower (HP) is one of the units of measurement used to express the power output of a lawn mower engine (or any engine, for that matter). Historically, this is probably the most widely used way of expressing power output. It dates back to the 18 th Century, when James Watt invented the steam engine. He worked out the power output of draft horses in order to compare ‘horse power’ with the steam power of his new engine.

There are different standards of horsepower including imperial and metric versions which differ slightly. But when comparing manufacturers’ information about lawn mower horsepower, it is probably most important to differentiate between two particular types of horsepower: gross horsepower and net horsepower.

Gross Horsepower vs Net Horsepower

Gross horsepower is the total power produced by an engine without taking into account any losses of power due to associated parts, such as an exhaust system or an air filter.

In contrast, net horsepower refers to the actual power output of the engine available for work when accounting for those losses. Net horsepower is therefore often lower than gross horsepower because of the power losses caused by the extra parts.

lawn, mower, engine, speed, doesn

For example, a lawn mower engine might have a gross horsepower of 6.5 HP, but if it were measured in net horsepower, it might be closer to 6.0 HP.

Net horsepower is a better indication of the power that will actually be available for driving the mower in practice.

How much lawn mower horsepower do you need for your conditions?

  • Small lawn:
  • A push mower would usually be okay for this type of lawn. These kinds of mowers are fairly lightweight and the engine does not drive the wheels. Therefore, an engine delivering between about 2.5 and 4.5 horsepower will suffice.
  • A self-propelled mower with a 4.5 to 7 horsepower will probably do the job on a medium lawn.
  • Here is when you are starting to get into the territory of riding mowers, zero-turn mowers and lawn tractors. You’ll need an increasing level of power with every acre you need to cut, e.g., 12 to 14 horsepower for 1/2 acre up to 24 horsepower plus for 3 acres or more.

Cubic capacity

Cubic capacity, also known as engine displacement, is measured in cubic centimetres (cc) or litres (L). It represents a measurement of the total volume of all the engine cylinders.

Technically, the cubic capacity of a cylinder is a measure of the total volume of air and fuel that is displaced by the piston as it rises up through the cylinder. If the engine has more that one cylinder (as riding mowers may do) the cubic capacity of both cylinders is added together to give the the total size of the engine.

Therefore, cubic capacity is not a measurement of engine power. Rather it indicates the size of the chambers central to the internal combustion process of the engine.

There is is no direct relationship between horsepower and cubic capacity because there may be other aspects of the way an engine is made that affect the power output. In other words, two engines with the same cubic capacity may produce different horsepower outputs.

But, all other things being equal, larger volume cylinders will produce more power. For example, a a 190cc lawn mower engine will likely have more power than 140cc one.

What size lawn mower engine in cc do you need for your conditions?

  • Small lawns :
  • A push mower with a 125cc to 140cc engine.
  • A self-propelled mower with a 140cc to 200cc engine.
  • You’ll need a riding mower, zero-turn mower and lawn tractor. The smallest of these have engines around 350cc, the largest have engines up to 750cc.


Kilowatts (KW) is another unit of measurement for power output, similar to horsepower. It is the accepted unit of power under the International System of Units. You will often see it referred to as the SI unit and tends to be used more by European or Asian lawn mower engine makers, such as Honda.

To convert horsepower to kilowatts and vice versa, you can use the following formula conversions:

Thus, an engine producing 5hp output, produces 5 x 0.7355 = 3.68 KW

This conversion might be necessary when comparing lawn mower engines from different manufacturers, as some may list power output in kilowatts and not in horsepower.

How much lawn mower power in KW do you need for your conditions?

  • Small lawn:
  • A push mower delivering between about 1.8KW and 3.3KW of power.
  • A self-propelled mower with a 3.3KW to 5.2KW engine.
  • Riding mowers, zero-turn mower or lawn tractor with increasing level of power with every acre you need to cut, e.g., 8KW to 10KW for 1/2 acre up to 18Kw to 20KW plus for 3 acres or more.


Torque is the rotational force that an engine produces and applies to the drive shaft. In cars, torque is an indicator of speed of acceleration. In lawn mower engines, it is also related to the spinning of the blades and, therefore, their effective cutting power.

Torque is usually measured in pound-feet (lb-ft) or Newton-metres (Nm). A higher torque value indicates a more powerful engine capable of working under tougher conditions, such as cutting through thick grass and tackling inclines.

For example, a lawn mower engine with a torque of 7.5 lb-ft might perform better in cutting long thick grass than one with a torque value of 6.0 lb-ft.

How much lawn mower torque do you need for your conditions?

  • Small lawn:
  • A push mower delivering between about 6.1 Nm (4.5 lb-ft) of torque.
  • A self-propelled mower with a 7.46 Nm (5.5 lb-ft) to 10.00 Nm (7.36 lb-ft) of torque.
  • Riding mowers, zero-turn mower or lawn tractor with increasing level of power with every acre you need to cut, e.g., 24 Nm (17.0 ft·lb) of torque at 2,500 rpm for 1/2 acre in a 350cc engine up to 50 Nm (36.88 ft-lb) plus for 3 acres or more in a 750cc engine.


Revolutions per minute are not a measure of power output. But they are relevant here because figures given by manufacturers for horsepower, Kws or torque are usually, but not always, published on the basis that the engine is running at a given number of rpms. Usually, but not always, this is 2,500 or 3,600 rpms.

Comparison of lawn mower engine power

With this information you should be better placed to make comparisons between the different engines that are used in the lawn mowers you might be interested in purchasing.

Obviously, it is useful to see the information side by side. But sometimes it is not possible to make direct comparisons because:

  • Some manufactures use net horsepower and some use gross horsepower. There is no direct formula for conversion from one measure to the other because the difference depends upon the additional parts and accessories that give rise to a net horsepower measure;
  • Manufactures don’t all publish the same information: some publish figures for horsepower and torque, some publish in KWs and torque, some publish cubic capacity and horsepower, and so on;
  • Power ratings may be given in the same units but at different rpms.

Nevertheless, as a general guide, here is what a comparison of different types of engine might look like:

lawn, mower, engine, speed, doesn

The two manufactures that make the vast majority of lawn mower engines are Honda and Briggs and Stratton. So below we’ll cover off the power ratings for some of their most popular engines

Honda lawn mower engine power

Here are the power ratings published by Honda for their popular lawn mower engines:

Briggs and Stratton lawn mower engine power

Here are the power ratings published by Briggs and Stratton for some of their popular lawn mower engines:

Engine Data From Briggs and Stratton

If you want to know what I think …

Obviously, the engine is only one part of the package when you are thinking of buying a lawn mower. You can check out the following post to help you choose the gas power lawn mower that is right for you:

And, by the way, if you want to know my opinion, based on my own experience, the Honda engine beat the Briggs and Stratton engine hands down.

Compared to the John Deere, Briggs and Stratton powered mower that I owned, the Honda mower runs much more smoothly, starts more reliably and is much less noisy. And all though it has a smaller engine (163cc vs 190cc) the Honda feels no less powerful.

Unless I had a small flat lawn, I’d choose a Honda mower every time:

What causes lawn mower engine surging?

Fuel system problems or vacuum leaks through the air intake manifold are typical causes of lawn mower surges.

Here’s a rundown of the specific issues causing small engines to quickly cycle between idle speed and full throttle:

  • Contaminated gasoline. Old gas or gasoline contaminated with water can cause engine surging.
  • Clogged fuel tank cap vent. The fuel tank cap vent can get clogged with dirt or debris. The fuel cap vent helps keep gas flowing smoothly to the carburetor. When it’s clogged, the engine can surge.
  • Dirty air filter. The carburetor won’t get a good supply of air when the air filter gets clogged with dust and dirt.
  • Worn or damaged air intake gasket. The mower surges when the carburetor sucks air through an unsealed gap in the air intake manifold instead of through the air filter.
  • Dirty carburetor. Clogged fuel jets inside the carburetor commonly cause the lawn mower engine to surge. Clogged jets can’t provide the right mix of air and fuel to the engine.

Troubleshooting a lawn mower that is surging

To find the cause of a surging engine, first check the basic issues described above.

  • If fuel in your mower’s gas tank is older than six months or you left the lawn mower out in the rain, move the mower to a well-ventilated area and drain the tank. If you see water in the drained gas, you’ll also need to remove the bowl from the bottom of the carburetor and dispose of that gas.
  • Check the air vent on the fuel tank cap. Clear the vent hole if it’s clogged.
  • Replace or clean the air filter if it’s dirty.

If these basic troubleshooting tips don’t help, then you’ll likely need to make one of the repairs described below.

How do you fix a pulsating lawn mower?

Move the lawn mower to a well-ventilated area and take these steps to fix the engine.

  • Remove the air filter and its housing. Check the air filter housing gasket and replace that gasket if it’s worn or damaged.
  • If that gasket is okay, check the gasket and seals between the carburetor and the engine. Replace any worn or damaged seals or gaskets.
  • If external carburetor seals and gaskets are okay, then you’ll need to rebuild or replace the carburetor. Replacing the carburetor is a much easier repair that rebuilding the carburetor. Follow the steps in our How to replace a lawn mower carburetor repair guide/video to install a new carburetor. If the replacement carburetor for your engine is unavailable, or if you prefer to rebuild the carburetor, order the rebuild kit for your carburetor and follow the steps in our How to rebuild a lawn mower carburetor repair guide/video to clean and rebuild the engine’s carburetor.

How much does fixing a surging lawn mower engine cost?

If you can troubleshoot the surging engine and don’t have to buy parts, the cost of fixing your lawn mower typically isn’t more than the price of replacing the gas.

If you have to buy parts such as an air filter, carburetor or carburetor rebuild kit and you complete the repair yourself, the cost of fixing a surging lawn mower won’t be much more than the cost of the repair parts. The most expensive part that you would have to buy would be the carburetor. You’ll pay about 50 for the replacement carburetor on many common lawn mower engines.

Hunting and Surging Lawnmower Guaranteed Fix Briggs and Stratton

When you have to take the lawn mower to a repair shop to get it fixed, then the repair will typically cost between 50 and 100 to fix a surging engine.

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