My Lawn Mower Loses Power When Cutting – Why?

You count on your lawn mower to help keep your yard in good shape. It’s like your right-hand man. So when you’re out there making your passes up and down the yard as per usual and the engine starts stuttering and losing power, it’s a problem. And most of the time this type of issue isn’t just going to “go away”. There’s usually a clear cause and you need to identify it and fix it to get your mower firing on full cylinders again.

If your lawn mower loses power when cutting, the most common causes are a clogged air filter, stale/contaminated gas, blocked fuel filter, too much/too little engine oil, dirty/damaged spark plug, blocked carburetor, clogged cutting deck or a dull blade.

Causes of Gas Lawn Mower Losing Power When Cutting

We need to differentiate between gas and electric here as some of the causes of a gas lawn mower that loses power when cutting wouldn’t apply to electric mowers.

A gas lawn mower engine needs 3 things to be “happy” and run efficiently to help you cut your lawn:

• Gas
• Air
• Spark

If you remove or restrict its access to any of these things, that’s when problems arise. Your mower may lose power or it may cut out altogether.

Here are a few common causes of a gas mower engine losing power:

Dirty/Clogged Air Filter

Your air filter is responsible for letting in the right amount of air into the engine for combustion to take place, while at the same time filtering out dirt and dust that could do the engine harm.

When was the last time you inspected it? It’s possible that it’s well overdue a good cleaning. Over time, the dirt and dust that it filters out can really start to build up and affect how it functions. When this happens, the amount of air that can pass through it into the carburetor is less than what is required, and this is when the engine might start to lose power or struggle.

Another possible cause is that you tipped your lawn mower the wrong way when doing some maintenance on the blade or cutting deck (maybe you were cleaning it or the mower blade was loose). This can cause problems with engine oil leaking out of its reservoir in the crankcase, and gravity brings it all the way to the air filter. And a lawn mower air filter soaked in oil is definitely not going to let sufficient air pass through it.

Solution: In the case of a dirty air filter, a thorough cleaning will likely be enough. Knock the debris off and replace it in its casing. If it’s extremely dirty, replacing it might be a better option. In the case of an air filter soaked in oil, a lot of the time a replacement will be necessary.

Contaminated or Stale Gas

This is a common problem with gas mowers, as a lot of folks don’t realize that gas goes stale and starts to break down in as little as 30-days if not stored correctly (also depends on the type of gas). Engine problems caused by gas are often experienced in the spring, when you dig the mower out from winter storage and use it for the first time. Especially if you forgot to winterize your lawn mower.

If the gas has gone stale and broken down, it may have left behind a gummy residue that may have clogged up the carburetor or coated internal components of the engine. And if you use gas that contains ethanol, it’s possible that you have water in your gas mower. This is because ethanol is like a magnet for moisture. It absorbs water and draws it into the fuel tank. If left for extended periods, this can cause corrosion in the tank, carburetor and internal engine. All over basically, and this can most definitely result in a lawn mower that loses power when cutting. It may even prevent it from starting up altogether.

Solution: There’s only one solution here. You’re going to need to remove all of the gas from the fuel tank (this will likely require you to detach the tank and give it a thorough clean) and refill it with fresh gas. To be on the safe side, you may also want to flush your fuel lines and clean the carburetor too.

Spark Plug is Dirty/Contaminated

We’ve talked about issues relating to air and fuel – now let’s talk about the third key ingredient to a “happy engine” – the spark.

It’s the spark plugs job (surprise, surprise) to generate this spark, and this is a component of every gas mower that needs to be regularly checked on and carefully maintained. As without that spark, nothing happens. And if it’s only firing at 50%, that could be why your gas mower loses power when cutting.

Over time they become dirty through use. That’s just part of the life of a spark plug. And if you’ve had a flooded lawn mower engine at any point, the gas could have contaminated the spark plug.

Solution: Carefully remove your spark plug and inspect it (let the mower cool down first if you’ve used it). Make sure it’s dry and if covered in carbon deposits, ash or oil build-up, give it a thorough clean with a wire brush. If it appears damaged or still doesn’t work after re-installing it, you will need to source and fit a replacement.

Too Much or Too Little Oil

It’s extremely important to only add as much oil to your mower as the manufacturer recommends. Far too many folks are really very blase about this, calculating this roughly by sight. My wife says I’m too much of a control freak, but believe me, when adding oil to a mower it really pays to be.

Running a mower with not enough oil is going to cause problems as the oil is there to lubricate all of the internal components of the engine. If they are not kept sufficiently lubricated, this can cause temporary seizures and increased friction between internal components, which causes the mower to lose power when you’re cutting. Too little oil for long enough will likely lead to your engine seizing up completely. And then you may as well send your mower to the scrap yard!

Adding too much oil isn’t any better for your mower though. This can lead to “frothing” with air entering the oil and reducing its effectiveness in lubricating the engine components.

Solution: This really is very simple. You add the amount of oil that the manufacturer recommends in your mower. No more, no less. Obviously, if you have been running your mower with too little oil, it’s just a case of adding a bit more. If you have too much oil in your mower, you’ll need to remove some to bring it down to the recommended level.

Clogged Fuel Filter

The fuel filter functions much the same as the air filter, except for gas instead of air. Its job is to prevent dirt and other contaminants from reaching the engine. But over time, as this filter does its job, it will become clogged, meaning less and less fuel will pass through to the engine. At some point, this restricted fuel flow will start to affect the engine’s ability to function normally, and that’s when it could start causing it to lose power while you’re cutting.

Solution: The fuel filter is another important part of your lawn mower that needs to be inspected on a regular basis and changed when necessary. Check yours now and see if it’s clogged up with dirt.

Blocked Carburetor

Issues such as an oil leak or gas going bad can cause a blockage in the carburetor which will then restrict the flow of fuel and air to the engine. And by now you already know what that means – poor engine performance. It may be that your lawn mower loses power when cutting, or you may not be able to start it at all.

Solution: The carburetor needs cleaning! Simple as that. First, you can just try spraying some carb cleaner in the throat of the carb and starting your mower. This combination may be enough to clear a partial blockage. If it doesn’t, you may need to detach your carb to do a deep cleaning.

Other Causes of a Lawn Mower Losing Power When Cutting

The causes that I’m going to outline below are not specific to gas mowers. These could also be reasons for an electric lawn mower that loses power when cutting.

Dull Lawn Mower Blade

A lawn mower blade has to be kept sharp enough to cut the grass effectively. Over time, you could hit things like small rocks sticks etc, and the blade will become duller. Even if you don’t hit anything, over time through use it will dull. And when that happens, it becomes less effective at cutting the grass, and to compensate your mower’s engine has to work much harder to deliver the cut, as your blade is now “tearing” your grass rather than cutting it. This can cause the mower to lose power.

Solution: Shut off your mower and once it’s cooled down (if it’s a gas mower), tip it over so you can get at the blade (here’s how to tilt a gas mower the right way), and inspect the blade. If you notice it’s really very dull, this could be the cause of the mower losing power. You’ll need to remove the blade and either sharpen it or replace it. How sharp should a mower blade be? It shouldn’t be so sharp that you’d cut your finger on it. If in doubt, have a look at a new blade, as they’re sharpened just right.

Cutting Deck Set Too Low

The unwritten rule in the world of lawn care that says you should never remove more than a 3^rd of the height of your lawn in any one session is primarily for the benefit of the lawn’s health. But observing this rule can also be the solution to the issue of a lawn mower losing power while cutting.

You see, if you have the cutting deck set super low and your grass is quite high, you’re going to really overwhelm your mower’s engine/motor. It’ll try its best to continue, as lawn mowers are loyal servants like that, but it’ll likely lose power at the very least, and may stall or cut out entirely.

Solution: Try raising the cutting deck and observe the 1/3^rd rule. If you’re using a relatively cheap electric mower, you’ll likely see that this really does help, as the motors on those mowers aren’t very powerful and quickly bog down in thicker or longer grass.

Cutting Deck is Clogged

This is why cleaning your lawn mower deck to so damn important. I know it adds one extra step to a chore you maybe already don’t want to do, but it can really help to keep your mower in good working order and prevent it from losing power.

You see when the deck is caked in dried-on clippings, the efficiency with which clippings are circulated around it when you’re mowing your lawn is much lower. As a result, more strain is put on your mower’s engine/motor and this may cause it to lose power.

Solution: Like with the mower blade, you’re going to need to tip your mower over to do this. Once you have access to the underside of the deck, work your way around it and inspect it for dried-on clippings – this is the main enemy here. You’ll need a scraper blade, putty knife or something of that nature to clean the deck as clippings dry on like cement. Once you have it clear, it’s recommended that you rinse your deck after every session to remove clippings before you store your mower. There are also certain products that you can spray onto your deck to help keep grass from sticking under the mower deck, such as Fluid Film.

Finding Out Why Your Lawn Mower Loses Power When Cutting

Like with a lot of things in life, there are lots of reasons why your mower may not be performing as well as it used to. What I’d recommend doing if you’re having this problem is to check the things that are less involved first, such as:

• Height of your cutting deck
• Condition of the blade
• Whether the cutting deck is caked in dried clippings

These steps shouldn’t take more than a few minutes. If you’ve got an electric mower that’s losing power when cutting, one of these things is usually the cause.

If you’ve got a gas mower, check all those things and if you are still experiencing a loss of power, check the following, one by one:

• Is the air filter dirty or clogged?
• Look in the fuel tank – any signs of corrosion or contamination (if you have water, it’ll look like there are bubbles in the gas)
• Is the fuel filter blocked?
• Is the spark plug dirty or broken?
• Do you have too much or too little oil?
• Is the carburetor dirty?

In my experience, 95% of the time the culprit is one of these things and often it’s just a part of routine maintenance that you’ve neglected.