My Lawn Mower is Cutting Unevenly: Why + How to Fix

If you’ve noticed that your lawn mower is cutting uneven, leaving some parts really very short, while others are left a lot longer, you can’t just stand back and do nothing. It’s a truly terrible look! But where do you start? What could be causing this issue? Will you need a new mower? Don’t worry, I’m going to help you get to the bottom of this. Based on experience, there are normally a few quick checks you can do to find the cause. More often than not, it’s actually a very easy fix.

Some of the most common causes of a lawn mower cutting unevenly include damaged wheels, a loose, bent or dull mower blade, a bent crankshaft, and a clogged or damaged cutting deck.

Most Common Causes of a Lawn Mower Cutting Uneven

Before taking drastic action and replacing your mower altogether (not necessary in most cases), check on the following parts of your mower. Many times, a lawn mower that is not cutting evenly can be fixed in just a few minutes with some very basic tools.

Wheels are Set to Different Heights

On some lawn mowers, the height of each wheel must be set individually. If just one wheel is set to a different height, no matter if it’s just one notch above or below the others, it can cause your lawn mower to cut unevenly. This can happen even if you initially set all wheels to the same height, as I’ve seen some height adjustment systems on cheaper mowers that are very flimsy, and banging into the corner of the shed or a fencepost is enough to cause one of the wheels to fall down to the setting below.

Solution: Carefully inspect all 4 wheels on your lawn mower to ensure that the height is on the same setting for each of them. If you notice that one is on a different setting, adjust it.

This problem is not likely to occur if your mower has a lever-based height adjustment system, as these typically adjust all 4 wheels together, or there’s a lever for the front two wheels and another for the back two wheels.

One or More Wheels is Damaged

Whether you’re using a walk-behind mower or a riding lawn mower, having all of the wheels function correctly is really important for obvious reasons. A broken/damaged wheel won’t just make your mower harder to push or drive, it’ll also likely cause your mower deck to “dip” on the side of the affected wheel, leading to an uneven cut.

Solution: Here you’re going to need to watch all of the wheels closely as the lawn mower moves. Do you notice any of them wobbling? This is the clearest sign of a damaged wheel. Most of the time the only solution is going to be to replace the wheel that is damaged. Just make sure the replacement that you source is of the exact same size.

Cutting Blade is Loose

Power tools need to be constantly maintained, as through use their condition worsens. The blade is the perfect example of this. It’s possible that over time, the bolt that attaches it to the crankshaft becomes loose and the blade is no longer fixed down securely, so it’s no longer kept 100% horizontal when in operation. Even a small change to the angle of the blade can cause your lawn mower to cut unevenly. A loose blade is often accompanied by a rattling sound.

Solution: It’s quick and easy to tell if a lawn mower blade is loose. You’ll need to tilt your mower for this and you might want to wear a leather glove to protect your fingers when touching the blade. Don’t forget to disconnect the spark plug if it’s a gas mower, and disconnect the power cord or battery if your mower is electric.

All you need to do now is hold the blade on its back edge, and see if there is any movement in it vertically. There shouldn’t be and you’ll instantly recognize if the blade is loose. If it is, you’ll need to grab a wrench and tighten it back up. While you have your mower flipped over, it’s also a good idea to see if the blade is sharp enough (how sharp should a mower blade be?), because that’s another potential reason why your lawn mower is cutting uneven…

Mower Blade is Dull

Your mower blade needs to meet two conditions to do its job: a) be completely horizontal, and b) be sharp enough to cut effectively. The reason why a dull blade can cause an uneven cut is because a dull blade does not actually “cut” the grass, it tears it. If you tear a piece of paper with your hands, it’s never perfectly level like it is when you cut the paper with scissors, and it’s the same thing here.

Solution: This is another quick fix to a lawn mower cutting uneven. You need to either remove your blade and sharpen it, or you need to buy a new one and install it. Just make sure you check which side of the lawn mower blade is up before you tighten everything up, as installing the blade the wrong way round will also lead to problems with your cut.

Mower Blade is Bent

One of the most common bent lawn mower blade symptoms is an uneven cut. You see, when you bend the blade (this typically occurs if you pass over something hard, like a rock or a metal pipe), it has a similar effect to a loose blade. The blade will no longer be spinning completely horizontally; the bent section of the blade will cause it to cut at an angle.

Solution: Some people say you can straighten a bent mower blade, but honestly, I think the chances of you being able to get it completely straight again are very slim, unless you have some serious knowledge and skill with this sort of work. So to avoid wasting too much time or putting your mower in further danger (continuing to use a blade that is even slightly bent can cause irreparable damage to the crankshaft), I’d simply buy a new blade.

Crankshaft is Bent

If you have bent your blade, it’s also possible that you could have damaged your crankshaft, and installing a new blade isn’t going to fix that. No sir! A bent crankshaft is big trouble. Want to know how to tell if a lawn mower crankshaft is bent? It’s really very easy. Disconnect the mower from the power source (spark plug for gas, power cable or battery for electric), flip the mower over and now you’re going to spin the blade, pushing it on its back edge (you might want to wear a leather glove for this). Once you spin it, you’re going to want to focus your attention on the center bolt that secures your blade in place. As it spins, does it wobble around or does it stay central as it spins? On a mower with a bent crankshaft, it will wobble all over the place.

Solution: If this is the reason why your lawn mower is cutting uneven, this might be painful to hear (financially painful), but you’re likely going to have a buy a new mower. A lot of the time a crankshaft can’t be effectively straightened again. And even if you do manage to straighten it, it’s potentially very dangerous to continue using the mower. A fracture on the crankshaft is a ticking time bomb. At any moment it could break completely and potentially send the blade flying out from under the deck. Not a risk worth taking in my opinion.

Cutting Deck is Clogged

A lot of people think that the only part of the underside of a lawn mower that matters is the blade. How wrong they are! The cutting deck itself is super important in creating the vacuum effect that makes the grass blades stand up tall so they can be cut level. If your mower deck hasn’t been cleaned in an age, it’s possible that dried-on clippings are clogging your deck and are affecting that vacuum, meaning some of the blades of grass are stood up tall when they’re cut, and others bent over double. The end result: an uneven cut.

Solution: You need to get your putty knife at the ready and be willing to put a bit of effort in. Free the underside of the cutting deck from all traces of clippings. If you want to take it a step further, there are some products you can buy to help prevent grass from sticking under the mower deck that you simply spray on every so often.

The Deck is Damaged

Over time, holes can form in a mower deck, whether it be from corrosion in the case of a metal deck or the accumulation of all of the bumps and bangs that you’ve given it over the years. Whatever the cause, if a deck has a hole in it, it can really affect the vacuum/suction effect that we just mentioned, causing the cut to be uneven.

Solution: You need to try to repair the damage to the deck and plug any holes. Sometimes it’s possible to patch a deck up, but if it’s not, you’re going to have to replace the deck. With a lot of cheaper mowers, it usually works out cheaper to buy a new mower altogether. Sad, but that’s the way it is.

A Mower that Cuts Unevenly: A Quick Recap

It’s not rocket science and it usually won’t take you long to get to the bottom of why your lawn mower is cutting unevenly. In my experience, in 95% of cases the problem lies with one of these components:

• Blade
• Crankshaft
• Deck
• Wheels

Work your way through each of the possible problems that I’ve outlined above, and I’m pretty sure you’ll find your solution. I’d put money on it if I were a betting man!