I was on my way to work and drove past a house with a freshly cut lawn. It looked horrible! I wanted to stop and tell the owner how to fix the bad cut, but I’m pretty sure he would have found it odd. So, I’m going to share my tips to solve a mower’s bad cut with you.
A lawn mower has an uneven or bad cut when the mower blade is dull, bent, or unbalanced; the mower deck is clogged with debris; the wheel is damaged; the crankshaft is bent; the engine speed is too slow, or there is damage deck shell.
While I discuss issues with a push mower here, you can find information on a bad-cutting experience with a riding mower or zero turn here.
Always remove the spark plug before working under the deck.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
Troubleshooting a Lawn Mower with an Uneven Cut
1. Blade is Worn or Dull
The grass can indicate you have worn or dull mower blades. When your blades are excessively worn, strips of grass can be left behind or your grass can develop brown tips a couple of days after mowing.
The brown tips come from the grass being beaten up by the blades instead of cut by sharp mower blades.
You must replace your lawn mower blades when they are worn. Sometimes, they are able to be sharpened. Read my article on sharpening mower blades.
2. Bent Mower Blade Will Cause Uneven Grass
Bent mower blades can affect the cut of your lawn. It may even leave a line of cut grass. Some bends in blades will be very obvious, but others will be a slight bend and hard to visually notice.
With a push mower, it’s easiest to determine whether you have a bent blade by removing it and placing it on top of a new blade. Check for any gaps between the old and new blade to see if you have a problem blade.
If you find a bend, install a new blade. Don’t attempt to straighten the blade. Doing so will weaken the metal and may pose a safety hazard if a piece of metal breaks off of the blade and is thrown out of the mower deck at very high speeds.
3. Blade Installed Incorrectly
A lawn mower blade must be installed with the sail, or fin of the blade pointed upward toward the deck. The sail is the high end of the mower blade.
Placing your blades upside down can cause your blades to hit the ground which can cause damage to the grass and your lawn mower.
The blade must be balanced before they are installed. They must have equal weight on each side of the blade or they can cause vibration in the deck and potential spindle damage. Read more about blade installation here.
Make sure the blade is securely installed so it is not loose or wobbly.
4. Mower Deck is Plugged with Dirt and Grass Clippings Will Cut Uneven
Grass buildup underneath your mower deck can disrupt your lawn mower’s cut. You may think it is a pain to scrape the deck to remove debris, but it is necessary if you want a nice-looking lawn. Plus a clogged deck can put extra strain on your engine.
The depth of the deck, along with the spinning blade, creates a suction in the deck to stand the grass tall so the blades can pass by and give you a nice cut.
If the cut grass ends up plugging up the space in the deck, you will lose the lift created by the suction in the deck.
Using a silicone spray designed for your mower deck will not completely solve the problem of debris sticking to your deck, but it will help.
Other things that will help with the grass buildup under the deck are not mowing when your grass is wet and mowing the lawn regularly to prevent your grass from clumping.
5. Wheel is Damaged
When the wheel is loose or damaged, the mower may no longer sit level along the ground when mowing. Damaged wheels will leave you with an uneven cut or step cut.
Tighten any loose hardware and replace a damaged wheel with a wheel of the same size.
6. Cutting Height is Not Set Correctly When Mowing
Most lawn mowers have several different cutting heights available. When the settings are not the same on both sides of the mower, one side will sit lower than the other. The mower deck will sit unleveled causing the mower to have a taller cut on one side.
Uneven cuts may be caused by mower deck damage or misadjustment. Check the height adjustment on the mower to ensure they are the same on both the left and right-hand sides.
Fix any damaged components that may affect cutting height.
7. Bent Crankshaft
A bent crankshaft will definitely cause your mower to cut unevenly. You can perform the following test to determine whether the crankshaft is bent. Be careful and protect your hands from the sharp blade.
Remove the spark plug, blade, and blade adapter. Turn the crankshaft and measure to make sure the shaft is remaining in the center.
You can use a dial indicator or some form of a stationary object to gauge whether the crankshaft is moving off-center indicating it is bent.
If it does not remain centered and shifts to one side, the crankshaft is bent and it’s time to replace your lawn mower.
Here’s the bad news! If you have a bent crankshaft it’s time to replace a lawn mower with a bent crankshaft.
Replacing the crankshaft is more expensive than purchasing a new mower.
Some choose to try to bend the crankshaft back in place, but this poses a significant safety risk. I highly recommend purchasing a new lawn mower.
8. Ground Speed is Too Fast
How fast you’re cutting the lawn can affect how well the lawn looks.
If you are mowing a weedy field with your mower, you’re probably not too worried about the quality of the cut and can mow faster. On the other hand, if you are mowing your front yard, you want that cut to look the best it can and will have to control your ground speed to move a little slower.
When mowing tall or thick grass, you need to slow down the ground speed of your lawn mower. If you move too fast, all of the grass does not get cut as some of the grass gets pushed over instead of cut.
It may also clog the deck and overwork the engine. Slow down your walking speed or self-propelled speed when you are mowing tall or thick grass.
9. Mower Deck Shell is Damaged
A deck shell can get tweaked by running your mower into objects like trees, rocks, and other hard surfaces. The deck shell is formed to create a suction to give you a good cut.
Your cut can be compromised by a mower deck that is damaged or bent. If this is the case, you can try to fix the deck to its original form.
10. Engine Speed is Too Low or the Engine is Damaged
The engine speed needs to run at full throttle for a good cut. If you have a push mower with a throttle lever, make sure you have it set at a fast speed (often shown with the image of a rabbit).
Don’t worry if you can’t find a throttle lever. Many push mowers automatically run the engine at full throttle as soon as you start it.
However, if you feel the engine is not running as strong as it once did, it may be time to have it checked by a small engine mechanic. Just note, if you have an engine problem, it will be cheaper to purchase a new push mower than replace the engine.
11. Lawn is Uneven Affecting Mowers Cut
Your lawn mower may scalp your yard in areas where your lawn isn’t completely flat. Uneven lawn sith mole hills is one thing that can make your lawn uneven affecting cuts.
I have this problem in one area of my lawn where moles are tunneling through the grown leaving raised areas in the yard.
If you have uneven ground, you can try to use a lawn roller to smooth them out. In my case, I’ll have to find a way to deter the moles from my yard because rolling will only be a short-lived solution to my problem.
In Summary: Mower Cutting Uneven
Stay on top of your mower deck needs by performing regular maintenance on your deck. Small items out of adjustment or not in good condition can cause you to have cutting or vibration problems in your deck.
Frequently Asked Questions
Mowing wet or damp grass will cause your lawn mower to leave behind piles of grass. The moisture in the grass will cause it to clump under the deck and on the lawn. Scrape the mower deck and avoid mowing wet grass to fix the problem.
The appearance of shredded grass is most likely due to a worn mower blade. A dull blade will tear and bruise the grass instead of a nice clean cut with a sharp blade. You will most likely notice the shredded ends a day or two after mowing the lawn.
Cutting the grass with a dull mower blade may make the lawn may appear brown. This is due to the worn blade edge bruising and tearing the grass leaving a ragged appearance a day or two after mowing.
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.