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 may provide a bad cut because the mower blade is dull, bent, or unbalanced; the mower deck is plugged with debris; the tire pressures are incorrect; the engine speed is too slow; or there is damage to the spindles or deck shell.
Keep reading and I’ll explain what you can do to make your lawn look good. For safety, always wear heavy work gloves when working with your mower blades. Also, remove the ignition key and spark plug wire from your mower to prevent it from turning on while you are working under the deck.
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Follow all safety instructions provided in your equipment operator’s manual prior to diagnosing, repairing or operating. Consult a professional if you don’t have the skills, knowledge or are not in the condition to perform the repair safely.
12 Reasons Your Mower Gives a Bad Cut
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 beat 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.
Blade is Bent
Bent mower blades can affect the cut of your lawn and leave you with an uneven cut. With your lawn mower parked on a flat surface, measure from the ground to one blade tip and then spin the blade 180 degrees and measure the opposite blade tip. If there is over and 1/8″ variance, replace with a new blade.
Repeat this step with other blades under your mower deck. Never attempt to straighten a mower blade.
Blades are 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 ends of the mower blade. Placing your blades on upside down can cause your blades to hit the ground which can cause damage to the grass and your lawn mower.
The blades 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.
Mower Deck is Plugged with Dirt
Grass build up 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 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 with debris sticking to your deck, but it will help. Other things that will help with the grass buildup under the deck is not mowing when your grass is wet and mowing the lawn regularly to prevent your grass from clumping.
Tire Pressures are Not Correct
Now that you’ve taken a look at the blades and mower deck, it’s time to look at your tires if your blades are not the cause for your mower’s bad cut. Make sure your tires have the correct tire pressure. When one tire is lower in pressure than the other, the deck can sit uneven and you guessed it…you will get an uneven cut.
Mower Deck is Not Level
If your lawn mower tires are set to manufacturer specification, the next step is to check the deck height. Make sure your lawn mower is parked on a flat surface. Measure the deck height on all four corners of the mower deck to ensure the measurements are all the same. If they are not, make adjustments according to your owner’s manual.
Blade Spindle is Bent
If your deck is level and you still have a bad cut, you need to check the lawn mower deck bearing(s) under your deck. Again, make sure the spark plug boots are off the spark plugs. Some small engines will have one spark plug while others have two.
Grab a hold of one blade at a time at each end of the blade and see if you can rock the blade up and down. Feel this motion and listen for a knock. If the blade moves or creates a noise you have a bad bearing in the spindle housing, or a quill assembly, as they are sometimes called.
Bad Mower Deck Belt or Pulleys
Check the deck belt and pulleys the belt runs on. The belt must be free of cracks and not showing signs of wear or it must be replaced with a new belt. Check the pulleys by turning each pulley slowly to see if you can feel any resistance. You will also be listening for any noise. Replace the pulley if you feel any resistance or hear noise as it is likely indication of a bad bearing in the pulley.
Engine Speed is Too Low or Ground Speed is Too Fast
The mower must run at full throttle to get the engine speed needed to operate the mower deck at its best. In addition to running at full throttle, you need to control how fast your lawn mower is moving.
If you are mowing a weedy field with your mower, you’re probably not too worried about the quality of cut and can mower 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.
Incorrect Overlapping Path
If you are leaving a strip of grass after checking all of the above items, it may come down to operator error. Make sure you are mowing in paths with a little overlapping so you aren’t leaving any uncut strips of grass between each pass. It is impossible to mow your lawn and cut all of the grass without overlapping your cuts.
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. If the deck has been damaged, your cut could also be compromised.
Lawn is Uneven
Your lawn mower may scalp your yard in areas where your lawn isn’t completely flat. Mole hills are one thing that can make your lawn uneven affecting cut. 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 areas in your lawn, 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.
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.
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t not come without its frustrations. Own a lawn mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting and overheating.
For a list of the most common lawn mower problems and items that can cause them, check out my guide “Common Lawn Mower Problems: Solved!“