Why Is Your Lawn Mower Not Cutting Evenly?




Why Is Your Lawn Mower Not Cutting Evenly

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Why Is Your Lawn Mower Not Cutting Evenly? : 7 Reasons

There is nothing worse than an uneven cut.

I know how frustrating this can be. When I had five vans on the road this happened to me regularly. 

I came up with a simple step-by-step method of diagnosing and fixing the issue. It worked well for me and I hope you will find it helpful as well.

So… Why Is Your Lawn Mower Not Cutting Evenly? Here are the most common reasons going from most likely to least likely.

  • Wheel height adjustment settings
  • Damaged mower blade
  • Blunt blade
  • Axle bushes worn
  • Wheel bearings are worn
  • Crankshaft is bent
  • Engine mount or crack in mower deck.

When someone told me that they had this problem I would run through the list of problems and fix it as set out below.

Wheel height adjustment.

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This might sound a bit obvious but it’s a good thing to check first.

Years ago I had a flatmate who was a refrigeration engineer and sometimes he was on call. The first question he asked when a call came in was “Is the unit plugged in and switched on?”. On the surface, this sounds like a silly question but if the answer after checking was “no” then he just saved some time and they saved a whole lot of money.

Check that the height adjustment on the lawnmower is correct, not damaged, and if you have interdependent wheel height adjustment make sure that they are all on the same level.

Now that you have ruled that out let’s move on.

Check the engine mounts and the body for cracks.

The easiest way to do this is to push down on the engine while the mower is level on the ground. Does the engine move or have any play? 

The engine should not have any movement on the body at all. If there is movement there, check the engine mounts. If they all look ok and secure then check the body for cracks.

If your body has cracks they will usually be around the engine mounts where the body connects to the engine or you can get cracks around the axles. Either of these will cause flex. This means that as you push the mower forward the body will dip, causing the blade to move closer to the ground and scalp areas of the lawn.

This actually has a different visual clue when you are mowing. Instead of cutting low on one side, the mower acts more erratically. It may mow fine for a bit and then scalp a piece of lawn for no apparent reason. This is a giveaway that you have worn engine mounts or a cracked body.

If the culprit is a worn engine mount then that can easily be replaced as demonstrated in the video below.

If the body is cracked, then you can have that welded but I usually write the body off. To me, that is a sign of the beginning of the end of the mower.

Are the wheel axle Bushes worn?

Check the hole in the body where the axle comes out. Is there any play? If so have a look at what is holding the axle. It will usually be a metal or plastic ring. If it is worn it will need replacing. You will need to search for your make and model but the part will look similar. Below is a video that will show you how to replace the bushes.

If the Bushes look fine then continue.

Are the wheel bearings worn?

Go around your mower and push and pull on the wheels. Is there any play? The wheels should spin but not move inwards or out. If this is an issue you will need to pull off the offending wheel and replace the bearing or bearings. Below is a video on how to do that.

Is the blade damaged or bent?

Bent blades are caused by hitting objects and you usually know it if you hit something hard enough to damage the lawn mower blade. 

It was always a bit harder for me with my worker’s mowers and they usually swore black and blue “Honestly I never hit anything”. But at the end of the day, blades don’t bend themselves.

It’s time to check the blade. Disconnect the spark plug and tilt your mower up with the spark plug facing the sky. Do not tip the mower on its side unless it is a two-stroke.

If you do not know if your mower is a two-stroke the easiest way to tell is to look for the dipstick. Two strokes don’t have them as the oil is mixed in with the fuel.

Once you can see the blade then make a mark on the inside of the mower body to mark when the end of the blade sits. Now turn the blade a full circle. Does the other side of the blade sit in the same place? It should.

If your blade fails this test then it may be bent. You will need to pull the blade off.

Once the blade is off. check to see if it is damaged or bent. I keep a spare blade and I put them together to test. You can also use an old undamaged blade to run this test. If you don’t have another blade to compare it to then put the blade on your workbench. Is it sitting evenly? If not it needs replacing. If it does not look damaged then move to the next step.

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Check your crankshaft. Is it bent?

The only way you can bend a crankshaft is to hit a solid object with your mower. There are usually a couple of aluminum notches on a blade holder that should break off before any damage is done to the blade as seen below but this does not always happen.

The two lugs you see on the base are designed to break off if you hit something therefore saving your blade.

image 1

There are a couple of ways to test if your crankshaft is bent. The easiest way is to get someone to pull the starter cord while you look at the end of the crankshaft. (remember to disconnect the sparkplug and remove the blade) If it spins in a perfect circle it is not bent. If it turns unevenly then your crankshaft is bent.

If you are not 100% sure then you could try placing a pair of vice grips on the end of the crankshaft and then getting someone to pull the pull start slowly. This should make any unevenness even more obvious.

If your crankshaft is bent I am sorry but there is no easy fix. You will need to take it to the shop. If the damage is not too bad then they may be able to bend it back. If not, then the mower is a write-off. It’s time to buy a new one.

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A final couple of other reasons your mower may be cutting unevenly.

The grass you are trying to cut is too long.

You will not get a nice cut if the lawn is up to your knees. Cut as high as possible. The main issue you are going to get here is that the mower is not going to cut properly where the wheels have pushed the grass down. This uncut grass is going to be sticking up again by the next day. This is why I would recommend doing a second cut slightly lower the next day.

This brings up another issue that I haven’t discussed here and that is engine speed. A surging engine may cause your lawn mower to cut unevenly. This should be obvious by the sound the mower makes. An overgrown thick lawn like the one in the photo below could make your mower do this. 

A Long New lawn with a mower in the middle

Why Is Your Lawn Mower Not Cutting Evenly – Summing up

This post covers the most common things I find that affect your mower cut. However, if you are using a riding lawn mower it may also pay to check your tire pressure. If you notice your lawn mower cutting unevenly and your machine runs two blades it may actually be a timing issue. I talk about how to troubleshoot blade timing issues here

This is the Briggs & Stratton guide to fixing those other issues quickly.

The grass is too wet.

If you are mowing wet grass then you would be better off waiting for the lawn to dry. However, if you are in a real hurry to mow the lawn then I suggest you read this post. “16 Tips to Cut Lawns on a Wet Day”

I hope this helps and happy lawn mowing people.

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