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Lawn Mower Leaving Uncut Grass – The complete fix with pics

By: Author John Cunningham. Published: 2019/01/22 at 8:35 pm

It’s a common source of frustration and ARGH!! But more than likely, the solution is a simple one, and you can fix it right now.

So why is my lawnmower leaving uncut grass? The most common cause of uncut grass is a dull blade, but it’s not the only possible reason:

  • Damaged Blade
  • Grass Too Tall / Wet
  • Deck Too Low
  • Throttle Too Low
  • Poor Engine Performance
  • Lawn Speed Too High
  • Grass Bag Too Full
  • Clutch slip

This problem has many possible causes, but a dull blade is a usual suspect. Check your engine performance; if you feel the engine doesn’t sound right or it’s sluggish, go ahead and solve engine-related issues first.

Very often, uncut grass is simply caused by a dull blade. If you need help inspecting and safely sharpening your blade, check out “How to sharpen mower blade video”

Check For Blade Damage

A defective blade will cause all kinds of problems in the grass-cutting and collection department. Examine your blade checking for loose bolts and damaged, misaligned, bent, or broken blade tips.
Mower blades have it tough, and hitting stones, sprinklers, stumps, and dog toys is all part of the job. Mower blades turn about 50 times a second – that’s 200 mph at the tip.
So when you hit something, it’s going to damage even hardened steel. Bending and gouging chunks from the metal blade will cause uneven cutting and a horrible vibration.

A blade may look OK, but they do wear. If it’s more than four seasons old, it’s probably worn out. The leading edge cuts the grass and is easy to see when it’s worn. The trailing edge stuffs the bag, and as that edge wears, it becomes much less efficient. The solution is to replace the blade.
Never attempt to repair or bend a blade; the metal has been specially treated, and interfering with this can cause it to shatter.

When replacing the blade, go ahead and get a new bolt and washer. They’re mower-specific and also specially treated, so a bolt from the local hardware store won’t be up to the job.
A torque wrench should be used to tighten the bolt to the correct specification. A quick check of your mower manual or on the dealer site will give the spec.

Blade Orientation

It’s possible to fit a blade backways, so if you fit a blade recently, just check that the orientation is correct. Hey, it could happen a Bishop, don’t worry about it!

Imagine looking down at the blade from above – the leading edge of the blade will turn clockwise.

Bent – A bent blade is dangerous, and it will cause lawn scalping and vibration and, if ignored, will damage the mower engine.

Replace – A new blade will solve many problems; when changing the blade, replace the bolt and washer too.

If you need video help replacing the blade, check out the “Replacing blade video” and if you need mower blades, check out the Amazon link below.

Amazon Lawnmower Blade

Dull Blade

Is your blade sharp? A dull blade is the number one reason for leaving uncut grass. The blunt blade will damage your lawn in no time at all; it tears the grass and leaves a jagged edge which turns the grass tips yellow.
The recommended way to repair the yellow grass damage – regularly cut with a sharp lawnmower blade. I tell my customers to sharpen at least once per season, and more depending on how often you cut and terrain type. A sharp blade is the secret to a healthy, beautiful green lawn.
Check out “Blade maintenance tools” here; they make the sharpening process a ton easier. And if you need video help sharpening the blade on or off the mower, check out the “Blade sharpening video”

Sharpen – Your blade needs a sharpening once per season, at least. If you file your blade regularly, it won’t take much effort to keep sharp.

What Blade Type?

You may have a blade that doesn’t suit your climate or your needs. So what’s in a blade? Quite a lot of clever engineering, actually. A blade looks pretty unimpressive, but change it out for a different type or a new one, and you’ll be surprised at the difference in cut and finish.
There are two main blade types, the lift blade and the mulching blade; each has its own strengths. How you intend to handle your clippings and your climate will likely dictate which blade suits you. 

Lift Blade

The Lift blade, also known as the 2 in 1 (collecting or discharging), vacuums the grass upright before cutting and moving it to the bag. These lift blades are designed for collecting grass and come in low, medium, and high-lift.
Lift means sucking power, and a higher lift blade will require a more powerful engine. The lift is created by curving upwards of the trailing edge of the blade; the steeper the curve, the more powerful the lift. These blades love to bag grass, wet or dry.

Lift blade – Also known as a 2-in-1, it loves to bag grass, wet or dry.

Mulching Blade

A true Mulching blade is designed to finely chop and disperse grass clippings, not collect them, a proper mulching mower won’t have a grass bag.
Many of the latest mowers are fitted with a hybrid mulching blade, also known as a 3-in-1 (collecting, discharging, or mulching). It’s sort of half lift blade, half mulching blade – Jack of all trades if you like.
These blades are not designed specifically to collect and are really best suited to very regular dry weather cutting; if the grass is tall, a 3 in 1 mulching blade may struggle to bag efficiently. 

Mulching – 3 in 1 blade is good but has limited success in more challenging conditions.

Check Engine Power

Check if the throttle is set correctly; it should be set to fast/run when cutting. Does your throttle cable need adjustment? Is the engine running as it should? If the engine power is reduced, the mower will not cut well, especially when it hits a patch of heavy grass.

Lawnmowers are generally very reliable; give them a tune-up and blade sharpening at the start of every season, regardless of how it’s running. The oil should be changed every 50 hours, and clean the air filter every 25 hours, and more often in dusty dry conditions.

Check out:

“Lawn tractor tune-up”

“Lawn mower tune-up”

Check throttle lever – If your engine seems to be a bit sluggish, first check that the throttle is set to full. The lever may be reading full throttle, but the cable may not be moving at the carburetor end.

Second, check the air filter is clean, try running the engine without the filter, and see if it makes any difference.

Gas – Old fuel is the number one cause of poor engine performance. Fuel older than a month goes stale and will gum up the inside of the carburetor.

This blocks the fuel feed ports and causes fuel starvation. This results in a sluggish engine and a poorly cut lawn.

Try using a fuel stabilizer; it will keep the gas fresh for up to two years, but more importantly, it will prevent the gumming up of the carburetor. Check out the video showing how to mix and add a gas stabilizer. The page includes a link to the gas stabilizer I use.
If you suspect bad gas is causing sluggish engine output, try draining the fuel tank and carburetor bowl and filling them with fresh gas. This will very often fix the issue, and if it doesn’t, a full carburetor cleanout will.
Check out:
“Lawn mower carburetor cleaning”
“Small engine carburetor troubleshooting”

Clutch Slip

If you’re using a tractor mower, you may have a clutch system fitted. The clutch or PTO (power take-off) transfers the engine power to the blades. When the clutch starts to fail, it slips, which means the blades are not turning with the same power or speed as normal. It will be especially noticeable in taller, heavier grass.
Some walk behind lawn mowers (Honda) use a very similar clutch setup, so if you have a separate control to engage the blade, then it’s likely you have some form of the clutch system, but note these clutch systems are cable operated and the tension on the cable may simply need to be adjusted.
If you don’t have a clutch fitted, but you do have a separate lever to control the blades, then your blades are possibly controlled by a tensioned belt. The Toro Time-master is a good example. (see below)
Pulling the bail lever tightens the tension on the belt and makes the blades spin; the tension on the belt may simply need to be adjusted.

Clutch – Three types of clutch, the tractor PTO, the Honda blade clutch, and finally, the belt-type clutch fitted to the Toro Time-master.

Tractor mower PTO clutch
Honda clutch
Toro belt clutch

Related Questions

What causes uneven grass cutting? The most common cause of uneven grass cutting is an uneven deck. Other possible causes include: 

  • Damaged blade
  • Deck wheels are set at different height
  • Tire pressures are low (tractor mower)
  • Deck height needs adjustment (tractor mower)
  • Anti-scalp wheels are missing (tractor mower)

You may find this post helpful:

Why is my lawn mower leaving a strip of grass?