Lawn Mower Leaving Uncut Grass

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 lawn mower 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 the 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.

 

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, 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 secound - that's 200mph 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 4 seasons old, it's probably wornout. The leading edge cuts the grass and is easy to see when its worn. The trailing edge stuffs the bag, and as that edge wears it becomes much less efficient. The solution - replace the blade.

 

Never attempt to repair or bend a blade, the metal has been specially treated, and interfering with this can cause them 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 back ways, so if you fitted a blade recently just check that the orientation is correct. Hey, it could happen a Bishop, don't worry about it!

 

Imagine looking down on the blade from above - the leading edge of the blade will turn clockwise.

 

 

 

mower blade mower blade

Bent

A bent blade is dangerous and it will cause lawn scalping, 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.


Mower Blades

Bent


mower blade mower blade damage

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

Replace


mower blades

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

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 - cut regularly with a sharp lawn mower 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.

 

 

 

mower battery

Sharpen

Sharpen your blade once per season and more if needed.

If you file your blade regularly, it won't take much effort to keep sharp.

Sharpen


mower blade

Sharpen your blade once per season and more if needed.

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's two main blade types, the lift blade and the mulching blade, each have their 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 up right, before cutting and moving to the bag. These lift blades are designed for collecting grass and come in low, medium and high lift.

 

Lift meaning sucking power, and a higher lift blade will require a more powerful engine. The lift is created by the curving upwards of the trailing edge of the blade, the steeper curve the more powerful the lift. These blades love to bag grass, wet or dry.

 

 

Mower lift blade

Lift

Or 2 in 1 loves to bag grass wet or dry.

Lift Blade


Mower lift blade

Or 2 in 1 loves to bag grass wet or dry.

Mulching Blade

A true Mulching blade is designed to finely chop and disperse grass clippings, not collect, infact a proper mulching mower won't have a grassbag.

 

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 is 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 Blade


Mower mulching blade

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

Mulching Blade

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.

 

Lawn mowers are generally very reliable, give them a tune-up and blade sharpening at the start of every season, regardless of how its 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"

 

 

 

Mower throttle lever Mower throttle lever

Check

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.

Check


Mower throttle lever Mower air filter

If your engine seems to be 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.

Check Gas


mower stale gas

Old stale gas causes lots of problems. Your mower may still run but lacks Oomph. 

Draining the gas and replacing should fix the sluggishness.

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 gumming up of the carburetor.

 

If you suspect bad gas is causing sluggish engine output, try draining the fuel tank, 

carburetor bowl and filling with fresh gas. This will very often fix the issue, and if it doesn't, a full carburetor clean out will.

 

Check out: "Lawn mower carburetor cleaning".

"Lawn tractor carburetor cleaning" cleaning"

 

 

 

Clutch Slip

If your 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, that 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 set up, so if you have a seperate control to engage the blade, then it's likely you have some form of 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 seperate lever to control the blades, then your blades are possibly controled by a tensioned belt. The Toro Timemaster is a good example.

 

The pulling of 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 Blades clutch and finally the belt type clutch fitted to Toro Timemaster.


Tractor mower pto Honda clutch Honda clutch

Clutch Types


Tractor mower clutch Honda mower clutch Toro mower clutch belt

Three types of clutch, the tractor PTO, the Honda Blades clutch and finally the belt type clutch fitted to Toro Timemaster.

Image

Welcome to Lawnmowerfixed.com. This is the site where I share everything I know about small engine repair.

I'm John Cunningham, qualified technician, I run my own repair shop.

The guides are free and I'm just an email away, Happy Hunting!

John Cunningham technician

Hi, I'm John Cunningham technician and gear head, welcome to Lawnmowerfixed.com.

This is the site where I share everything I know about small engine repair.

The guides are free and I'm just an email away, Happy Hunting!

John Cunningham technician

Hi, I'm John Cunningham technician and gear head, welcome to Lawnmowerfixed.com.

This is the site where I share everything I know about small engine repair.

The guides are free and I'm just an email away, Happy Hunting!

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)

 

Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.