If you’re wondering why is my lawn mower spitting out grass, you’re not alone. This common issue is frustrating and leaves your lawn looking messy. A lawn mower spitting out grass is often telling you the deck needs cleaning or the blade has gone dull. But those are just two of several potential causes. This guide will walk you through every reason and show you exactly how to fix it.
Why Is My Lawn Mower Spitting Out Grass
The main reason your mower spits out grass clumps is a disruption in the cutting and discharge system. For a clean cut and even dispersal, the blade must be sharp, the deck clear, and the grass conditions ideal. When one part of this system fails, the mower cannot process the grass clippings properly, leading to spitting or clogging. Let’s break down each component.
The Primary Culprit: A Clogged Mower Deck
A clogged deck is the most frequent cause of grass spitting. The underside of your mower deck is designed to be a smooth, empty chamber where grass is cut, lifted by airflow, and ejected out the side chute or into the bag. When wet grass, dirt, and old clippings build up, they disrupt this entire process.
The sticky layer acts like a speed bump, preventing clippings from moving smoothly to the discharge area. Instead, they tumble around, get chopped again, and form dense clumps that the mower then spits out onto your lawn. This buildup also traps moisture, promoting rust and making the problem worse over time.
How To Clean Your Mower Deck
Cleaning the deck is a straightforward task you should do regularly. Follow these steps for a thorough clean.
- Disconnect the spark plug wire to ensure the mower cannot start accidentally.
- Tip the mower on its side, with the air filter and carburetor facing up to prevent oil or fuel leaks.
- Use a putty knife, scraper, or a stiff brush to remove all caked-on grass and debris. A wooden paint stirrer works well too and won’t scratch the deck.
- For stubborn grime, use a garden hose with a spray nozzle. Some people use a pressure washer carefully from a distance, but avoid direct, close-up spraying on bearings or seals.
- Let the deck dry completely before you use the mower again. You can spray a light silicone lubricant on the clean deck to prevent future grass buildup.
A Dull Or Damaged Mower Blade
A sharp blade slices grass cleanly. A dull blade tears and shreds it. These ragged clippings are longer, wetter, and more likely to clump together inside the deck. Instead of being finely chopped and dispersed, they stick together and get ejected as unsightly wads.
You should check your mower blade for sharpness and balance every 20-25 hours of use, or at least once a season. A damaged blade, such as one with deep nicks or bends, will also cause poor cutting and uneven discharge.
How To Sharpen and Balance a Mower Blade
You can sharpen a blade yourself with a few tools. Here is the process.
- Disconnect the spark plug wire for safety.
- Secure the blade by blocking it with a piece of wood. Then, use a wrench to remove the bolt holding the blade to the engine shaft.
- Inspect the blade for severe damage. If it’s badly bent or has large chunks missing, replace it instead of sharpening.
- Use a metal file or a bench grinder to sharpen the blade. Only file the cutting edge at the existing angle, removing an equal amount of metal from each end.
- Check the blade’s balance. Hang it on a nail through the center hole. If one end dips, file a little more metal from the heavier end until it hangs level. An unbalanced blade causes vibration and can damage the engine.
- Reinstall the blade tightly, making sure the cutting edges face toward the deck.
Mowing Wet Or Overgrown Grass
Even a perfectly tuned mower will struggle in poor conditions. Mowing wet grass is a leading cause of clumping and spitting. The moisture causes clippings to stick together and to the inside of the deck, quickly forming clogs.
Similarly, cutting too much grass at once—either because the lawn is too tall or you’re cutting too much off the top—overwhelms the mower’s capacity. The deck cannot process the volume, leading to blockages and spitting.
Best Mowing Practices To Prevent Spitting
- Always mow when the grass is dry, ideally in the late afternoon after morning dew has evaporated.
- Follow the “one-third rule”: Never remove more than one-third of the grass blade height in a single mowing. If your grass is very tall, mow it down over several days.
- Cut at the correct height for your grass type. Cutting too low stresses the grass and creates more clipping volume.
- Overlap your mowing passes slightly to ensure even cutting and discharge.
Problems With The Discharge Chute Or Bag
The discharge chute is the exit route for grass clippings. If it’s blocked or damaged, clippings have nowhere to go but back into the deck or out in messy clumps. On bagger mowers, a full or clogged bag restricts airflow, causing the system to back up.
Check the chute for obstructions like stuck grass, twigs, or even a small animal nest. Also, inspect the plastic chute or bag attachment for cracks or breaks that disrupt the airflow path. A damaged chute deflector can also cause grass to be thrown incorrectly.
Incorrect Mowing Speed And Engine RPM
Mowing too fast doesn’t give the blade enough time to cut grass cleanly or the deck enough time to process clippings. The result is ragged cuts and uneven discharge. Furthermore, if your engine is running at a low speed (low RPM), the blade isn’t spinning fast enough to create the strong airflow needed to lift and eject clippings effectively.
This often happens if the engine governor is set too low or if the engine is struggling due to other maintenance issues. Listen to your mower; it should run at a consistent, high-pitched speed when the throttle is set to maximum.
Worn Deck Components
The mower deck is more than just a metal shell. Over time, components can wear out and affect performance. The deck belt on a riding mower can become stretched or glazed, reducing the blade’s spinning speed. The mandrel or spindle that the blade attaches to can develop bad bearings, causing the blade to wobble instead of spinning true.
A warped or damaged deck itself can also ruin the critical airflow pattern. If the deck has been hit by a hidden rock or stump, it may be bent out of shape, preventing grass from flowing correctly to the discharge chute.
How To Check For Worn Spindle Bearings
- With the spark plug disconnected, try to wiggle the blade side-to-side.
- If there is noticeable play or a rocking motion beyond the flex of the blade itself, the spindle bearings are likely worn.
- Also, spin the blade by hand. It should rotate freely and smoothly without any grinding noises. A rough feel or noise indicates bearing failure.
Airflow And Lift Issues
The blade doesn’t just cut; it’s also a fan that creates a vacuum to stand the grass up and a stream of air to carry clippings out. This airflow is crucial for a clean cut and discharge. Several factors can disrupt it: a clogged deck, a dull blade, low engine RPM, or incorrect blade installation. Some mowers use special “high-lift” or “mulching” blades designed with enhanced wings to create stronger airflow. Using the wrong type of blade for your mower can reduce this lifting action.
Step-By-Step Diagnostic Guide
When your mower starts spitting grass, follow this systematic approach to find and fix the problem.
Step 1: Immediate Safety Check
Before touching anything, always disconnect the spark plug wire. This prevents the engine from starting accidentally while your hands are near the blade.
Step 2: Inspect Grass Conditions And Mowing Technique
Ask yourself: Is the grass wet? Did I let it get too tall? Am I mowing too fast? Correcting these issues is often the fastest solution.
Step 3: Visually Inspect The Discharge Area
Look at the chute and bagger attachment. Clear any visible blockages by hand. Check for obvious damage to the chute or deflector.
Step 4: Check The Mower Blade
Tip the mower safely and examine the blade. Is it dull, nicked, or bent? Check for balance and tightness. Sharpen or replace as needed.
Step 5: Clean The Mower Deck Thoroughly
Even if it doesn’t look extremely clogged, give the deck a complete cleaning. Remove all debris to restore smooth airflow.
Step 6: Assess Engine Performance
Start the mower and listen. Does the engine sound strong and run at full speed? If it bogs down or runs unevenly, you may have an engine maintenance issue affecting blade RPM.
Step 7: Check For Mechanical Wear
Inspect the spindle for wobble, listen for bearing noise, and check the deck for damage. On a riding mower, inspect the deck drive belt for wear.
Preventive Maintenance Schedule
Preventing grass spitting is easier than fixing it. Stick to this simple maintenance routine.
Before Each Use
- Check that the grass is dry.
- Walk the lawn to remove sticks, toys, and rocks.
- Do a quick visual check for debris under the deck.
Every 10 Hours Of Use
- Clean the mower deck thoroughly.
- Check the blade for sharpness and damage.
At The Start Of The Season
- Sharpen and balance the blade (or install a new one).
- Change the engine oil (for gas mowers).
- Replace the air filter and spark plug.
- Check all nuts, bolts, and belts for tightness and wear.
- Lubricate any necessary fittings.
At The End Of The Season
- Give the mower a complete cleaning.
- Run the engine to burn off fuel or use a fuel stabilizer.
- Store the mower in a dry place.
FAQ Section
Why Does My Lawn Mower Throw Out Clumps Of Grass?
Your mower throws clumps primarily because it’s mowing wet grass or grass that is too tall. The excess moisture and volume cause clippings to stick together inside the deck instead of being finely chopped and dispersed. A dull blade or clogged deck makes this problem much worse.
How Do I Stop My Lawn Mower From Spitting Grass?
To stop your mower from spitting grass, first ensure you are mowing under dry conditions and not cutting off too much length. Then, perform this three-part fix: clean the underside of the deck completely, sharpen the mower blade, and check that the discharge chute is not blocked. This addresses 90% of cases.
Is It Bad For A Lawn Mower To Spit Grass?
Yes, it is bad for the mower. Spitting grass indicates a problem that strains the engine, promotes rust from trapped moisture under the deck, and can lead to more serious mechanical failures like damaged spindles or belts. It also harms your lawn by leaving clumps that can smother the grass underneath.
Why Is My New Lawn Mower Spitting Grass?
A new mower might spit grass if the factory blade isn’t sharp enough or if the deck has a protective coating that grass sticks to. It could also be that you’re using it in wet conditions right away. Check the blade sharpness and clean the deck, even though it’s new, to ensure optimal performance.
Can A Dirty Air Filter Cause A Mower To Spit Grass?
Indirectly, yes. A dirty air filter restricts airflow to the engine, causing it to run poorly and at a lower RPM. If the engine isn’t running at full power, the blade spins slower, reducing cutting efficiency and the airflow needed to discharge clippings properly. This can lead to clumping and spitting.
Figuring out why your lawn mower is spitting out grass is usually a simple process of elimination. Start with the easiest fixes: check your mowing conditions and clean the deck. Most of the time, that’s all it takes. Regular maintenance, like blade sharpening and seasonal checkups, will prevent most of these issues from happening in the first place. By understanding how your mower works and giving it a little care, you can ensure a clean cut and a healthy, good-looking lawn every time you mow.