My Lawnmower makes a large clunking sound when starting. This is how you diagnose and fix it.


Lawnmowers are mechanical devices that already produce a lot of noise. This is also the case when you start your mower. But if you hear strange sounds, you should be alarmed. This should also be the case if you hear unusual clunking sounds. It can happen when metal parts strike with one another and is usually an indicator that something is not working correctly. But it can also have a different cause. If you aren’t sure how to fix this issue, we will help you with this blog post. We will look at the different causes and fixes when a lawnmower makes clunking sounds when starting.

My lawnmower makes a large clunking sound when starting. This is how you diagnose and fix it:

  • Step 1. Remove debris accumulated under the deck
  • Step 2: Check the mower deck
  • Step 3. Ensure that the blade is tight enough
  • Step 4. Check the Lawnmower muffler
  • Step 5. Check the engine oil
  • Step 6. Check the spark plug
  • Step 7. Check and Clean the dirty air filter

The clunking sounds can arise due to various factors. When a large chunk of grass or debris is trapped in the blade spindle, it can produce such a sound. The muffler might also be a cause. Or it may be that the blades are loose and producing such sound. If the fault is not fixed, it can cause permanent damage to certain components, and you might need to take your device to a mechanic who may charge you a hefty sum for replacements. Hence, it is better to fix such a problem once it’s started to restore the mower to its initial efficiency. This blog post will look into more detail about the various reasons and how to fix them.

Fixing Lawnmower Engine Clunking Sound:

You should follow all these steps to troubleshoot the issue with your lawnmower.

Step 1: Remove Dirt from the Lawnmower Deck

By continuous usage, dirt and debris get accumulated under the deck of the mower. This accumulation reduces the space for grass. Thus, the grass disturbs the blades resulting in the production of a clunking sound.

To remove the dirt, first, unplug the sparkplug wire. This is to ensure that the engine can not unintentionally start. If your mower has a water hose attachment, you can use it to attach a hose and clean the deck. Let the mower run for a few min. If your mower does not have this and it can be cleaned with water (check the manual), you can use a strong hose. Or a power washer on its lowest setting. Alternatively, you can scrape the dirt off using a scraper. And clean with a little bit of water.

Once all the ground is removed, spray some silicon spray of a lubricant (like WD40) on the deck, preventing further soil accumulation under the deck.

Step 2: Check the mower deck

One of the more frequent reasons for a clunking sound is a loose sitting lawnmower deck. When the engine is starting, it makes the mower shake in a certain way that can produce a clunking sound. Turn the mower off and disconnect the spark plug cable. This to ensure that the engine can not unintentionally start when you are working on or close to the blades.

Move the mower deck with your hand, and check if it is loose. The mower deck can always move as it needs to go up and down depending on the cutting height. Check if there is more movement than normal. If that is the case, check the various nuts and bolts that keep the mowing deck in place. If one is loose, tighten it. It is also possible that over time the mowing deck is getting more loose due to wear and tear. If that is the reason, you can try to find the most affected connections and replace the bolts with some slightly thicker ones.

Step 3: Tightening the blades and make sure they are balanced

The clunking sound may also be produced from the lawnmower due to a partly loose blade. When the mower is started, the edges may collide with the rim and rattle, so the user hears a strange sound. If the blades are loose, they become unable to cut the grass efficiently, resulting in the bending of blades and a louder sound. 

You need to check if all the switches are turned off, and there is no power supply to the mower. Disconnect the sparkplug cables. This prevents any accidental starting of the mower when you are working on or near the blades. Tip the mower to its side and tighten up the bolt that holds the blades using a wrench. It is also a good idea to check if the blades are correctly balanced. The only way to test this is to remove the blades. Hang them on a nail and check if the balance is ok. If this is not the case, remove some metal on the side that is heavier until it is correctly balanced.

Step 4: Check the Lawnmower Muffler

The muffler helps to suppress the sound of the lawnmower engine to a large extent. But if the muffler is damaged or rusted, it may not function properly. This can lead to a louder sound, and depending on the muffler, also some clunking sound.

If the muffler is really damaged, it needs to be replaced. Make sure you let your mower cool down before touching it. Check the muffler as best as you can. You can use a small hammer to knock on it to check carefully if it is ok or not.

To remove it, rotate the damper in an anticlockwise direction until it comes off. If you do not know the correct type, use the muffler to buy a new one. Ensure that it has the same size, and fits perfectly on the lawnmower. Sometimes, the muffler may stick into the outlet, so you need to use a wrench to take it out. Reattach the new muffler and check if it has helped.

Step 5: Check the Lawnmower Engine Oil

Engine oil enables the smooth working of the lawnmower engine. If there is low engine oil in the lawnmower, it may produce a knocking or clunking sound. Regularly check the oil level, and if needed, add some oil. Also, ensure that you use the correct oil for your mower.

If you have recently mowed the lawn, wait for it to cool down. Remove the oil fill cap and dipstick. Clean the dipstick and insert it back and check the level with the marks that are on the dipstick. When the level is under the mark, add some oil.

Step 6: Check the spark plug

Sometimes there is a misfiring from the engine. This can be due to a bad sparkplug. Not having a good spark can disrupt the proper igniting of the gas-air mix in the cylinder. As a result of this misfiring, a clunking sound can be heard.

To check, remove the sparkplug and check its condition. Clean it if it is dirty, replace it when it looks really bad.

Step 7: Clean the filter

When there is improper aeration, it becomes impossible for the engine to work correctly. This can happen when the air filter is dirty. The dirt blocks the flow of air, and less than the needed amount of air can reach the carburetor.

To check and clean the air filter, remove the cover. Remove the air filter. If you have a foam air filter remove the bigger dirt first. Then you can wash and dry it. If the filter is damaged, replace it. If your mower uses a paper air filter, you have no other option than to replace it. After cleaning, put the filter back in its place and close the cover.

Some Precautionary Measures:

Following are some preventive measures that should be considered:

  • Turn the engine off: While fixing any problem, make sure the power is turned off.
  • Let the engine cool: The mover should be cooled down before operating it.
  • Use protection: Gloves should be worn while handling the blades or other sharp objects since.
  • Don’t overdue the WD-40: The layer of WD-40 should not be applied thickly as it drips while mowing the lawn.
  • Protect the engine: While removing the filter, make sure no dirt particle enters the engine.

Frequently Asked Questions:

1. What to do when there is less oil in the engine?

When there is less oil in the engine, the engine does not function properly. It can produce a dull sound. The engine oil needs to be checked regularly. Oil should be added when the level is low. A good routine would be to do this before you start mowing. Always ensure that there is enough good quality oil to lubricate and protect the engine. Most lawnmowers can use SAE 10W-30.

2. The muffler is replaced, still hearing the clunking sound. What to do?

There are chances that despite changing the lawnmower’s muffler, the clunking sound does not go away. This may be due to a low quality of damper used in the lawnmower that cannot suppress the noise. One more possible could also be that the muffler’s size in the mower is not compatible with the lawnmower’s size. In that case, it is somewhat loose. If you do not know the exact type that your machine needs, take the worn-out damper when you buy a new one. Always go for a good quality muffler to minimize the noise level.

3. The dirt is not coming off from the lawnmower filter. What to do?

Sometimes, the dirt sticks to the filter so bad that it blocks the small pores present in the filter. The minute pores are filled with small dust particles. It prevents airflow through the filter; thus, there is insufficient air for the engine to light up. If it cannot be adequately cleaned, replace it.

Final Remarks

Although lawnmowers are noisy, any unusual sound should alarm you. In that case, try to find the reason why it sounds. In this blog post, we have looked at the situation when you hear a clunking sound when you start the lawnmower. In most cases, it can easily be fixed. Always try to maintain your mower well. This decreases the chance for this kind of problem. Clean the mower after you use it. And check the oil level and blads before you start mowing.

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