If your lawn mower makes a loud clunking sound when starting, then it’s best to stop your engine and see what’s going on. Hopefully, it’s just a simple fix, but there could be something going on that is causing irreversible damage if the knocking is allowed to carry on for too long. So let’s take a look at some of the common reasons why your lawn mower might be knocking and see if you can fix it.
Why Does My Lawn Mower Make a Knocking Sound? (The Short Answer)
A lawn mower knocking sound is either from something damaged inside or outside the engine. Here are 6 different scenarios I have experienced that have caused my lawn mower to make a loud clunking sound when starting up.
- Low Oil
- Damaged Blade
- Broken Blade Adapter
- Damaged Belt Covers
- Obstruction in the Deck
- Loose Engine Mounts
6 Possible Causes of This Issue
There are actually lots of different reasons that might be causing your lawn mower engine to knock. But there are a few that seem to be more common than others. Here are 6 of the most common issues people talk about.
Low Oil
On the top of the list is your engine could be low on oil. Low oil will cause internal components of the engine to overheat and expand, making them knock against each other. The quickest way to tell is by removing the oil dipstick and checking how much oil is in your mower engine.
Now you could just top off the oil, but if the knocking is really bad, then it would be a good idea to carry out an oil change and inspect the oil a bit closer. You might find some metal filing in the oil, which will indicate you have a bigger problem.
Damaged Blade
Next on the list is a damaged blade. If you hit something like a rock or the edge of a driveway, your cutting blade will suffer damage. The outcome is that the blade can become bent causing the blade to hit the cutting deck.
So, pop off the spark plug cable, tip over the lawn mower so that the air filter is pointing up, and then look at the blade. If you spin the blade by hand, you’ll be able to see if it’s striking the inside of the deck. If it is, then you’ll need to replace the blade.
Even though it’s a damaged blade, the knocking noise can sound like it’s coming from the engine because, in a lot of cases, the blade is attached to the engine.
Broken Blade Adapter
Some lawn mowers with a blade that connects directly to the driveshaft and doesn’t use a pulley system have a blade adapter instead. So if you did hit something hard, then the adapter could be damaged.
I’ve had a lawn mower with this issue which would make a knocking sound when I started up my lawn mower and when I switched it off. The knocking sound seemed to be worse when the blade spun slowly. This is why the knocking noise was more apparent when the engine turned on and off.
So, you’ll need to remove the blade adapter and see if anything looks damaged. Also, you’ll need to make sure that the key inside the adapter isn’t broken. The key is a little piece of metal that locks the adapter in position, so it doesn’t spin on the driveshaft. Because the adapter is attached to the engine, it sounds like there’s an issue with the actual engine itself.
Damaged Belt Covers
Another issue I’ve had is the belt slapping against a damaged belt cover. Different mowers come with different types of belt covers. For example, ride-on lawn mowers usually have covers on the cutting decks, and some push mowers that use belts have covers under the deck.
So, you want to inspect the covers to ensure they are not damaged and are securely attached to your lawn mower. Usually, I find that the covers have come loose and just need to be resecured.
Obstruction in the Deck
Another common cause of a lawn mower knocking when starting is that something obstructs the blade in the cutting deck. Now I’ve seen sticks do this, chunks of old grass, rocks jammed in the deck, and even broken shrouds designed to direct the grass clipping and increase the airflow.
So you’ll want to inspect the deck in just the same way as if you were checking for a damaged blade. If you spin the blade by hand, you might find the obstruction causing the knocking.
Loose Engine Mounts
Loose engine mounts are actually more common than you might think, especially on single-cylinder engines. This is because, with every engine cycle, the engine tries to rock back and forth, loosening the engine mounting bolt. Twin-cylinder engines don’t rock back and forth as much because the two cylinders balance and cancel out the rocking motion. But you will still get loose bolts.
So, take hold of your engine and tug at it. Just make sure that your engine is cold first. If you see that the engine can move around on its mounts, then you’ll need to grab a wrench.
How to Fix a Lawn Mower Engine That Makes a Knocking Sound
Now that you have discovered why your lawn mower engine is knocking, let’s look at the fixes you can follow to get things back to normal and get rid of the pesky sound.
Replacing & Checking the Engine Oil
If you are doing regular oil changes, then you probably won’t run into this problem as you have your eye on the oil level most of the time. But if you have a knocking sound, then this is a potential symptom of low oil.
So, you could just try to top off the oil by adding fresh oil into the oil filler, then try the engine again to see if the knocking has stopped. But if it’s particularly bad, it’s always a good idea to remove the oil and use a magnet to see if you find any metal filings.
Also, you might need to run the oil through some type of sieve. This is because some lawn mowers use plastic parts in their engines. Here are the steps you can follow to check and replace the oil.
Steps for Changing & Checking Lawn Mower Oil
- Remove the spark plug cable and tuck it out of the way
- Remove the oil cap
- Position oil pan next to the lawn mower
- Tip over the lawn mower
- Pour oil out of the oil filler cap
- Catch old oil in the oil pan
- Pass a strong magnet through the old oil and look for metal filings
- Pour the old oil through a sieve and look for plastic pieces
- Upright the lawn mower
- Fill with fresh oil
- Replace the oil filler cap
- Reinstall the spark plug cable
If you find filings or plastic pieces and the engine won’t stop knocking after an oil change, something is damaged inside the engine. Well, this is a problem. So you could strip down the engine and look for the damaged part, or you could consider a replacement or have a service technician take a look. This really comes down to the price.
If it’s going to cost a lot to repair, then you’ll want to make sure it’s not more than the value of the engine. It might be best to replace a small push mower, but fixing a big tractor might be worth it.
Tools & Parts to Change & Check Lawn Mower Engine
- Oil Pan
- Fresh Oil
- Strong Magnet
- Sieve
Replacing a Damaged Blade
The only thing you can do with a bent lawn mower blade is replace it. This is because it’s very difficult to straighten out a bent blade so that it’s perfectly straight. A blade bent even just a little will cause excessive vibration, potentially bend the crankshaft, and cause a bunch of other bent blade symptoms that you really want to avoid. So, changing the blade is the best approach. Here are the steps to change a lawn mower blade.
Steps for Replacing a Lawn Mower Blade
- Remove the spark plug cable and tuck it out of the way
- Tip over the lawn mower the right way
- Brack the blade so that it can’t turn
- Remove the center blade bolt
- Remove the old blade
- Position the new lawn mower blade
- Install & tighten the blade center bolt to the correct torque setting
- Remove the blade brace
- Upright the lawn mower
- Reinstall the spark plug cable
Tools & Parts to Replace a Lawn Mower Blade
- Blade Brace
- New Blade
- Socket Wrench with Torque Adjustment Ability
Replacing a Damaged Blade Adapter
The blade adapter is the weak link between the blade and your lawn mower’s crankshaft. Thankfully the blade adapter usually breaks before any damage is done to the crankshaft when you hit something hard. But unless you know where to look and what to do, you’ll probably have a persistent knocking noise coming from your lawn mower engine.
So if you find that your blade adapter is loose and damaged, you’ll want to replace it with a new one. I’ve found it tricky to replace a blade adapter because it is usually jammed in place. This is where a harmonic puller comes in handy.
Instead of bashing the adapter off with a hammer, try using a harmonic balanced puller. Here are all the steps, tools, and parts you’ll need to replace your blade adapter.
Steps for Replacing a Lawn Mower Blade Adapter
- Remove the spark plug cable and tuck it out of the way
- Tip over the lawn mower the right way
- Brack the blade so that it can’t turn
- Remove the blade bolts
- Remove the blade
- Remove the blade adapter center bolt
- Attach the harmonic balanced puller to the adapter
- Tighten the harmonic balanced puller and remove the old adapter
- Remove the harmonic balanced puller
- Install the new blade adapter
- Install the adapter center bolt
- Position the lawn mower blade onto the adapter
- Install & tighten the blade bolts
- Remove the blade brace
- Upright the lawn mower
- Reinstall the spark plug cable
Tools & Parts to Replace a Lawn Mower Blade Adapter
- Blade Brace
- New Blade Adapter
- Socket Wrench with Torque Adjustment Ability
- Harmonic Balanced Puller
Securing the Belt Covers
A wobbly belt cover is something that I come across more often than I would like. With my lawn mower’s vibration, the fixings loosen every few months. So, if you are experiencing this issue, then all you need to do is grab a screwdriver or a wrench and tighten the fixings back up.
Also, you can remove the bolts and use a threadlocker. This is a glue that stops nuts and bolts from loosening themselves off with vibration. So, here are the steps you’ll need to follow to stop fixings from loosening and causing the engine knocking sound from the belt covers.
Steps for Securing Belt cover Bolts
- Remove the loose bolt or nut
- Coat the bolt or nut with a threadlocker
- Reinstall the bolt or nut and tighten
- Allow the threadlocker to fully dry
Tools & Parts to Secure Loose Bolts or Nuts
- Socket Wrench
- Screwdriver
- Threadlocker
Cleaning Out the Deck
Depending on the obstruction in your deck, you’ll need to use a few different approaches. If there is something wrapped around the blade and crankshaft, just try removing the obstruction with your hand. However, if the obstruction is really wrapped around the driveshaft, then it might be easier to remove the blade.
Now, if there is junk in the deck causing the knocking, like dirt and grass build-up, you can use a scraper or even a power washer to clean the deck. You’ll be aiming to turn the blade without it coming into contact with anything, which will stop the knocking sound from being transferred from the blade up into the engine.
There isn’t really any particular step to follow here because there are so many different ways a deck can become obstructed. But once the deck is cleared out, you might want to use a PTFE spray or some other non-stick spray to stop material like grass from sticking to your cutting deck. Just remember to remove the spark plug and tip your lawn mower the right way before cleaning.
Tightening The Engine Bolts
To tighten up the engine mounting bolt, you can use the exact same step as if you were tightening up belt covers. Again, you can use a threadlocker to stop the fixings from loosening in the future. So, remove the fixings, coat them with the threadlocker, and then reinstall them tightly back on your lawn mower. This will stop your engine from rocking from side to side, causing the engine to knock.
Tools & Parts to Secure Engine Mounts
- Socket Wrench
- Threadlocker
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