Stop Petrol Lawnmower Vibrating-Help And Repair.

Why Is My Petrol Lawnmower Vibrating So Much?

When I first started to Repair Lawnmowers For Profit I couldn’t believe just how many lawnmowers had a terrible vibration problem. Luckily through experience, I’ve come to learn the reasons why and also how to eliminate the problem.

Why Does My Lawnmower Vibrate?

A petrol lawnmower usually has excess vibration if

the blade is out of balance, this can happen if the blade becomes bent or it’s not sat correctly on the blade adapter. It can also happen if the blade has come loose or the lawnmower has incorrect ignition timing. It can also happen if the crankshaft is bent or if the actual lawnmower engine is not correctly attached to the lawnmower deck.

How Can I Stop My Lawnmower Vibrating?

Fortunately, there are a few simple checks and repairs we can do to stop your lawnmower from vibrating. In this article, I’m going to cover all the tips and tricks I’ve learned from repairing hundreds of lawnmowers.

If you have a large area of grass to cut having a vibrating problem can be a real pain so let’s take a look at some of the more common reasons for lawnmower vibration.

Feel free to look in my online parts store for all your spares

Remove The Spark Plug

Before you do any repair or inspection work on a lawnmower be absolutely sure to remove the spark plug from the lawnmower. This is the only way to completely eliminate any accidents. Also when you do remove it consider replacement with a new one and be sure to check the gap in the spark plug.

Occasionally a spark plug can be faulty and cause hunting and surging problems which in turn may cause a lawnmower to vibrate more than it should.

Check To See If The Blade Is Bent!

With the spark plug completely removed tip the lawnmower up from the side with the carburettor facing up and the exhaust side facing the floor. One easy way to tip a lawnmower is to lean the handle against the wall whilst tipped so it stays in position.

Have a look at the blade for any signs of obvious damage. Take look at the outside edges of the blade, there should be an even gap at both sides between the end of the blade and where the lawnmower deck starts.

If there isn’t then the blade is bent and will need replacing. Unfortunately, if this is the case you may have caused further damage to the lawnmower so be sure to read the remainder of this article before purchasing a new blade.

If the blade is bent you will no doubt have terrible vibration throughout the lawnmower as it is no longer balanced. I will discuss how to balance a lawnmower blade later in the article.

Check The Crankshaft Isn’t Bent

The easiest way to check if the crankshaft is bent is to remove the spark plug and slowly turn the blade.

Take a look at the blade bolt in the centre as you turn the blade. It should remain in a central position.

If it appears to move across or down as you turn the blade the crankshaft may be bent. It’s fairly unusual to bend the crankshaft on a modern petrol lawnmower as it’s fitted with a flywheel key to prevent this from happening.

You will also see a keyway on the bottom of the crankshaft if you remove the blade adapter. If the crankshaft is bent or either keyway is damaged you will get terrible vibration if the lawnmower does actually run at all.

Check The Blade Adapter Pins

Take a close look at how the blade is mounted onto the blade adapter. On a lot of lawnmowers, there are usually two holes located towards the centre of the blade. They should have what I refer to as pins sitting in the holes.

They usually sit level with the blade when everything is correct. Often I see lawnmowers where one pin or both have broken off and therefore the blade doesn’t sit straight against the blade adapter. The idea of these little pins is once again to avoid damaging the crankshaft.

They are designed to break first to protect the crankshaft. If the blade is not sat on the pins correctly and has become tightened at a different angle you will suffer vibration through the lawnmower.

Check The Flywheel Key

If the blade on your lawnmower has hit something hard such as a tree stump or drain cover etc there is a good chance the flywheel key or the keyway on the blade adapter has sheared.

To check the flywheel key you will need to remove the starter recoil cover and use a flywheel puller to remove the flywheel. You can find more information on how to do this in this article.

Petrol Lawnmower Wont Start-Help And Repair

If the flywheel key has any damage whatsoever, it will massively affect the timing of the lawnmower engine and you may experience vibration throughout the lawnmower. It is also worth checking the keyway on the bottom of the crankshaft.

To do this you will need to remove the blade and the blade adapter. You will see a small key that sits in the crankshaft, make sure this is in perfect condition also.

It is really important that both keys are in perfect condition to avoid any running and kickback problems and hopefully cure any vibration problems.

Take a look at this video on how to check a flywheel key.

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Check The Blade Is Balanced

A lot of lawnmowers have terrible vibration if the blade is balanced incorrectly. Sometimes the end of the blade may even be missing causing the blade to become unbalanced. To balance a  lawnmower blade you can use a blade balancing tool.

If necessary sharpen the blade while you have it removed and if you don’t have a cheap blade balancing tool then hang the blade on a nail to check that one side doesn’t hang lower than the other when balancing. This does make a difference to the vibration on a lawnmower so it’s highly recommended that you balance the blade whilst sharpening it.

Check The Valves Are Adjusted Correctly

Take a look at the front of your lawnmower and look for a cover that has OHV stamped on it.  If you see this it means your lawnmower has overhead valves on that can be adjusted. If the valves need adjusting usually you will have difficulty starting the lawnmower or kickback as the starter recoil rope is difficult to pull over.

If the valves need adjusting to the correct specifications then you may experience vibrating on your petrol lawnmower.

If you do not see the OHV cover anywhere then it’s likely your lawnmower has a flathead engine.

You may be able to check or replace the valves if necessary, however, this task is much more in-depth when compared to working with overhead valves. I would suggest purchasing a lawnmower in the future with overhead valves.

You can see a video on how to set the valves here

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Check The Engine Is Tightly Bolted To The Deck

I have seen on occasion lawnmower engines that have become loose from the lawnmower deck. This does not happen all too often but sometimes people swap lawnmower engines onto different lawnmower decks. Take a look at where the engine bolts to the lawnmower deck and ensure it’s tightly fastened.

Check The Lawnmower Handles

Often engine vibration can be mistaken for the actual handles of the lawnmower becoming loose. Over time the handles do become loose and often need re-tightening. See where the handle attaches to the actual lawnmower deck and make sure that everything is tight.

Also, have a look at the weld just below where the handles attach as from time to time this part of the deck can start to split and cause vibration.

Check The Exhaust

The exhaust on a gas lawnmower is usually covered over by an exhaust guard. Wait until the engine is completely cold and remove the exhaust cover. Check to see if the exhaust is bolted tightly to the engine.

It may sound odd to check the exhaust but if you are picking up lawnmowers from auctions or eBay sellers there is a good chance they have had a play about with all sorts of the lawnmower before selling it on. Believe me, I’ve seen all sorts! Check the exhaust is bolted on correctly and also the exhaust guard is securely fastened when replaced.

Final Thoughts

To avoid having vibration problems in the future take extra care not to hit anything hard with the blade of your lawnmower.

It is really easy to catch the blade on a drain cover or the edge of a pathway or tee stump for example. This is the main fault when it comes to vibration but don’t disregard all the other instances listed previously in this article.

Make sure both keyways are in perfect order as well and particularly sure that the blade is sat on the blade adapter correctly.

If the pins are broken on the blade adapter then replace it but be sure to buy the correct one. Blade adapters vary on whether your lawnmower has a belt for the self-drive, so be sure to choose the right one!

Check out my current recommendations for this year’s best lawnmower here

I really hope this helps anyone having difficulty with lawnmower vibration.

Happy Mowing


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