Lawn Mower Kickback Help And Repair

There are various reasons that lawnmowers kickback. In this article, I’m going to share some of the common reasons and repairs I have done over the years while repairing lawnmowers for profit.

Usually, there is a simple reason your lawnmower tries to rip your arm off when attempting to start your lawnmower. The first of those is that the flywheel key has sheared, however, there are other reasons also. Let’s take a look at how to eliminate kickback and some of the reasons it happens.

Why Does My Petrol Lawnmower Kickback

Most petrol lawnmowers kickback because the flywheel key in the lawnmower crankshaft has sheared causing the engine to be mistimed. In short, the engine fires at the incorrect time and causes the pull cord to jerk back in your hand. This is called kickback. It can be eliminated by checking the crankshaft flywheel key and replacing it.

Lawnmower Pull Cord Jerks Back-Help

If your lawnmower pull cord jerks back when you attempt to start up the lawnmower you can almost certainly be sure that at some point the blade has hit a hard object such as a tree stump or perhaps the edge of a paving stone.

It really doesn’t take much to slightly throw the timing off a lawnmower. This happens when the flywheel is in the incorrect position.

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What Is A Lawn Mower Flywheel Key?

A flywheel key is a little rectangular piece of metal that sits at the top of the crankshaft on your petrol lawnmower. There is also a key at the bottom of the crankshaft where the blade adapter sits.

How Does A Lawnmower Flywheel Key Work?

The flywheel key is a small metal rectangular piece of metal that keeps the crankshaft and flywheel lined up when the flywheel is bolted to the engine. This is what sets the timing on the lawnmower.

Take a look at my video on how to replace a sheared flywheel key here

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The idea of a flywheel key is to save more expensive parts of the engine from being damaged, The flywheel key is designed to shear if the mower blade hits an object like a paving stone or tree stump hard enough to move the flywheel into an incorrect position with the crankshaft. It is there to prevent the crankshaft from becoming bent.

If the flywheel key breaks or becomes sheared then your lawnmower will have poor timing or perhaps not start at all. Chances are if you try to start the lawnmower with a sheared flywheel key you will experience kickback.

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Where Is The Flywheel Key?

The flywheel key is located underneath the flywheel. It is not visible fully until the flywheel itself is fully removed.

You can, however, remove the starter recoil and inspect the flywheel key from above. It should look perfectly in line with the crankshaft.

To inspect it properly, however, it’s essential to remove the flywheel and inspect the whole part as even the slightest mark in the flywheel key can massively alter the timing and cause the lawnmower to kickback.

How Difficult Is It To Replace A Lawnmower Flywheel Key?

It’s very simple to replace the flywheel key itself, however getting access to be able to remove it can be a little time consuming and frustrating if you don’t have the correct equipment. You will need to remove the flywheel on the lawnmower first so let’s take a look at how to do that.

How To Access The Lawnmower Flywheel Key?

To access the flywheel key you will first need to remove the spark plug. This should always be the first stage of any lawnmower repair. Now you will need to remove the starter recoil which is usually straightforward as it’s normally a case of just removing 3 bolts and lifting the recoil cover off.

Next, if the lawnmower has a brake on it that connects to the flywheel use the lever at the top of the handle to move it away from the flywheel.

Note be absolutely sure to completely remove the spark plug first
What we now need to do is completely remove the flywheel.

Start by undoing the nut on top of the flywheel. If you have an impact tool then simply remove it using that. If you don’t have an impact tool you will need to wedge a piece of wood under the blade to stop the flywheel moving when you attempt to undo the nut.

I used to ask my local car garage occasionally to do this for me as it took seconds with the right tools.

With the nut loosened take great care to protect the top of the crankshaft as it’s threaded. You do not want to be hitting any part of this with a hammer!!

You will need a flywheel puller to take off the flywheel the correct way.

This tool screws into the holes on top of the flywheel and pulls the flywheel up and free from the crankshaft. If you don’t want to buy a flywheel puller I would (but shouldn’t) suggest taking a heavy rubber mallet and tapping around the flywheel and underneath to loosen it and remove it.

Although this is not the best idea I have seen it done on many occasions. With the flywheel removed you will see the top of the crankshaft and a little cut out are where the flywheel key sits.

This job is made much easier with an impact tool, here’s the one I use.

Is The Flywheel Key Sheared Or Damaged?

Take a very good luck at this tiny little flywheel key. This part is important. Any small line in the flywheel key or any broken pieces will massively affect the timing of your lawnmower.

The flywheel key sets the exact position of the actual flywheel. I this is slightly out of alignment the magnets on the flywheel will work in conjunction with the ignition coil and will spark at the wrong time causing your lawnmower to kickback.

Where To Buy A New Flywheel Key?

You can purchase new flywheel keys in the online parts store here https://repairlawnmowersforprofit.com/recommended-products/

Other Reasons For Lawnmower Kickback

I have picked up a lot of lawnmowers to repair for profit from action centres that have terrible kick back, even to the point where I have seen people break the starter recoil.

If you have checked the flywheel key on the crankshaft and it looks fine then you may need to adjust the valves on your petrol lawnmower.

This makes a huge difference. If you set up the valves correctly the lawnmower will pull over much more easily and it will be running with the correct timing, assuming the crankshaft flywheel key is in perfect condition.

Adjusting The Valves On An Overhead Valve Engine -OHV

Take a look at the lawnmower from the front. If your lawnmower has overhead valves you will see a cover (usually silver) stamped with the marking OHV. This is where you can set the valves. I will cover how to do this in a future article but for now, just take a look on my youtube channel and you will see some videos on exactly how to do it.

Adjusting The Valves On A Flathead Engine

If you don’t see the OHV marked on the engine then it is likely to be a flathead engine. These valves can’t be adjusted as such but you can still check the gap using a feeler gauge and place them if worn.

This is a hassle as sometimes the valve stems will be worn but the valve seat is worn and you get an incorrect clearance on the valves. This is something to bear in mind when purchasing a new lawnmower. I would advise purchasing a lawnmower with overhead valves (OHV) that you can set yourself.

Take a look at this video for a demonstration on how to adjust the valves on an (OHV) engine.

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How To Replace The Flywheel Key

Replacing the flywheel key is simple. Just swap the old one for new and slide it down the crankshaft. Replace the flywheel and line everything up. The flywheel should line up evenly with the flywheel key evenly when re-assembled.

You will then need to re-tighten the nut to hold the flywheel in place. The easiest way to do this is with an impact wrench. If you don’t have one then wedge a piece of wood underneath the blade to stop it turning and re-tighten the nut. You will then need to replace the starter recoil and spark plug.

Check The Crankshaft Isn’t Bent.

One last thing to check is that the crankshaft isn’t bent. Remove the spark plug and tip the lawnmower carburettor side up. Take a look at the ends of the blade and make sure there is an even gap.

Take a look at this playlist of videos, it will help you resolve almost any problems you have.

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Things To Look For When Purchasing A New Lawnmower

If you are looking for a lawnmower that you will be able to service when the valves wear then you need to look for one with overhead valves. This way your lawnmower is easy to adjust and should last a lot longer than a flat head valve engine.

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Happy Mowing.