Engine Kicks Back When Starting

Engine Kicks Back When Starting

Engine Kicks Back When Starting

When you experience kickback you won't mistake it for anything else. Kickback is when you pull start your mower and the pull cord is snapped from your hand. It can sometimes whip you as it does so, which is Oouch!!.

Engine Kicks Back When Starting

Why does the mower engine kickback when starting? Lawn mower engine kicks back because the blade has hit something solid which caused the engine to come to a sudden stop and in turn breaking the flywheel key.

Symptoms include:

  • Engine won't start
  • Pull cord snapping back
  • Engine backfiring
  • Oil leaks
  • Broken pull cord / handle
  • Loose / damaged blade

Before I describe the repair, it may be helpful to know a little about engine timing. Timing is maintaining a mechanical set relationship between internal engine components, and firing the spark plug at the optimum time. When the timing is out/off you run the risk of severely damaging the engine.


Timing is important for a few reasons, when timing is out a lot, the mower won't start. If the timing is out a little, your mower may run, but run rough.


A broken shear-key is as you now know the most likely cause of engine kickback. The repair procedure isn't complex but a few procedures need to be followed to nail this repair successfully.

If you need video help, check out "Shear-key replacing video". It walks you through the whole process step by step, from checking the shear-key, tapping & removing the flywheel, fitting the shear-key, torqueing the flywheel and setting the armature air gap.




The recoiling of the pull cord stings a lot.

What Is A Four Stroke Engine?

Most lawn mowers are fitted with a four stroke engine, and like all engines, they have a set sequence of operation. The main components involved with timing the engine are: crankshaft; piston; flywheel; camshaft; valves; coil; spark plug.


Four stroke engines are more reliable, live longer, quieter and cleaner than a two stroke engine. So called four stroke, as it has four distinct stages in a complete cycle.

How does four stroke Work?

First Induction - piston travels down the cylinder and draws air/fuel mix in through the open inlet valve.

Second Compression - the piston starts to travel back up the cylinder and closing the inlet valve, creates a sealed, air/fuel compressed cylinder. 


Third Power - The piston is now at the top dead center (TDC) and starting to turn back down the cylinder. As soon as the piston is past TDC, spark plug fires which sends the piston down the cylinder under power. 


Forth Exhaust - The piston turns back up the cylinder, the exhaust valve opens, expelling the spent gases out the muffler and the sequence starts over.  

mower engine valves


Valves open and close as engine moves through the cycle.

What Is Engine Timing?

So when the timing is out, lots of things don't happen when they're supposed to, like valves don't open or aren't closed. But the most important one, the plug doesn't fire at the correct time. 


The engine is designed to run clockwise, and to fire the plug only when the piston is past TDC. Firing the plug before TDC causes the piston to try and go back down the cylinder, working against clockwise crankshaft momentum.


That's when the pull cord is snapped from your hand. This has the potential to bend or break internal engine components.

mower carburetor


When the timing is off, it means the plug is firing outside its narrow window of operation.

What Is A Shear key?

The Shear key is a small block of aluminum, so called a key, because it fits in a slot (key way) cut from both the crank shaft and flywheel (see pic). The whole point of a shear key is to shear (break) when needed. Sounds odd I know.


When a mower blade, attached to the bottom of the crankshaft, hits something solid, the engine stops dead. The flywheel attached to the top of the crankshaft has mass and inertia will force it to turn, it's at this point the shear key does its job and uncouples the flywheel from the crankshaft by shearing.


If the shear key wasn't fitted, the crankshaft would most likely twist, this puts the timing out and the only fix is to replace the whole crankshaft. This isn't a small job and really the engine is junk.


How To Replace The Shear Key

Now the good news, the flywheel shear key is probably the cheapest part you can buy for your mower. The fact the key has sheared means your engine has been saved, of course the timing will need to be reset and fitting a new shear key will sort this.


Fitting is mostly straight forward, however you'll need a socket set and a flywheel pullers would be great. It's possible to remove the flywheel without the pullers, but you must use caution as the engine casing is a soft metal.


If you prefer to have the right tool for the right job, then go ahead and rent, buy or borrow one. I have the Briggs and Stratton pullers, which you can buy on Amazon, but you can also use a universal puller set that will pull any flywheel.

mower engine mower cover

Wire Off

Remove the plug wire to disable the mower and turn off fuel.

Remove plastic engine cover, couple of screws usually.

mower engine


Remove pull cord assembly. Two bolts at rear and one at front.

No need to remove the dip stick on this model, it comes off with the assembly.

Mower flywheel


Mark the nut in relation to the crankshaft, this lets you know how much to tighten, in reassembly.

Mower on its side


Use a block of wood to wedge against blade to prevent engine from turning as you loosen flywheel nut.
Mower flywheel Mower flywheel


Loosen the flywheel nut, and remove, using a socket set.

At this point you will be able to confirm that the shear key is your problem, the key ways don't align. 

However, I have worked on mowers with kickback and found the shear key to be good. This unfortunately means that the crankshaft has twisted, this is rare.

Replacing a crankshaft may not be economically viable for some mowers. 

Spray the key way with WD40 and let it soak a while.

Mower flywheel


Remove the coil by removing two bolts. Set the coil to one side no need to remove the wire connector.

When reassembling, an air gap must be maintained. Check out "Fitting a Coil"

mower flywheel mower flywheel


Ideally, use a specialized tool called a pullers. I am going to assume you don't have, so try this, but use caution as you can damage the engine.

Ask a helper to apply the bail lever or use zip tie or tape. Put the flywheel nut back on by hand and back it off 1 turn.

Using two hammers and some safety goggles, place one hammer on the flywheel nut and strike it sharply with the other, once is enough, don't over do it.

mower flywheel


Now go ahead and remove the nut.

Striking it with the hammer helps loosen its grip. If this hack doesn't work, you'll need to invest in a flywheel puller set. You can check them out here.

Mower flywheel Mower flywheel


Remove the flywheel and remove the old shear key from the key way. Check also the key way on the engine.

If you can't get the flywheel to move, don't worry it happens.

Go ahead and buy, rent or borrow a flywheel pullers, it makes this job look easy.

mower shear key mower key

Shear Key

New and old shear key.

mower shear key


Fit new shear key by simply pushing the new key into the key way on the crankshaft.

mower flywheel

Push On

Align flywheel key way and push on.

Mower flywheel


Tighten nut to your mark.

Reassemble in reverse order and check out "Fitting a coil".

mower blade boss mower blade


Check your blade and blade boss for damage.

Make sure its secure. A symptom of a bent blade is excessive vibration.

That's it, Nice Work!

Related Questions

Will a lawn mower start without a blade? No, most mowers won't start without the blade. The force of the blade turning (inertia) has been fractured into the design of the mower engine.



Lawn mower string won't rewind? The pull cord recoil spring is worn or broken. A new spring can be fitted, often replacing a complete pull assembly is easier.



Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.