Engine Kicks Back
when Starting

Engine Kicks Back
when Starting

Your Problem Will Likely Be Loose Battery Connections, Flat/Faulty Battery Or Faulty Starter Solenoid.

SYMPTOMS

01. Engine won't start

02. Pull Cord Whiping

03. Enging Backfiring

04. Oil Leaks

05. Broken Pull Cord/Handle

06. Loose/Damaged Blade

Most Common Cause

The likely reason for kickback - the flywheel key has sheared. This puts the timing out on the engine. And the reason the timing is out - the blade has hit something, causing the engine to come to a sudden stop.

 

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 painful.

 

 

 

What's Timing

Timing is maintaining a 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.

 

 

 

Mower

Kickback

The recoiling of the pull cord stings a lot.

Kickback


mower

The recoiling of the pull cord stings a lot.

4 Stroke Engine

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

 

 

 

How's it Work?

So called four stroke, as it has four distinct stages in a complete cycle.

 

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.  

 

 

 

 

 

Valves


Mower valves

Valves open and close as engine moves through the cycle.

mower engine valves

Valves

Valves open and close as engine moves through the cycle.

Timing Off

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

Firing

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

Firing


mower flywheel

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

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.

 

 

 

Replacing 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 prefare 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.

Remove

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 engine
Mower flywheel

Paint

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

Plug Wire Off


Mower plug wire Mower engine cover

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

Remove plastic engine cover, couple of screws usually.

Choke Off


Mower engine cover

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.

Paint


Mower flywheel

Your mower may not have a choke, it may instead have a priming bulb.  

It has the same end result as the choke plate - gives the engine extra gas for cold engine starts.

Block


mower blade

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

Mower on its side

Block

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

Loosen


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.

Loosen

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 Mower flywheel

Remove

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

Remove Coil


mower carburetor float and needle

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"

Pullers

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 mower flywheel

Pullers


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. 

Remove

Now go ahead and remove the nut.

Striking it with the hammer helps loosen its grip.


mower flywheel
mower gas off

Gently

Sometimes it will be necessary to use a long screwdriver to lever down on the engine. Lever from one side to the other to remove flywheel.

The word here is finesse, please be gentle.

Remove

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 flywheel Mower flywheel
mower shear key mower key

Shear Key

New and old shear key.

Fit

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


mower shear key
mower flywheel

Push On

Align flywheel key way and push on.

Tighten

Tighten nut to your mark.

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


Mower flywheel
mower blade boss mower blade

Blade

Check your blade and blade boss for damage.

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

Remove


Mower flywheel

Now go ahead and remove the nut. Striking it with the hammer helps loosen its grip.

Gently


mower flywheel

Sometimes it will be necessary to use a long screwdriver to lever down on the engine. Lever from one side to the other to remove flywheel.

The word here is finesse, please be gentle.

Remove 


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.

Shear key


mower shear key mower shear key

New and old shear key.

Fit


mower shear key

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

Push On


mower flywheel

Align flywheel key way and push on. 

Tighten


Mower flywheel

Tighten nut to your mark.

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

Check


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.