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!!.
A lawnmower engine commonly kicks back when the shear key breaks. The shear key commonly breaks because the blade has kit a solid object and come to a sudden stop. Common symptoms of broken shear key 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, don’t worry, I’ll keep it short.
Timing maintains 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 vital for a few reasons. When the 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 techniques 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, torquing the flywheel, and setting the armature air gap.
Kickback – 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 two-stroke engines. 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.
The 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 past Top Dead Center (TDC) and starting to turn back down the cylinder as soon as the piston is past TDC, the 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 – Rockers open and close valves as the engine moves through the cycle.
When Engine Timing is Out
So when the timing is out, lots of things don’t happen when they’re supposed to. Valves don’t open or aren’t closed. But most importantly, the plug doesn’t fire at the correct time. The engine is designed to run clockwise and fire the plug only when the piston is past TDC (third stroke power).
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 not to mention your arm.
Firing too soon – When the timing is off, it means the plug is firing outside its narrow window of operation.
It’s firing too soon and that causes the piston to reverse, causing the crank to turn counterclockwise and snap the pull cord from your hand as it does so, Ouch!
What Is A Shear Key?
The Shear key is a small block of aluminum, so-called a key because it fits in a slot (keyway) cut from both the crankshaft and flywheel (see pic). The whole point of a shear key is to shear (break) when needed. Sounds odd I know.
Keyway – Crankshaft and flywheel keyway
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.
Shear key – New and damaged
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 straightforward. However, you’ll need a socket set, torque wrench, and a flywheel puller. While it is possible to remove the flywheel without the pullers, it risks damaging the engine and isn’t always successful. In any event, a puller isn’t expensive, and you can find a link to a quality Briggs & Stratton puller here on the “Small engine tool page” or check out the Amazon link below.
Here’s the process:
Wire Off – Remove the plug wire to disable the mower and turn off fuel. Remove plastic engine cover, couple of screws usually.
Remove – Remove engine cover and blower housing assembly.
Paint – Mark the nut in relation to the crankshaft. This lets you know how far to tighten if you’re not going to use a torque wrench. (A torque wrench is preferred)
Block – Use a block of wood to wedge against the blade to prevent the engine from turning as you loosen the flywheel nut.
Alternatively use an impact tool or use a piston stop tool, you’ll find all these tools on the “Small engine tools page”.
Loosen – Loosen the flywheel nut, and remove, using a socket set.
Check – At this point, you can confirm a shear key is your problem, the key ways don’t align. However, I’ve worked on mowers with kickback and found the shear key to be good. This, unfortunately, means that the crankshaft has twisted, it’s rare. Replacing a crankshaft may not be economically viable for some mowers.
Spray the keyway with WD40 and let it soak while you remove the coil.
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. It’s all covered here, check out “Fitting a Coil”.
Pullers – Time to use the pullers. The pullers thread into the flywheel, however, some flywheels aren’t tapped, meaning an extra step is required to tap threads into the flywheel. A pain? Yes, but I have you covered with the “Shear key video”.
Tighten – Now go ahead and tighten the pullers, give the puller bolt a tap with a hammer if it’s not playing ball.
Remove – Remove the flywheel and remove the old shear key debris from the keyway. Check also the keyway on the engine.
Fit – Align flywheel and crank keyways, now push the new shear key into the keyway. Nice!
Tighten – Tighten nut to your mark. Or use torque wrench. You’ll need to swap the block of wood the other way to prevent the crank from spinning while tightening.
Blade – Check your blade and blade boss (blade mount) for damage. Make sure it’s secure. A common symptom of a bent blade is excessive vibration. Check out the blade replacing video here.
That’s it. Nice Work!
Will a lawnmower start without a blade? Most mowers won’t start without the blade. The engine requires the force of the blade turning (inertia) in order to run as designed.
Lawnmower 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.
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.