Snowblower Pull Cord Snaps Back

Ouch!! Many of us have experienced the bite of a mistimed engine, and it stings… a lot!. While it may seem like the engine is out to get you, it’s just the engine's way of letting you know all is not as it should be. A few minutes from now, you’ll be wise!.

Snowblower pull cord commonly snaps back because the flywheel shear key has broken. The shear key helps maintain the flywheel crankshaft relationship. When the shear key breaks, the engine timing is off which causes the pull cord to snap back sharply. Replacing entails removing the flywheel and installing a new key.

In this post you’ll learn why your snowblower pull cord snaps back. You’ll learn how to replace the shear key and why it breaks.

Mower gas cap

Vented Gas Cap

What Is A Shear Key

A shear key is a small block of alloy used to locate the flywheel on the crankshaft. Both the crankshaft and the flywheel sport a machined keyway, the shear key fits neatly into the keyway and aligns both components.

It’s important that the crankshaft and flywheel are aligned correctly, any variation in their alignment will cause the timing of the engine to be off.

The shear key has a second important job though. Ok so we know it aligns the flywheel and crankshaft but it also has the task of sacrificing itself to protect the crankshaft.

The shear key is made from alloy and if called upon it will break and allow for flywheel crankshaft movement.

Mower gas tank

Winged & Threaded caps

Diagnosing

A pull cord that snaps back is a sure sign of a broken shear key. But to know for sure, we’ll need to remove the pull starter assembly and remove the flywheel nut.

Doing so allows us to view the keyway, and a misaligned key way is easy to spot.

Common symptoms include:

  • Pull cord hard to pull
  • Pull cord snaps back
  • No start engine
  • Broken pull cord handle
  • Oil leaks from engine
Vented mower gas caps

Vented gas caps

Checking Snowblower Shear key

In the workshop I use an impact wrench which makes flywheel nut removal easy. If you don’t have one, you’ll need a piston locking tool, old timers stuffed rope into the cylinder to lock the crank, but I wouldn’t advise it.

You can check out the piston locking tool I recommend here “Snowblower maintenance tools page”

The following steps assume you don’t have an impact wrench, if you have, great! Skip the piston tool steps.

You should note, a flywheel nut should be torqued to specification and that requires a torque wrench. You’ll find one here on the “Small engine repair tools”.

Removing Snowblower Flywheel Nut

  • Remove the pull start assembly
  • Remove the plug wire
  • Remove the spark plug
  • Fit the piston stop tool
  • Rotate the crankshaft counterclockwise until hits the stop
  • Using a ratchet and long reach socket loosen the flywheel nut (turn counterclockwise)
  • Remove the nut, washer and pull start receiver

The keyway and shear key are now visible. If the flywheel and crankshaft keyways don’t align, your shear key has indeed broken.

Replacing Snowblower Shear key

Replacing the shear key itself is easy, the hard part is accessing it. The flywheel can be stubborn to remove and will likely require a tool known as a flywheel puller. It’s not an expensive tool but it is, I’m afraid, essential

Prying or hammering on the flywheel is a no no, it will damage the flywheel and possibly the engine. You can check out the flywheel tool I recommend here on the “Small engine tools page”.

Here’s the stepped process, starting where we left off above (flywheel nut off). Note, your engine may be slightly different and so some steps may differ slightly.

Removing Snowblower Flywheel

  • Remove the armature fasteners and set armature (coil) aside Turn the flywheel clockwise until it hits the piston stop
  • Fit the puller tool (some models will require tapping the flywheel first)Using a ratchet and socket tighten the flywheel puller tool
  • Tighten a little and using a hammer tap on the top of the tool (repeat until flywheel comes loose)With the flywheel free, remove the pullers
  • Remove the remnants of the broken shear key from both the flywheel and the crankshaft keyways

Fitting & reassembly

  • Refit the flywheel and align keyways
  • Slide new shear key into the keyway
  • Fit pull start receiver washer and nut
  • Tighten nut to spec using torque wrench
  • Remove the piston stop tool

Fitting Armature

A special procedure is required to fit the armature, it’s not complex but it is important. A tool called a feeler gauge is required, and ideally the specification of the armature air gap should be referenced

The gauge isn’t essential and I’ll show you a mechanics hack that will do the job also. However if you want the gauge, you can find the feeler gauge here on the “Small engine tools page”.

The armature fitting process as follows:

  • Fit the armature in place
  • Fit the fasteners in place but don’t tighten just yet
  • Place the feeler gauge between the flywheel and the armature (check spec). Alternatively use a business card (hack)Push the armature towards the flywheel while tightening the fasteners
  • Remove the feeler gauge or business card

Now go ahead and rebuild in reverse order and get out there and clear that driveway! Good Job!

Share

Auto Technician and Writer at Lawnmowerfixed | 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.