This is a dangerous condition and can easily cut your hand or worse, break a bone in it. Do not continue to try and pull start the lawn mower until you have it checked out by a lawn mower repair professional.
Now you are probably wondering why this happens in the first place right? Well, most of the time it happens because you have hit something with the blade or someone has been working on the engine internals and did not set the timing up right on the cam.
When you hit something or have a blade strike as it is called in the industry, it will cause the shear key in the flywheel to move or “shear” as it is designed to do. When this flywheel key shears, it causes the timing of the ignition firing to be off a number of degrees. When the spark plug fires at the wrong time, it will cause the piston to move violently in the downward motion and the pull cord is still trying to move the piston in the upward position.
The result is the force you feel when the pull cord is whipped out of your hand and the handle slams into the engine cover.
So it is wise to get your lawn mower checked out by a professional that can diagnose the problem and repair it so you have a safe to operate mower once again.
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