A “blade strike” as it is called can do some damage to the lawn mower, but engineers have built in some safety features that help contain that damage.
What happens during most blade strikes is when the blade contacts a rock or root, the impact causes the blade to stop and a shear key located on top of the engine to shear either partially or fully. This causes the ignition timing to fire incorrectly.
The result could be that the pull cord wants to rip out of your hand on a push or self-propelled lawn mower. If you are lucky, you will not injure your hand or shoulder!
The only way to tell if the flywheel key has been sheared is to remove the flywheel cover and in most cases, the flywheel as well. Then you can see if the square key has been partially sheared or is fully sheared.
Replacing the sheared flywheel is solves this problem. It must be the factory key as it is made of a material that will shear away and not solid steel that will damage parts if you hit something else.
If you need a professional lawn mower repair shop to troubleshoot and correct this problem, visit our Locations page for help.