So you hit something and the mower will not start, or starts VERY hard, or maybe even jerks back and pulls the rope out of your hand. You need to check the blade and shaft and make certain that you haven't bent either of them, but a hidden fault is the flywheel key. They make the key out of soft metal so that it will sheer INSTEAD of the flywheel's inertia spinning your crank in half when you stop the blade suddenly, so "It's a good thing".
I'll check the key when I know I have a bent blade, or just can't
out why something will not start. You can often see if the key is
sheared by just removing the starter pulley on top of the flywheel, but
you may have to pull the flywheel to check.
<>Here's a Robin key that was broken when the mower hit a stump. That's the keyway to the right and the top of the key to the left. It had actually moved about 10 degrees and welded itself back to the flywheel shaft. The mower would start and run eventually, but was a REAL pain to get going!
Here's what a Tecumseh key looks like when it's about 15 degrees
off. Mower ran but very fast and erratic. The Techumseh key looks like
a big L, with the large part in the flywheel and the small part in the