Broken Flywheel Key

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 figure 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.


Typical B&S - it would run, but very roughly.

<>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 shaft.

Flywheel key problems will make the engine run rough, pull the rope out of your hand, or not run at all.

Usually you can check it by removing the nut on the top of the flywheel and looking at the notch in the flywheel and the shaft.  Just a little offset can make it run rough, 10 degrees can make it snatch the rope out of your hands.

To repair it you have to get a new key and pull the flywheel off the shaft.  Sometimes that can get tough if it has smeared the (aluminum) key when it sheared.

You want to remove the sparkplug when you do that, because you want to compress the safety lever on the handle.  The lever connects to a brake that runs on the inside of the flywheel, and if that's on it makes flywheel removal difficult.  You want to remove the sparkplug so it doesn't accidentally start while you're doing this!

Pulling the flywheel can be a problem.  They sell inexpensive  pullers to do it, but I've always been successful by levering carefully underneath it while thumping on the engine shaft.  That system works good if you can get somebody to hold the mower off the ground by the flywheel while you're doing this.