Our lawn mower was running ROUGH! Not a little rough, but dislocate-your-shoulder-trying-to-start-it rough. Eventually we could usually get it started, but would most likely get injured in the process.
Starting: We would yank the cord furiously trying to make it start and would have little luck, and every once in a while it would just lock and the cord wouldn’t pull like you’d expect. Ouch. With some persistence it would start though.
Running: It ran rough. It seemed louder than when we had first bought it. There was an occasional clanking sound like metal hitting metal. Also, It couldn’t handle thick grass like it used to. It would get bogged down and stall really easily.
Cause: We suspected at least 2 footballs had been run over as well as a tree root (possibly more than one).
We really were not sure what to do about this. We thought we’d have to go buy a new mower. We really didn’t want to since this one was only a couple years old to begin with. Fortunately, doing some research online led us to other people with similar problems and an easy (and cheap) solution!
It turned out the problem was a tiny little piece of aluminum (or something soft like that) called a flywheel key. We went to a local small engine shop and bought a 2 pack of flywheel keys for $2. Put one in and the engine was running MUCH, MUCH better!
The video below is a good explanation of how to get at and replace the flywheel key. My only comment is that before you take a hammer to your mower and risk damage. Try just grabbing your flywheel on opposite sites and giving it a pull. Mine came right off without the need to bash it. If that doesn’t work, then give his hammer trick a whirl. His is better (less damaging) than others I’ve seen.
Since we had the mower mostly apart we actually decided to go a little further and we cleaned out the cylinder head and muffler with carb cleaner. This is because we had been getting some black smoke when starting the engine sometimes. We suspect that this is a result of running our engine rough for so long (about a year!)
All said and done we fixed our problem for $2 dollars and went the extra mile for the cost of half a can of carb cleaner.