Originally Posted By: exranger06
Old oil would not cause a mower to not start. An engine needs 3 things to run: Fuel, spark, and compression. If you're missing any of those things, it won't run. Compression is most likely fine and your problem is almost certainly fuel or spark.
Testing for spark is easy. You can buy a cheap spark tester, or you can remove the spark plug, plug the wire onto it, and touch the base of the plug to the engine. Have someone crank the mower while you look at the plug. You should be able to see a spark when the engine turns over.
If you have spark, you have a fuel issue. Inspect the carburetor.
I don't think I can do much better than this, except to add that I would guess it has a spark and either the carb is clogged/plugged or the diaphragm is ripped or not sealing. The diaphragm is probably a 4 dollar part, the cleaning you can do on your own pretty easily. Modern Briggs are easy to work on and/or difficult to put back together incorrectly.