Small Engines (Lawn Mowers, etc.): Snapper riding mower wont turn over, carborator, cable ends

Question
When I turn the key, it makes a sustained racing, buzzing noise--I assume in the starter--but the engine will not turn over.  I've replaced the spark plug, cleaned the air filter, and checked for clogs in the gas line.  There is gas in the carborator.  It just won't catch.  What next?

Answer
Have you had the battery tested with a load tester?
Have you removed and cleaned the battery cable ends and terminals with a wirebrush?  They may look clean but we fix a lot of starting problems by just cleaning the connections.

Will the starter turn the engine over with the spark plug removed?
Have you removed the starter cover and checked the plastic starter pinion gear?
Let me know.
Eric