Small Engines (Lawn Mowers, etc.): Spark plug carbon covered, riding lawn mower, float bowl

Question
Eric,
My 5-year-old Craftsmen riding lawn mower with a 17 OHV B&S engine runs well – when it runs.  The problem started this year – after running the engine for 30 to 40 minutes the engines stops, the spark plug is covered with a crusty (carbon-like grayish color, not wet or oily) substance.  I pull the plug; wire brush clean the plug, reinstall and the engines runs for a while, then only runs at idle or backfires. The engine has good spark. Even with a clean plug, the engine would not start this last time, I removed the small cylinder, small wire connection to the bottom of this cylinder, (I think nikki carb) under the carb float bowl, work the plunger a bit – reinstalled and the engine ran great – for 40 minutes with the crusty spark plug problem recurring.  What is causing the problem of the crusty plug recurring and the “needs gas” problem?
Thanks Eric  

Answer
Is the engine consuming/burning oil?

If it's not burning oil then I would check all the gaskets in the intake system; something maybe leaking leaning the engine and allowing dust to enter causing the grayish carbon.

Has the engine ever had a major tune-up including a valve check and carb cleaning?
Let me know.
Eric