Small Engines (Lawn Mowers, etc.): B&S 15.5HP backfires under load, riding mower, fuel flow

Question
I have a craftsmen riding mower with the engine type mentioned.  It starts fine, but wavers a bit for about the first 20 seconds it is running.  It smooths out fine, and then when it is warmed up and I engage the blades it starts to backfire constantly.  First thing I did was change out the gas.  That didn't work, so I checked the plug and it was blackened so I replaced it.  Still no luck.  The air filter is clean.  This thing was running fine just a few weeks ago.  I'm thinking the carb might need cleaning but don't want to do anything unnecessary.  Any advice?  Thanks for any help you can give.  

Answer
Hi AJ,

The backfiring might indicate a valve problem.  I would do a compression test.  You could test for a lack of fuel flow by squirting fuel or starting fluid into the carb to see if the engine's power picks up.

Let me know how you make out.
Michael