Small Engines (Lawn Mowers, etc.): B&S 25 HP ELS -Suddenly runs rough and backfires, craftsman lawn tractor, adjustment screws

Question
Hello. I have a Craftsman lawn tractor with a B&S 25hp engine (ENGINE MODEL NUMBER 446777, TYPE NUMBER 0246-E1). I have kept the engine maintained with oil / filter changes and air filter changes. The engine has never given me any trouble until recently.  

The last time I used the mower it was running fine. A week later I cranked it up and it was running rough. Engine sputters a little was an occasional backfire. Some smoke (black). The gasoline I had added was fresh, oil looks good, air filter and pre-filter are clean. I drained the gas, replaced fuel filter, and added new gas. I have also checked the spark plugs - they were very, very cruddy with carbon buildup. Changed the plugs and still have the problem. Used carburetor cleaner and checked the choke and all linkage. Everything looks good. There are no adjustment screws. Engine does run but lacks enough power to drive and mow. After running, the plugs have dry black residue.

Any ideas?
Thanks in advance.


Answer
The engine is running too rich.  My guess would be the fuel pick-ups.  I would start by rebuilding the carburetor.  It will need to be thoroughly cleaned in a soak can or ultrasonically and a new carb kit used.    Let me know if you have any questions.  Thanks, PK.