Small Engines (Lawn Mowers, etc.): Engine backfire, briggs stratton engine, briggs stratton

Question
14.5HP Briggs & Stratton engine (Model 287707)on 42" cut Murray rider.  Engine begins to sputter after about 30 minutes run time.  Ocassional backfire with real fire out of muffler.  What's happening and what do I need to have fixed?

Answer
Ron

Problem: Engine backfires.
Possible causes:

Carburetor set too lean.
Magneto is not timed correctly.
Valves are sticking.

Problem: Engine operates erratically, surges, and runs unevenly.
Possible causes:

Fuel line or fuel tank cap vent is clogged.
There is water in the fuel.
Fuel pump is defective.
Governor is not set properly, sticking, or binding.
Carburetor needs adjustment or service.
Loose carburetor/intake pipe resulting in vacuum leak