Small Engines (Lawn Mowers, etc.): Honda engine backfiring, craftsman riding lawn mower, riding lawn mower

Question
QUESTION: Hi, I have a three year old Craftsman riding lawn mower with a 16.5 HP Honda Model GXV530 engine (Type EXA1).  It began backfiring, so I changed the spark plugs.  This seemed to fixed the problem, but after about 30 minutes it again started to backfire.  Each time it runs fine after start up, but in 20 to 30 minutes of mowing, the backfires return.  Any ideas?

ANSWER: Have the valves ever been adjusted?  Has the carburetor been cleaned?  How old it the mower?

Did you recently change fuel grades?
Let me know.
Eric

---------- FOLLOW-UP ----------

QUESTION: Eric, The mower is three years old.  The valves have never been adjusted. The carburetor has never been cleaned.  (I use the mower year around and average about two hours per week) I haven't changed fuel grades.  John

Answer
Are you familar with over-head valve engines?  I would first remove, disassemble and clean the carburetor with compressed air and carburetor cleaner.  After cleaning, see if it still backfires.  If so, I would check the valve clearances.
Are all the intake and carburetor bolts tight?  Are the gaskets in good shape?
I believe the engine is either running lean or the timing/valves are off causing the backfire.
Try the carb first, then the valves.
Let me know if the carb cleaning fixes the problem.
Eric