Small Engines (Lawn Mowers, etc.): JD 265 Kawasaki 17 hp motor

Question
Engine would run fine from cold about 45 mins then grind to a halt...after cooling repeated.  Finally wont start.  Electrical and plug are fine, fuel pump and carb fine and clean.  Engine when cranked now sputters white smoke wont start.  Think its valves...maybe you have an idea?

Answer
When were the valves last checked?

Did you replace the spark plug?

When was the oil last changed?  What oil are you using?

Did you download the engine service manual?
http://www.mymowerparts.com/pdf/Kawasaki-Service-and-Repair-Manuals/

Eric