Small Engines (Lawn Mowers, etc.): hard warm start, valve job, riding mower

Question
I have a riding mower with a 16hp Kohler motor. I just tuned it up and rebuilt the carb and it runs great. The problem is sometimes after cutting the grass for a while, if I shut it off it has a hard time starting. It cranks good but I can't figure out if it needs gas or not, to choke or not. It just seems like I luck out when it starts. What could it be and how can I adjust it?

Answer
Because the problem seems to be related to heat, I suggest you first check for spark when it is difficult to start.  If you have no spark, you will have to troubleshoot the ignition system.  If the problem is related to compression (burnt valves, improper tappet clearance, etc), you will need to listen to engine as you crank it over.  If the valves are leaking you should be able to hear a "whisping" sound from the exhaust.  If this is the case, you will need to do a complete valve job and adjust the tappet clearance.  Let me know what you find and I can help further.  Hope this helps.