Small Engines (Lawn Mowers, etc.): 15hp Kohler runs for 20 min. then loses spark, hydraulic lifters, 10w30

Question
John,

 I have a z-42 Cub cadet model 53AC305G100 with a Kohler 15hp pro series, model number CV15S spec # 41535.  
 What happens is that the mower will run fine for 20 minutes then it dies.  It worked fine the first of the year but the last two times I have mowed it started doing this.  I have checked, it is not a fuel problem.  When I check the spark plug it has no spark.  I have tried changing the plug and get the same result.  If I let the mower set for an hour or so it starts right up and runs fine for another 15-20 minutes then dies again.  There seems to be a direct relation to how long I let it set and how long it runs.  The longer I let it set, the longer it runs.
 It seems to be heat related.  I am wondering if this could be a coil problem?  I have had someone say it could also be an exhaust valve problem?  How do I determine which it is and how involved is fixing an exhaust valve?

Thank You for your time,

Tony

Answer
Looking at the engine blowup, you have hydraulic lifters, and if
there was a valve problem, it likely wouldn't run at all, unless it is a loose seat, but after a couple of times, wouldn't start again.

Coil could be breaking down, but I see your system has an oil switch
which could be a big possibility.

If it is due, change the oil and filter, use 10w30 and make sure the oil level is at or slightly above the full mark, and see if the problem gets better.