Small Engines (Lawn Mowers, etc.): jd stx38, fuel cap, fuel tank

Question
This mower will run beautiful for 20 minutes, then cut out and quit. (kohler 12.5hp) I check the spark after it quits and it jumps fine on my screwdriver.   I think the problem is in the fuel system? Leaving the fuel cap loose doesnt help either?  Any ideas?

Answer
 Hello John:

 There are 2 possible causes for your problem. 1-You are picking up Trash from the Fuel tank. Clean and Overhaul the Carb.  Clean the Fuel Tank. 2- The Valves need to be ReGround and seated. Hope this Helps.

 Good Luck

 Respectfully

 John