Small Engines (Lawn Mowers, etc.): Kohler M18QS needs choke to run, or dies after short run, air mixture, limp home

Question
My Power King 2418 tractor's Kohler M18QS engine has a Walbro carb.  
Recently, the engine began refusing to run unless the choke was fully
on.  Following suggestions here, I have stripped, cleaned, and
reassembled the carb 3 times, including a rebuild kit and removal/
cleaning/replacement of welch plugs.  Now, starting is iffy, but if I can
get it running with the choke off (starting with low throttle setting seems
to help)  it will run flawlessly for 15-30 minutes, then began slowly
dying.  I can usually get it to limp home with a few choke-on sessions,
but it seems to grow weaker and weaker, dying out a few seconds
sooner each time.  I did check the fuel pump by removing it and
manually operating the cam lever, which squirts out fuel.  I suspect a
heat-related problem in addition to any unresolved carb issues, but I'm
baffled.  Any suggestions?

Answer
Hello Marc:

1- Check the Gas Tank Cap. Ensure it is Venting the Tank Properly. 2- Does this Carburetor have the Air Mixture Screws? If Yes, Let me Know and I can Send the Instructions for Setting these, 3- Does this Carburetor have the Gas Solenoid? If Yes, this can be Heating Up and Restricting the Gas FLow to the Carburetor Jet. 4- Check the Engine Compression. The Valves May Require Adjusting. If the Valve Clearence is Too Close, when the Engine Get Hot the Valves Leak the Compression and Cause this Problem. I'm Thinking the Gas Cap or the Carburetor. Hope this Helps. I am Here if You Require More Assistance. Let Me Know What Happens, Please. Thanks.

Good Luck

Respectfully  

John