Small Engines (Lawn Mowers, etc.): Engine wont turn over when hot, hp briggs, oil viscosity

Question
Engine is 16.5 HP Briggs Model 42A707, Type 2238-E1.  Item was picked up from salvage yard to replace the original unit that blew up.  

Engine rotates and starts easily when cold, but after running about 20 minutes and being shut off, the starter won't rotate the engine fast enough to start it.  Engine may be getting hotter than it should, it's hard to tell, but it has been run up to 30 minutes and has plenty of power and runs fine.  It does "lope" on part throttle conditions, and starts to shut down under very low throttle.

Unit in question has carburetor, magneto, and starter off original engine, which did not have this problem.  Battery is good.

Answer
Jerry

Possible causes:

  1. Magneto is not timed properly.
  2. Carburetor set too lean.
  3. Air intake or cooling fins are clogged.
  4. Shroud or blower housing missing.
  5. Excessive load.
  6. Insufficient or excessive oil.
  7. Improper oil viscosity (4 stroke) or mixture (2 stroke)
  8. Valve clearance is too small.
  9. Excessive carbon buildup in combustion chamber.