Small Engines (Lawn Mowers, etc.): Kohler 141T wont rev after Hot, head bolts, engine throttle

Question
K141T powered air compressor. Friend complained engine very hard start and would sputter and not rev at all.

Engine has very low hours on it. I replaced pitted dirty points plug condenser. engine started right up..adjusted idle and hi speed. Everyting worked fine till engine got very warmed up. When the unloader released, the engine throttle advanced but engine would sputter and and puff black smoke. I noticed that the governor shaft was loose in the lever. I wasn;t able to tighten it enough so I removed it and manually ran the engine.. ran fine until it warmed up. Idles perfectly smoothly. even after warm. Any thoughts?  

Answer
Did you replace the spark plug?  Are the carb to engine intake gaskets in good shape?  Have you inspected the carburetor butterfly shaft for wear?

Have you re-torqued the head bolts?
These problems can be difficult to fix due to the age of the engine and numerous small parts that may be work.  Individually these parts appear fine but collectively they can cause some odd problems.
Has the engine ever had a valve job?
Let me know.
Eric