I had a B&S single with exact symptoms mentioned. Adjusted valves for final resolution. Correct valve lash is paramount for compression release to function properly.
I didn't read all 7 pages of replies so I apologize if this has been offered: Back in the 70's I had a single cylinder Kohler motor with same symptoms as OP. Worked on it for several days - Final resolution was exhaust valve guide was coked up. Engine would run long enough to get hot, block...