My 12.5 HP vertical shaft B&S is running a Craftsman lawn tractor and is approximately 17 years old. I performed my usual spring maintenance on my push mower and tractor which is change the oil, replace the gas filter, air filter, replace spark plug,inspect the belts, and run each for 4-5 minutes with new gas. The tractor ran for the entire time without any malfunctions until I tried to engage the blades, after 100 ft (+/-) of mowing the engine stalled and would not start. After the engine stalls it feels as though the engine is seized although I can turn the engine by hand with difficulty, the electric start does not budge it. After I allow the engine to cool it will start and run again for about 5-6 minutes, just before it stalls there is a smoke exiting the carburetor from the crank vent located between the carburetor and the block. The engine was running fine when I put it up last season.

Thank You
OrdO

When the engine feels like it is seized, it's because it ended on the compression stroke. The valves are loose. Tighten them and it will end that problem

Thanks sounds good, it still feels seized even when I remove the spark plug?