When brand new spark plugs soon get sooty and fail to produce combustion in the cylinder, and the ignition appears to be strong with a properly timed fat healthy spark with brand new plugs, and the compression is acceptable along with proper valve clearances, and the carb is clean and properly jetted, remember to perform the clear tube test to determine the existing fuel level, because a too high fuel level can itself alone result in a quickly sooting and failing spark plug even though everything else is "perfect."
Good Fortune!