Kohler 12.5 HP engine (CH12.5) on my welder runs rough - hard to stay cranked. Cleaned carburetor & fuel lines, drained fuel tank & filled with fresh, clean fuel (ethanol-free), installed new spark plug. Battery is good. Still runs rough & won't stay cranked.

I am unsure by what you mean by "hard to stay cranked" can you explain further? As well as "won't stay cranked"