runs rough

Hi all - wondering if there are any of you who may have a good conceptual understanding of the elements in play for my engine problem. The problem is that the machine struggles to start and runs rough. A quick check of the plug confirms rich too dark and not tan colour. Today I went into it in...

Simplicity Regent 12 witha 12.5hp Briggs 289707 1 cylinder 4 stroke running incredibly rough, smoking, stuttering. On lower power settings it will smooth out for about 3-5 seconds then die, on high power settings it stutters and runs extremely rough, never smooths out. Has an integrated...