Another rough running issue

Looking for ideas with my rough running 912uls. Runs slightly rough at 2500-3000 rpm static. Carbs were just checked at Rotech Canada, balanced with carb mate spot on. The interesting thing is the plugs on number one cylinder replaced new get black after a short warm up and carb balance. All other plugs including number three cylinder on the same side are fine. The other thing that’s happening is on first startup the engine runs perfectly fine for about a minute before starting to stumble, almost like it’s loading up. Differential test and compression test are perfect as well. Mag drop normal. Currently getting the gearbox serviced as it’s overdue at 700 hrs. New throttle and carb cables all checked and good.