Vacuum leaks would make it lean, not rich. Try cleaning the plugs, removing the air filter, and going for a quick ride without it and see if they carbon up again. If not, your air cleaner's past it's use-by date. It's kinda hard to make them suddenly run rich, unless a jet's come loose or the airflow's restricted or something. Hmm you're not leaving the choke on or something stupid like that? Do our carbs have choke butterfly valves, and if so could one be stuck shut? It's not likely to be a stuck float valve, you'd notice fuel running out of the breather pipes on the bottom of the bike (unless the squirrel condoms are still on? Take them off, see if fuel runs out!).
