Might be an idea to ultra sonic clean them, whilst giving better economy is the cb550k3 carbs forte, they do block easier so I might think an ultra sonic bath would be a good start for them followed by getting fuel with redex in there a good few days before trying to start it. I just bough a commercial mower with a caked up carb, started off as a total none starter, everthing gummed up badly, that was running fairly well after jetting carb cleaner into the carb and redex and new fuel into the tank the afternoon it arrived and leaving it to stand overnight. It arrived with the throttle leavers stuck fast by gummy old fuel, even the choke needed freeing off with carb cleaner and a pry with a screwdriver.