Ugh...those Encarwi carbs are just the worst....best thing I ever did to my M56 was put a SHA 15.15 on it (treats has them, you'd need the different intake too).
Running a fuel filter? How rusty is the tank? You could be pulling crap into the carb every time you clean it. Either way I always recommend a fuel filter, no matter the state of the tank, keep that crap out of your carb/jug.
Check for air leaks?