from my experience with my mk iv super snipe carbys (have the same issue), i came to the conclusion that the leak might be coming form one or more of the mini-welsh plug type fittings in the lower body of the carb, i went through a few carbs & they all did the same thing... years ago i remember talking to a retired carby rebuilder, & apparently the super snipes (with that big carby) commonly did it... i was going to get mine rebuilt, i think maybe a carby rebuilder would need to drill out the old ones & put new ones in... not game to do it myself, & i never got around to getting mine done yet as life got in the way... i will get around to looking into it again one day... until then mine will have to sit in the shed, as it drips petrol onto the manifolds when driving, & costs a bit in petrol, not counting the possible flammable factor.... another common problem was that the screws did sometimes strip the thread in the body of the carb, making it leak around the thread, as the carby metal is soft.....