What way is the regulator diaphragm oriented ? This is the one that has the metal disk that covers the fuel chamber (needle and seat). The disk should be facing the needle. Verify that the needle arm is parallel with the carb body. Did you bend the arm at any point in time ? Make sure the return line is clear all the way to the fuel tank. Somewhere there is a restrictor in the return system, but I'm not remembering where exactly it is. Could be in the carbs or in line. All that really matters right now is there is a path for excess fuel back to the tank. Did you by chance verify the needle pop off pressure ? Are you sure you are adjusting the mixture screws correctly (ie, the low speed screw is typically the one closest to the base of the carb)? Did you by chance get the mixture screws mixed up (IE, rich and low, they are not the same) ? When you changed the tank vent, did you install it correctly (not a carb related question) ?
Seeing as it seems to run like a stripped ass ape wide open, I'm ruling out ignition problems. On a side not, have you verified that the power valves are operating correctly ?
Trust me, I know how frustrated you are, but you can do this. Think logically. On a carbed automotive engine that was running that rich, what would be the first thing you would look at ? Principles are still the same, they have just added a couple of extra components to the Walbro style carb system on pwc's. It's just a over grown version of the carb on your trimmer.