Sorry,I didn't read your post carefully.So you mean it leaks from the border between carb. and crankcase?If so,I guess the lower o-ring was broken.There are two o-rings on the carb.'s bottom.You can get the o-ring set from CRC.
Don't screw the fixing nut too tight!It's only fix the carb. But it dosen't have ability to seal the fuel.If you screw it too tight,you may damage the carb.
If you still have leaking problem after any try,please mail me : firstname.lastname@example.org