Well, this is a pretty common set-up in many motorcycle carbs. The main jet screws into the bottom of the needle jet and as long as that special washer is between them, the main jet holds the needle jet in the carb body. Without the washer, you could still screw the main jet up into the needle jet, but there would be nothing stopping the two from popping up and out of the carb body into the main bore. So, that washer is pretty important.