I got my kit today and I have my 2.75 carbs on the work bench ready to disassemble. I thought I’d experiment with one of the spare carbs before I dove into the "good"one. However the good carb has something neither of the other two have. There appears to be some kind of valve perched on the rear top of the air horn . What does it do and do I really need it ? Remember this is a "74 carb and I am not a fanatic about originality. Bob