Not Ranked
I don't think I would look at it as if it makes it richer. As I understand it they are calibrated expecting a certain range of fuel from each of the systems, idle, transfer and main. If the plates are open to far at idle then you are getting more fuel from the transfer system and less from the idle system than was intended. Creating an imbalance. With my some what limited knowledge, that is the best I can explain it. On my 351 I had that issue. What first tipped me off was after adjusting the idle mixture for best idle, the idle screws where only about 1/2 a turn out. Normally a good starting position is 1 1/2 to 2 turns out. This indicated to me that I was getting extra fuel from some where. In extreme cases you may even see fuel coming out of the mains at idle!
If you can, replace the secondary set screw with some type that can be adjusted with the carb on the car. Set the primary butterflys where they should be and from there on out adjust your idle speed with the secondary butterflys. Honestly, I don't know why they don't just come like this.
Rick
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