340 Edelbrock Carb Adjustment
Couldn't really tell you why it idles smooth, as i adjusted it and it idled best at that setting. It would seem too lean if it was 1/2 to 3/4 of a turn out. I think the general setting was screw the knobs all of the way in, then turn them both out three turns and adjust in 1/2 turn increments until it runs to your liking. It seems to work for me. I have close to the same setting on my 318 in my Duster and it runs well too. I don't see the smoke on it though and it is set straight out of the box too; Edelbrock 600. The idle is fine on it as well.
Believe me, they idle smooth, and I'm very particular about getting the motor to hit on all eight like it should, so i would notice any misfires. I put my ear right up to the tailpipe and listen when i tune. Lol.