Exactly. More initial will allow you to need less throttle opening to idle, then your idle mixture screws should be more effective. BTW, when you say "shaking", you don't mean that you're developing a "miss" on certain cylinders, do you?........because since you mentioned that this has happened with 2 different carbs, it sounds a bit strange. Better check that out.......if you've got a lean condition, the spark may be "jumping out" somewhere before it gets to the plug.