There are idle adjustment screw on Webers. They are kind of hidden but they are there. Look at the primary "crank" (?) that attaches to the linkage. You should be able to find a screw with a spring that contacts it. Usually not easy to get a screw driver on it.
Make sure you rotate the front and rear equally to maintain a smooth idle. I always use the ear check method to make sure they are sucking equal air.