i have a weber also and i couldn't get it to idle smooth like yours. i only used the gaskets that were given and no silicone stuff. i finally realized the butterfly valves (i think that is what its called) weren't closing all the way and put a really stiff spring on the close lever. after that the engine ran smooth as butter and had real good power. just adding my experience with a weber.
I remember now. I had that problem with the throttle shaft being too tight so loosened it off a touch and put a second nut on it as a jamb nut and it was fixed.