when you say 'carb mixture screw' are you speaking of the set screw behind the small expansion plug on the side of the mixer?
Yes there is a small expansion plug you have to remove to get to the dwell set screw.

IF this is where you are trying to adjust and still cannot achieve your setting you may want to look at the mixer diaphram, make sure its clean and in good condition. Also make sure all your vacum lines are connected and in good condition. And as edward mentioned, that fuel valve is working properly, it should have a chatter or buzz when it is operating, it may start and stop the buzzing but when the engine is running it should be making that noise.