I would not drill anything in there . The air cut valve is designed to stop a lean condition and backfiring when the throttle is closed abruptly. Some guys on the DOHC forum block that circuit off when rebuilding if the diaphragms are worn out, since they are expensive when you need to replace 4 pcs.
There should be two holes in that circuit the one in the outer body that is represented by the small hole on the outside. The second is possibly in the centre of the cutoff where the diaphragm metal post sits, or in the internal outer part of the diaphragm area.
I would continue to use a cleaning product to clear the passages and if you have two clear passages from the air cutoff area then you are good. Blow air into the air cutoff hole and,if your getting air into the carb then you are good.
That carb is still pretty dirty especially in the air cut area. That needs to be cleaned up nice and clean internally.
That brass pipe may be a dead end with another small hole exiting up or down and not straight back as you would think is does.
