Tuning an Edelbrock 650AVS on a stock 340

Agree, 9" of vac is too low. You need to find out why. Could be the cam itself, faulty in some way. Wrong cam put in box?
I would imagine with the low vac that the pri t/blades are cranked open too far exposing too much T-slot. Would also cause unstable idle. I would also expect that the met rod pistons are not staying DOWN at idle, causing more A/F instability.
The key is finding out why vac is so low & remedying that.

Acc pump shot. I have been working on these carbs since the 70s. Have yet to find an engine that needed the pump rod to be in any other hole other than the outer hole. That is because of the superior metering design.