I would first triple check the intake seals for leaking, and check the airhorn gasket at the airbox baseplate (since you said it was just re-built).
The static timing could be fine but the advance curve may be lacking (check with ECMDroid and/or your 'race' program).
Could just be lean in that spot. The AFV is a global correction and would't usually show that. Instead of doing the right thing and putting a Wide band O2 sensor on it, you could go to live data on ECMDroid and datalog the stock NB O2 voltages to see if its lean there.