The video appears to show the car sitting still, transmission in neutral.
The air valve is not opening too soon, it is not opening at all. This is correct - secondaries shouldn't be called upon to rev an engine in neutral!

The description of 'backing off on the throttle' matches a too lean situation. Inadequate pump shot is the first thing to look at AFTER setting the timing to spec. My advice is also check the timing advance to see if it matches the FSM. (Slight adjustment may be needed later on due to changes in fuel, but start with a known baseline. This all assumes a true resto, nothing hopped up)