Lean Spike with High Load

Hey everyone, I have a Holley 750 Double pumper with mechanical secondaries and i'm having a carb tuning problem.

I'f i'm cruising in 3rd gear, punch it, the car will downshift but will have an initial lean spike (up to 16-17 AFR) but will then settle back down to 12-13 AFR within a second or two. Also, it will backfire through the carb.

Is that something to worry about? I was thinking that maybe it needs a different power valve but I'm not sure if I would need to replace the one on the primary or secondary side (maybe both?)

This seems to be the only issue while driving as all other driving conditions yield a consistent and steady AFR of about 11-12 AFR. For example, from a dead stop, if I punch it all the way through the gears its totally fine.

I'm a bit of a newbie with it comes to carbs so any advice is appreciated. Thanks