OH crap I thought I saw in the video, you pulling the secondaries closed by hand. And my brain got stuck on Metering rod carb. Sorry Will
With this type of carb setting up the transfer port sync is critical for tip in response. I'm pretty sure you know this.
Idle the engine up to 2000 rpm or thereabouts. Reset your mixture screws for highest rpm. If they do nothing, idle it back down 200rpm and try again. After you get that set, idle the engine back down to normal.If she idles like crap now, it's cuz the transfers are not right.
Or you can just flip the carb over and look.
IMO, it may be that the 20* initial is causing the sync to be off , and the transfers are drying up at idle.Then when you tip the throttle in they take a few microseconds to get going again. I see that all the time. The pumpshot without the transfers is just not enough.
There is a proof for this and it is right there in the video. Several times you tip it in from curb idle, and there is the stumble. But a couple of times you tipped it in from some higher rpm with the throttle not yet returned to curb idle, and then the stumble was reduced.
So, maybe you can humor me; reset the timing to 12 degrees, make sure the secondaries are fully closed,idle her back up to 700, and reset the mixture screws. You may have to readjust the accelerator arm preload to initiate the shot as soon as tip-in occurs. Now try it.
Or you can just flip the carb over and look.