Before playing with the carbs, the ignition timing must be spot-on.
Are you using 'Triumph' thick O-rings? Not the skinny Amal ones. Do you have the correct washers, cups, etc for the mounts?
Use a thin smear of grease to hold the O-rings in place.
Although 1 1/2 turns out for the air/mixture screw is nominal, the set-up must be tuned.
*note the gap between the flanges, with the thick O-rings at the start of the video.
Tuning about 3 1/2 minutes into this: