just my thoughts:
1) You are aware of how to use the choke correctly ? (Silly question, sorry for that ;))
2) Engine off, ignition cover removed: Check the timing advancer of the ignition. Does it move freely approx 15° when turned by hand ? Turn it from stop to stop. Does it stick in any of these positions ?
3) Check tightness of intake boots (false air intake?). Turn engine on and let it idle. Spray some “starter spray” or just simple brake cleaner spray onto the intake boots (between engine and carbs). Are the revs still constant or are they increasing rapidly ?
This test will work at any engine condition: running on choke or not, engine cold or hot, doesn’t matter.
4) Have you tried to run the bike with the fuelcock in “PRI” position ?
5) Have you tried to adjust the “idle screw” ? Does that help or is it getting even more worse ?
6) Is your Twin in regular use or may there be an issue with the carbs ?
Dirt, debris, resin ?
It doesn’t sound like a typical carb issue to me.
7) But it might be an carb issue !
If there is resin in the carbs (typical after long time of “parked standby”), the idle system is the first victim.
Your bike may start with the help of the choke to enrich the fuel/air mixture. Idle system is blocked, so idle speed is set to run on main system (5.000rpm). As long as the engine is cold and you run it with choke it may work a kind of “correctly”.
As engine warms up and the choke is disabled, the drama as you described it starts. May run at high revs, but stalls below a certain “rev number” and can’t be rescued by pulling the throttle cable beyond that point. That would be typical for a blocked idle system.
Might be the culprit ?
Looking forward to your answers.