My car did the same thing with not wanting to idle back down after letting off the gas. What worked for me was lubricating the lever on each carb. Also, the nut that holds the lever on each carb was too tight, I loosened it and then lightly tightened it back. Fixed that problem.

When choking the idle speed should go up some, but your cable sounds like it needs adjusting if a barely touching the choke causes that big of a difference. Take off your air filters and look down in carbs as someone gradually applies the choke. The butterflies in there should be pretty close to vertical with no choke, and as choke is applied gradually close to a horizontal position.

The choke cable is seriously sticking (age I assume), so I will try this!

Thanks