My 411/3 sold some years ago did not have any half throttle kickdown response but when I decided to fit an Edelbrock Performer Carb and Manual choke set to the car and adjust all the linkages as the installation instructions suggested all was well and I had full and part throttle kickdown no expensive mods and I remember reading years ago that Bristol had set up the linkages on their cars of this period to exclude the half or part throttle kick down both of which operate on my 603.
This video which I have just come across may offer some insight into the issue or not as I got distracted when viewing it but but decided to post the link anyway as we may all learn something.
Geoff.
https://www.motortrend.com/how-to/th...-down-linkage/