I have carbureted big blocks and with the throttle levers at the idle stops the idle sometimes hovers around 1,000 when the engines are warm. When I manually push on the throttle arm (on the carburetor), I get an almost inperceptible movement of the arm (up against the idle speed screw adjuster) and the engines settle to about 850 rpms (perfect).
Therefore, I would like to put an ever so slight additional pressure on the arm (in the direction of the white arrow).
I am asking if I can unbolt the red cable connector and is that threaded so that I could "add" a mm or so of length...I believe this would give me a little more throttle lever throw to allow for complete throttle arm motion onto the idle screw stop.

If you're that close, it might only take a 1/2 turn or turn. I also usually run another spring to the big "opening" at the bottom of the throttle lever to one of the holes in my cable bracket. I would definitely check after your adjustment to make sure you are getting a complete butterfly OPEN when fully throttled.

I thought about an additional spring...thanks for the recommendation. The throttle levers seem to have a few mm of idle detent play...so I figured I would mechanically (with the screw adjustment) take some of that play out of the system.

does the red cable end fit tight on the bolt, meaning hole and bolt diameter the same

Standard procedure (for me at least) is to adjust cable so it just slides in place, then remove and give it one more turn to apply some pressure against the stop

Carb of EFI is the same

I did not notice any movement/slop in the cable to bolt interface. This is a professionally built turn-key motor set-up.

I like the way you think...that is essentially what I am thinking. I am pretty gentle/deliberate with my throttle movements so I do not anticipate putting much stress on the cable/connection.

Yup, adjust idle with engine fully warm and running and cable not attached to carb, then do above. This way the cable does not effect idle speed as it is with you. These cable are solid and don't flex, unlike most vehicles thin braided cable.

just curious because the washer doesnt seem to fit the bolt

Good point...I will check to see if the washer rattles around and will check for play...but I did not notice any slop in the movement of the connection.