Knowing that the throttle shafts on my HS6's were a bit sloppy and having trouble getting the mixture right, particularly on the rear carb (20 flats vs. 12 on the front), I tore them off thinking I might rebush them myself. (Rhys did my son's HIF6's and they are divine but my better half thinks food is more important than carb rebuilds. Go figure).
When I got them apart, it occurred to me that a temporary fix might be in order, something to get me through the summer. So I took 2 small elastic bands, each about 1" in diameter and wound one twice around each throttle shaft just behind the actuator yoke, the thing Volvo calls "lever on throttle flap spindle".
I then reinstalled the carbs and did a rough tune. I was surprised to find that I had much better tunability and had good balance and mixture with 12 - 13 flats on both carbs. I went for a spin and the car ran well and idled well, although the presence of the elastics kind of overpowered the throttle return springs and it took a minute or so for the idle to fall below 1000 rpm at a stoplight.
I share this experience strictly as a diagnostic tool. I certainly wouldn't recommend it as even a medium term solution to worn shafts/bushings. I figure based on the way the elastics deform under throttle, and with the under-hood heat, they might last a week.

Cheers,
--
Barry -- 1967 122S 'Betty'