I've never had a Volvo with an automatic tranny so I'm not much
help on that but I think the cable is misidentified. It is probably
also misinstalled because what it is probably supposed to do is to make
the transmission downshift when you tromp the gas pedal to the floor.
Normally they have enough slack in them that they won't work the other
way but if it is hanging up on the shift linkage or something, it could
certainly yank the throttles wide open. Have you looked at it?
Did the mechanic test drive it? By all means it should go back and be
made right or you should get a refund.

What I do know about is the carbs. Zenith Stromberg carbs CAN be adjusted
but it is difficult and it takes someone who really knows what he is doing,
as well as a special factory tool that presses the jet up and down in the
body of the carb.

The easy way out is to change to SU carbs. Either HS6 or HIF6 carbs will
work well, but the HIF6 comes closer to meeting any emissions requirement
that may exist. I've been very happy with the ones on my 69 164, which I
installed about a dozen years ago.
--
George Downs Bartlesville, Oklahoma