rough idle increase in neutral

have 82 380sl with slightly high idle 8oorpm at park.
in neutral after running the idle goes to 2000.
new plugs, wires, cleaned idle control valve
does this sound like a vacuum problem.
the vacuum lines are original and dry rotted.
will replacing them help?
when replacing do you have to use to small plastic tubing that was originally on the car or can you just use regular vacuum tubing all same width.
can't find the idle adjustment on the 82 380sl my 76 slc has a large plasic screw in the front to adjust.
where is the adjustment for the 82 380sl?
can't tell from the cd manual where it is can't seem to find it.