As Chuck said the settings of idle speed and mixture are somewhat interconnected, but you can do the following to set the throttle plate gap.

FIRSTLY, (this is important) make sure the throttle body venturi is cleaned out, it`s probably all gummed up with oil deposits.

Refering to the pic below, (this is a B200 but basically the same)

If you have a throttle switch slacken the screws "B"
Loosen the locknut and back off the throttle valve stop screw "A" to allow the plate to close FULLY.
If it doesn`t shut completely you may have to adjust the link "C" and/or the throttle cable.

Turn in screw "A" a until it touches the arm, then turn a further 1/4 to 1/2 turn and tighten the locknut.

Set the switch so that it clicks immediately the throttle starts to open.

You should now be able to set the idle speed within a reasonable range by turning the the knob "B".

Take the car to someone with a CO meter to get the mixture set.
The adjustment is VERY sensitve and if you have been messing around with it it will be way out of adjustment.

Hope this helps,
Colin.

1990 740SE B200E/M47, remote C/Locking.