I'm giving you the idle adjustment procedure right from the Motronic manual titled "911 Carrera DME Testing Plan" this manual covers the Motronic system and howto fix it.
Here is the page on setting the base idle:
Please be aware that you CAN NOT alter the idle by setting the idle screw! The DME controlls the idle in the Carrera and the only way to change the idle is to alter the Chip. I know this for sure since I burn these chips often and my 84 Carrera idle is set to 920RPM because of the 20/21 Web Cams I had to increase the idle. I did this by re-programming the chip.
The 86 USA chip has it's idle pre-set in the chip to 800RPMs. The idea for seting the base idle with B&C jumpered is to get the base idle in the ballpark. I ussually set the base 25RPMs higher than the chip idle. So for your car dissconnect the O2 sensor (O2 MUST be disconnected) jumper the B&C pins and set the idle to 825RPM or so then pull the jumper and the DME will command the ICV to pull the idle down to 800RPM.
When you jumper the B&C pins it tells the DME to center or park the ICV at it's center point (50% opened). This insures that the ICV is half opened and has headroom to pull the idle down or up.
Many folks think you can somehow alter your idle by just adjusting the idle set screw but this is not possible! The idle set screw is only used to get the idle in the ballpark when the ICV is parked or centered.
Also, notice that the Procedure for base idle set should be done along with the CO setting but setting CO is a little more detailed and I'll assume you are OK here so just try to set the idle as described.
If you really want the details on a simple CO set procedure that can be done with just a Digital Volt meter and the existing O2 sensor I can post it later. You can get the CO in the ballpark with just a digital volt meter. But the best method is with a wideband O2 sensor, I have one of these installed in my car but they are expensive.
Hope this helps you better understand your idle.