Black smoke, high idle and other symptoms could indicate a rich condition. Is the fueling system stock? If yes, you need a set of CIS fuel pressure testing gauges to troubleshoot the fuel system. There is a device known as the WUR (warm up regulator) that is critical. Your 930 also has two high pressure fuel pumps, both of which must be working properly to deliver the correct volume and pressure of fuel to the CIS fuel distributor.
This is a good read on how CIS works and how to test it.
CIS Primer for the Porsche 911