1989 740 turbo At about 3000 rpm it start to spudder and black smoke comes out of tail pipe,
can someone please help me here?

 

sounds like clogs in th fuel system. Whether the injectors need to be cleaned or the throttle body is gunked up or the turbo needs to be serviced. I would start at the fuel injection and replpace the plugs,wirres, and check the voltage coming out of te coil. It cou not be giving enough current to give the sufficient amount of spark to combust the fuel at the plugs.

 

You may also consider testing the Air Mass Meter for signs of failure. Check all air induction conduits, that means all the hoses for holes or tears or excessive wear. I don't know for sure, but another item may be the Engine Coolant Temperature sensor, check this function or replace if you havent in a while. And of course make sure you have done a "stage 0"service interval.