loss of power on 300td

300TD 1985.

I have a problem with power loss and black smoke out of the tail pipe. After is move up to about 40mph the car will not accellerate. Once I hit 40 and give it all shes got the power immediatly is lost and black somke out the back. Some thing of a james bond trick.

I used to think it was the turbo but I have no idea.