case-poclain excavator purchase

[DrRick]

I may buy a 1989 case-poclain 170c excavator w/ a deutz engine. I am concerned that the engine smokes at startup and under use. It is a black smoke, not blue. The machine sounds good and digs OK, When I move the machine in low speed it turns and moves great. In high speed it will move forward or back, but will stop if I remove either left or right pedals. Is this a condition of weak hydraulics? Or is it considered normal at lower rpm's. I can't find much on these machines on the net which makes me concerned.
r. roberts