You are not getting much help,most of the time low compression like Geoff said is the problem but injectors leaking can cause a problem with it smoking that bad take it to a dealer or run it hard for 50 miles see if it cleans up
I don't know if it will take 50 miles but I do agree with the idea. If this engine has been doing a lot of idleing/loafing along it may load up and smoke some. Take it out an run the snot out of it then observe the smoke and performance of the engine. This advice holds true for gas engines also. Dealing with old equipment and trucks I often see an engine which appears to be sick only to be cured by a good hard run. I just came off of a job where we had a towboat built in 1958 with 2 8V-71s for power. One engine was pretty fresh but still put of a good amount of smoke after ideling for a few hours. The worn out engine could put a smoke screen that 007 would be proud of. I also observed that the good engine cranked slowly and would be running in 1-2 revolutions no matter what the temperature was. The worn engine cranked pretty fast and ...... they bought either by the case. See how it behaves after a hard run.