Can you answer some questions?

1. How much smoke is coming out of the pipe? Alot, a little, or none? Any change when you give it throttle? More smoke comes out?

2. How hot is the engine after the 10 or 15mins of running? If you spit on the head, does it sizzle right off, boil away after a few seconds, or just sit there?

3. What type of glow plug are you using?

Alot of things could be the problem, because as a used engine, you cant be sure how worn out it is. These things are very easy to damage if not run right. If the previous owner ran it too hot, he could have worn it out, and when the engine gets hot...the piston sleeve expands and creates blow by.