I'm going to have to tear it down again to find out why. This one did it when I bought it ober 10 years ago because the cylinder was out of round from wear but I had the cylinder rebored and I rebuilt the engine. This last time I took it apart to change out the tensioner and replace the headgasket and it started smoking like this but it didn't smoke at all before I took it apart so this is something I did I think.