My batavus with bosch points setup did this to me for a while. I realized later that the flywheel was all messed up. Could have been my fault, not sure actually, but the flywheel had a crack. Like in the keyway, along the length of the keyway. Hope that makes sense. But when I would adjust the timing it was like nothing would happen. I think it was because as the flywheel would spin that crack would open up, and no matter what I did it would stay super advanced.