I had a Big twin that did that really bad. I think it was a 1955 25 if I remember correctly. I had come to the conclusion (at the time) that it was the oil that the customer was using. I remember changing to a different oil and that helped it clear up some. It was so bad, I was convinced something was wrong, till i tried a different tank with different oil and fuel mix. It did not affect the operation of the motor.