Reminder about White Rotary Tension
Just a reminder from manual: "The tension cannot be regulated when the lifter is up because the Releaser is operated by the presser bar lifter" Instructions for use and care of the No. 49 Rotary Sewing Machine Edition 4-2-37. (I have found true also on my 1912 White Rotary treadle).
Translated: Have your foot down when adjusting the tension.