could be associated with the fact that my back wheel and engine chain sproket aren't exactly in -line
I addressed this in a previous thread.
I think that I could have resolved the alignment with a "real" vise and a large crescent wrench.
Instead, as it was the last operation and horny to ride, I fired up the torch and bent the tension/idler about 7 degrees to be perpendicular with the rear axle.
Hope this will help,
DC