Yours is the magneto model, Correct? Years ago I had a similar problem with my 1961 Starflite, but mine is the 12 volt distributer version. After much “chasing” the issue, it turned out that the cog belt sprocket on the distributor shaft was loose and the key sloppy. So where as all the timing looked OK to the marks, in fact the distributer point cam was retarded. I don’t know if the same condition could exist on the magneto model.
Joe B