Further research from several repair sites confirms that it is a very good idea not to put too much tension on the spring drum, specially with vintage machines.  Too much tension can lead to damage of the toothed carriage rack, since each movement of the carriage along it will knock the teeth too firmly, and may result in the loss of a tooth and consequent skipping - requiring a major, and perhaps impossible, repair. 

It is unecessary to have more tension than is needed to move the carriage successfully all the way from right to left.  This also produces a quieter machine, and makes the act of returning the carriage a little easier. 

Manufacturers often seemed to tension the carriage more strongly than was really necessary.  I don't know why that was, unless, perhaps, to allow for stretching of the original fibre draw-band over time.

Last edited by beak (30-4-2013 08:44:30)

Just jumped into my first drawstring repair.  This was helpful- thanks!  ~Tom~