Ladies and Gentlemen: I'm pleased to report that the behaviour of
the roll motor in my Steck Half Duo-Art is much improved.  I followed
the suggestions of several MMDers, and made observations of actual
speed versus indicated speed at a range of vacuum levels and made
these tests several times, altering the tension in the motor governor
spring between tests.

I used Excel to plot the results, and this provided a clear view of
how things were improving (or not).  I also ran tests with the motor
running at a constant vacuum level (7" on a water manometer) to get
plots of indicated speed versus actual speed.

I believe the motor is now behaving as well as I can achieve.  It goes
too fast at the lowest speeds on the horizontal dial, and too slowly
at the highest speeds, but these speeds are rarely called for.  In the
middle, the speeds correspond to the dial quite well, and the motor
is almost independent of the vacuum level.

I obtained my vacuum using an old barrel vacuum cleaner running from
an autotransformer.  I'm aware that some people regard this as risky;
however, I've been doing it for years, and the cleaner has never
over-heated.

I believe that I've thanked all my respondents for their advice, but,
just to be sure, thank you again, MMD readers.

John Phillips
Hobart, Tasmania