[Mgs] manifold vacuum

[Paul Hunt]

Sounds about right, it depends on the damping as to how much flutter you get.
My Redex gauge has a damper screw on the pipe on the back of the gauge to
compress the tube and eliminate the fluttering for driving, you open this up
when tuning and diagnosing for greater sensitivity.  It's difficult to
describe the difference between flutter and pulsing due to a defect, except
the fluttering is regular whereas a downward pulse occurring just one beat in
four is a cylinder defect.  If you have fluttering and pulsing then you
probably do have a problem.

If you have manifold vacuum then *any* distributor will increase the speed and
advance with the vacuum connected at idle compared to disconnected, this is
because there is near full vacuum at idle with manifold vacuum, as opposed to
zero with carb vacuum.  16-17 degrees of vacuum is more than the maximum point
for any MGB distributor, which varies from 8 in Hg. to 15 in Hg, so you will
be at maximum vacuum advance, which can be anything from 6 to 24 *additional*
degrees.

PaulH.
  ----- Original Message -----

  What's the expected manifold vacuum on the MGB at 900 rpm? Mine's reading
  16-17. Should it be rock steady or is a variation of 1 inch acceptable?

  Anybody out there using a Pertronix distributor? With the vacuum advance
  connected, "idle" speed goes to 2000 and ignition advance to 30 degrees from
  a 7 degree advance when disconnected!