Vacuum Advance

[simon.matthews@st.com]

The purpose of the Vacuum Advance is to advance the ignition timing
under LOW LOAD conditions -- ie, when the throttle is not wide open.

Advancing the ignition timing should make the engine run more efficiently
-- but I very much doubt that failure of the vacuum advance could cause 
backfire -- much more likely to be air leaking into the manifold some-
where, casing the mixture to be way too lean.

Regards,
      Simon