Classic/Antique Car Repair: Timing on a 79 MGB, mercury capri, vacuum level

Question
Should the vacuum advance retard the timing and if so how come when theres no vacuum to the dist. does the timing retard

Answer
Doug:

It's called vacuum advance because it advances the timing. On some motors (notably Cologne-built Mercury Capri V6s), the mechanism both advances and retards, but on the MGB, it merely advances.

In addition to this, you'll have centrifugal weights which advance the timing based upon engine speed. So, your total advance will be a combination of engine load (vacuum) and crankshaft speed (centrifugal).

In addition, if you want to scope the engine, the characteristics of each module are stamped on the cover's upper casing. A typical one is 7-15-8, meaning that the advance comes in at 7 in. Hg, hits its max at 15 in Hg., and the max number of degrees at that vacuum level is 8°.

--Paul