Sucking on the pipe will NOT turn the rotor arm.
Sucking hard on the pipe SHOULD make the breaker plate turn a little. It is the rotation of the "static" breaker plate in relation to the rotating distributor cam lobes that alters the dynamic timing of the engine on the run.
Remember every degree the breaker plate moves is TWO degrees on the crankshaft.
There are various configurations of vacuum advance and retard mechanisms, but in a modern engine such as the Rover V-8 you can be pretty sure that it works along these lines ....
When there is NO vacuum on the pipe (wide open throttle) the ignition timing is RETARDED by quite a lot.
When there is HIGH vacuum (light throttle) on the pipe the ignition is ADVANCED quite a lot
and then infinite variations in between depending on throttle opening and engine speed.
The idea is to have the ignition timing ADVANCED as far as possible when cruising on light throttle (high vacuum) but to RETARD the ignition by an appropriate angle when you shove the throttle down to make the engine work harder.
If the vacuum system is jammed or disabled when you set the timing in the usual way, you can be sure there's going to be trouble as soon as you drive the car, ESPECIALLY when you try to apply power. PINKING (detonation of fuel-air mix) is a certainty and piston failure will occur sooner or later. It can happen very soon indeed.
Interestingly there is very little vacuum on the system when you start the engine so it starts on retarded ignition (= good) but as soon as it fires up to tickover speed it MAY advance the timing DEPENDING WHERE THE VACUUM PIPE TAKE-OFF NIPPLE IS LOCATED. Sometimes they are fitted right at the throttle butterfly to prevent this happening, and to make it work only at driving speeds when the throttle is more open than tickover. So, the vacuum pipe must be fitted to the correct nipple.
All very clever, IF IT WORKS!
CharlesY
