Originally Posted by Bobbber
The vacuum advance on the distributor controls the... "ADVANCE" of the rotor arm.
This is important. It has nothing to do with fuel economy. It's to do with ensuring the spark fires at a point pertinent to the speed of the crankshaft.
Now - the next bit. The vacuum is GREATEST when the car is at idle. Hard to believe eh? So when at IDLE the vacuum (being high) retards the point of ignition (Ben NZ, a very intelligent young man - yes, a timing light is all important to check this).
So, vacuum pipe missing :
1. On fuel injected cars, unmonitored air into the inlet manifold.
2. On all cars - the timing is advanced when at idle - your car will NEVER idle correctly.
In a car with a mechanical distributor which controls advance (some modern cars have a distributor with ecu advance control):
1) The mechanical counter weights system in the distributor control the ignition advance in order to ensure that the cylinder is fired at "optimal" point in the cycle.
2) Vaccum advance is only used to improve fuel economy. Many cars did not even have a vacuum advance mechaism.
3) If the vacuum advance hose is missing or "holed" and the hole into the inlet is not blocked then air will leak into inlet sustem and cause poor idling etc.
i.e. Any air leak into inlet reack will cause poor idling and has nothing to do whether car has vacuum advance or not.
4) A car will idle perfectly without vacuum advance as long as there is no "exteraneous" air leaks into manifold. (see 3 above)
5)Low vacuum occurs when foot flat to the floor under hard acceleration. On a car with added vacuum advance , ignition follows mechanical curve.
6) Performance of a car can be modified by changing the springs and counter wights ina distrubutor. ie so called "Emission cars" often had a mechanical advance curve which was not optimal for performance...example US Alfa Romeosfrom late to early 1980's had mechanical advance curves aimed at reducing emmisions rather than performance.