On our Volvo 140 '72 (B20E) there is a distributor which has centrifugal weights and a vaucuum system (as usual) to control the ignition timing under the different speed and load conditions of the engine. How and why the centrifugal works is pretty straightforward. When the engine speed is higher the air/fuel mixture must be ignited earlier because the piston is moving faster.
But I've never quite understood why the vacuum system works as it does on our B20E. The theory says that when there is a lean mixture (and also low pressure) in the sylinder, the burning prosess takes longer time. This is the case when the throttle is (almost) closed. Vacuum builds up in the intake manifoil and also in the sylinder. With the throttle closed there is a lean mixture of gasoline in the sylinder.
The reason why the vacuum capsule was invented is (from what I've read) that for best possible burning, the ignition timing should consider the amount of fuel (and pressure) in the sylinder to "calculate" when the spark on the plug should occur. If the mixture burns slow, the spark should occur earlier and vice versa. The point is always to get maximum pressure when the piston is at top. Since the pressure in the intake manifoil represents the pressure and fuel amount in the sylinder, the ignition is partly controlled by this vacuum.
Shouldn't it be so that a high vaccum in the intake manifoil (means low sylinder pressure and low amount of fuel) should give an advanced spark? This is not the case on our volvo. The hose for the vacuum capsule is conected between the throttle and the engine so it "feels" the intake pressure. Higher vacuum retards the ignition in our case. This is wrong? If the throttle then opens (lets say at a low engine speed) then suddenly the ignition would advance (go earlier) which is not right. It should retard (be later) since a richer fuel mixture need less time to burn. Is it suppose to be like this?
The Haynes manual also says that "an increase in engine vaccum results in a retarded igntion" (not directly from the book) for the B20E and B20F engines. It also says that "in this case the vacuum capsule is connected to a point between the throttle and the engine". Could it be that the distributor for the car is wrong? Since the car is fuel injected (which is only on the 'E' and 'F' engines, i think), the distributor has an extra connector so the control unit can measure engine speed. If it's the wrong distributor it probably can only come from an 'F' engine.