ummm no, vac advance isn't affected by load it is by rpm you are neglecting the fact that the advance is changed by the throwout weights once it comes up off idle speeds You only get advance on extremely low rpm such as idle once you hit the 2 grand region the weights most definitely are all the way out. An the vac advance IS a retard system. you have full vacuum at idle bringing the timing into full advance however as your engine speed increases, due to the throttle opening vacuum drops and this causes the timing to retard, However, this is where the throw out weights take over, turning the rotor allowing the timing to advance or retard as the throttle position changes therefore varying the engine vacuum. You are trying to say that it is the load that is making these changes, This is a complete misnomer, it is the vacuum based off throttle position. As you know a wide open throttle yields zero vacuum, a closed (for the sake of discussion since the throttle is never really closed fully) throttle yields the highest vac. Since there is the least amount of air/fuel being taken in when the throttle is closed the ignition timing needs to be advanced as far as possible (to TDC ) to ensure the engine stays running smoothly. The choke held the throttle open enough to supply more fuel to enable the engine to start at the retarded point due to lack of vacuum and no mechanical advance due to low speed rotation of the distributor. The faster the engine turns the more the rotor is adjusted by the throw out weights due to centrifugal force of the distributor. Again this is solely the action of RPM not load, and that is the result of throttle position (which adds or removes vacuum from the distributor as well). Again this removes the load from the equation and puts it back on the engine RPM the timing changing all over when you rev your engine is the result of what is called propagation delay. Slow that speed change down and reconnect the vac line to the distributor ad you will be able to actually chart out the advance curve of the distributor. That's why they make timing lights that allow you to advance or retard the trigger point of the light, so that you can follow the actual curve.
You need to keep in mind that the vac advance is in full advance at idle and retards as the engine speed increases due to the opening throttle which causes loss of vacuum, this is where the weights take over, and their function is solely based on the RPM of the engine, as the faster the distributor spins the faster and farther the weights turn the rotor to the next pole in the cap. Load has no bearing other than your need to use more or less throttle.
