Let's break this up into two posts- - - -I don't know if the forum imposes a text length limit, and old schoolteachers have a tendency to walk off and leave their mouths running! The speed-related timing advance is handled by a system of weights and springs under the breaker plate called the "centrifugal advance ". As speed increases the weights overcome the resistance of the springs and move the distributor cam clockwise, making the points open sooner in relation to the crankshaft. An advance of up to 35 crankshaft degrees or a little more can be accomplished this way. Maximum centrifugal advance usually comes in around 2500 RPM or so. Then there's a system operated by air velocity in the carburetor called "vacuum advance". Its purpose is to add a few degrees of extra advance under light throttle cruise conditions, usually 8-10 degrees. If the throttle is opened for passing, hill climbing, or load pulling, the vacuum drops and that advance goes away. Centrifugal advance is engineered for maximum power under load. Vacuum advance is there to increase fuel economy at light throttle cruise. Both the centrifugal and vacuum advance numbers are engineered into the distributor when it's manufactured, so the only practical way to change them would be to do internal modifications, or find a distributor that's calibrated differently.
There are timing lights with a "delay flash" system that allows the timing mark to be electronically dialed back to the zero point to monitor the operation of the advance systems. To do really accurate tuning, one of those lights is a must-have tool. My current light was bought on Ebay for a little under $100.00. It has paid for itself many times over! If you don't have that kind of light, about all you can do is monitor the position of the initial timing setting. There is a way to do a fairly accurate "vacuum gauge" timing, but it's done at cruise RPM with the vacuum advance disconnected, not at idle. Most people who advocate setting timing with a vacuum gauge have no understanding of what's actually happening- - - - -they're just parroting things they've heard without actually knowing what's going on in the process. That's just my decidedly biased opinion, but I can provide facts and figures to back up my Bovine Scatology!
Jerry
