If you're only getting 5-8 crankshaft degrees with your centrifugal advance, something is restricting the movement of your distributor cam and/or the flyweights. There should be about 15-20 degrees of centrifugal advance, plus another 10 degrees or so from the vacuum chamber. Most engines, 6 or V8, run best at around 36 or 37 degrees of initial plus centrifugal advance. The extra advance from the vacuum unit happens at light throttle cruise and is only for increased fuel economy. It goes away as soon as passing or other acceleration demands are made by opening the throttle,and the advance drops back to whatever is being provided by engine speed.
If you're going to use a vacuum gauge to estimate ignition timing, do so with the vacuum advance disconnected, and adjust the timing for the highest vacuum reading at no-load cruise RPM. Then reconnect the vacuum, and do a test drive to see if the engine detonates under slight acceleration at cruise RPM. In case of detonation, retard the timing a degree or two at a time until the detonation disappears.
Jerry
