Sorry for the length of the dissertation. For a dual advance vacuum canister, you can use nipple on the outboard side of the diaphram, and not use the inboard nipple that functioned to retard the timing. You might try to set your initial timing at 10 degrees or more. If you get pinging, back off the timing. You may also be able to adjust the spring tension on the diaphram in the vacuum cannister by inserting a hex key into the nipple. Many cannisters allow this adjustment. It will change the level of vacuum at which the diaphram starts to move and when it finishes. As noted before, it can lead to idle problems if a vacuum cannister requires 16" of vacuum to fully advance, and your engine only has 14" of vacuum at idle. Pull the cap off of the distributor and use a vacuum pump and gauge to see the amount of vacuum at which the vacuum advance starts and finishes. Ignition timing has 3 components that make up the total advance: Initial, vacuum, and centrifugal. You set the initial advance and then the combination of the other two, vacuum & centrifugal, ultimately determine your total timing. Since the centrifugal does not contribute until you have achieved some minimum engine speed, vacuum advance is used to bridge the gap between idle speed and the engine speed when centrifugal dominates the timing. Centrifugal may start advancing the timing at 1200 to 1600 rpm and may not be fully advanced until 3000 rpm. Some old cars did not have provisions for vacuum advance. However, these special h.p. engines had a power band in the 3500 to 6500 rpm range, well above where all of the centrifugal advance would be fully deployed. Street drivability and fuel economy would be enhanced if it had vacuum advance. Engines do not expel the exhaust well at low rpm’s, especially with long duration cams. More ignition advance is needed in these diluted mixtures because the flame front progresses at slower speeds.
Manifold sources provide maximum vacuum and should be used if possible. It provides the greatest source of vacuum. Ported sources provide less vacuum and less advance at idle, because it is vented above the throttle plates. If there is less advance at idle, then the gap between the initial timing and the point where centrifugal timing takes over is even greater. Ported sources were incorporated in the 60’s in order to reduce the amount of advance at idle, thereby increasing temperatures and reducing emissions. At idle, total timing may be about 25 degrees (10 degree initial, 15 deg vacuum). Punch the throttle wide open at low engine rpm’s and the engine may bog in part because the vacuum advance component has fallen and the centrifugal component is not up to speed yet. The fuel mixture is relatively lean because the vacuum signal to the carb was reduced. The dilute mixture would ideally require more advance. Accelerator pump circuits help provide more fuel to overcome the loss of signal to the carb, but the ignition advance still suffers. At wide open throttle, above say 3000 rpm, you will likely have 35 degrees of total timing (10 deg initial, 25 deg centr). At cruising speed, the total timing consists of some combination of all three components and may run 45 to 50 degrees but the engine can handle larger timing advance at higher rpm’s. It is all a compromise with the old distributors.
Another note, distributors from emission era cars that used EGR valves etc, may have more advance designed into the timing curves than what a non-emission vehicle may need. The EGR valves dilute the combustion mixture, which required more ignition advance. They also typically had a larger portion of the timing contributed by the vacuum advance curves. These distributors may need to be re-curved to you specific application. New cars have a computer-mapped timing curve that is less of a compromise and does a better job of optimizing the timing under all load and engine speed variations.
