Hi Brit - sorry to hear you have been having problems with your MSD ignition. I've read through your thread and it isn't clear whether you have the vacuum advance connected when you check the maximum advance at 4500 rpm. The vacuum advance should be disconnected (and plugged, so that the open port doesn't affect the fuel/air mix) when checking the mechanical advance at all engine speeds - with the vacuum advance connected the ignition can advance beyond 34° when the engine is not under load.
If you checked the advance at 34° with the vacuum advance connected, and retarded the ignition accordingly, then you would be too far retarded
, not too far advanced. This would stack up with your idle advance being less than 8°. Sooty exhaust smoke is also indicative of retardation, but can also occur if the ignition is over-advanced as well.
The vacuum advance is an economy feature fitted to road cars. It is intended to improve fuel economy at part-throttle settings. The vacuum port is placed in the inlet manifold, below the throttle butterfly. With the throttle is closed (engine at idle), the difference in pressure above and below the butterfly is at its maximum, and vacuum is high - although at idle the engine is not creating much manifold depression anyway. But if you are running on the flat at a part-throttle opening, you can see that there will still be an appreciable pressure differential above and below the butterfly. This creates a partial vacuum in the advance canister which pulls the distributor baseplate around, advancing the ignition. This advance will be in addition
to any mechanical advance caused by the springs and bob weights (mechanical advance is determined by engine rpm alone).
Advancing the ignition results in a leaner burn, which saves fuel. This is how electronic fuel injection works - there is an accoustic knock sensor fitted to the block, a sensitive microphone if you like, which listens for the tell-tale "ringing" caused by detonation. The ECU advances the ignition until the knocking starts, then backs off slightly, and it does this hundreds of times a second, under all conditions of engine speed, temperature and throttle position. I like to think this is why EFI equipped cars make that tinkling sound under hard acceleration. They are operating right on the ragged edge, all the time. But to get back to our distributor story...
OK, say you decide to floor the throttle. As soon as you open the throttle wide, all of the pressure differential above and below the butterfly disappears. Whatever vacuum-induced advance that was present before you floored it is lost and the distributor baseplate snaps back to a setting determined only by the mechanical advance. As the engine speed builds, centrifugal force throws the mechanical bob weights out against the return springs, advancing the ignition until they reach the limit of their travel, which should be at 34°. There is no contribution from the vacuum advance at all when running at WOT (Wide Open Throttle). This is why racing engines usually have the vacuum advance removed - they are always operating at, or close to, WOT, so have no need of the fuel economy benefits of vacuum advance. The downside is fuel consumption, as I discovered with my old Droop Snoot (which had a Group 2 race engine), which for a 2.3 litre slant-4 produced an eye-wateringly poor 9 mpg
I usually test the vacuum canister by sucking on the end of the vacuum tube. You can usually see some movement of the distributor advance plate which shows that the diaphragm is not perforated, which is the usual failure mode.
A quick check of the mechanical advance is to grasp the rotor arm and try and rotate it in the normal direction of rotation. There should be a small amount of movement, which corresponds to the bob weights being forced out against the return spring pressure. The rotor arm should return to its original position when you let go. It doesn't tell you if the springs have stretched or if one is broken, but it is an easy check to perform. The bob weights need occasional lubrication with light machine oil, otherwise they can stick. If one or both return springs are broken, you could be over-advancing the ignition when the engine is under load, and that might explain the detonation you are experiencing. If everything checks out I would question the octane rating of the fuel.
God, I don't half go on sometimes...
Hope you get things sorted soon!