Originally Posted by subzali
...Mechanical advance is just rotating the dizzy to set the nominal advance with the bb on the flywheel right? Right now I have that a little over 7* IIRC.
Yes and no; mostly no. You are correct, setting the timing is a "mechanical" action, i.e. you turn the dizzy. Each distributor has a "mechanical advance" system built in. As the engine RPMs increase the dizzy's built in advance system ( "mechanical advance") changes the timing - it advances it. The basic principle is that at high RPM, each cylinder can spark a little earlier than at low RPM - so that power and fuel usage are "maximized". In the evolution of things, a vacuum advance system was added to use ported vacuum to further maximize power and fuel usage under certain conditions, i.e. when ported vacuum is high.
Originally Posted by subzali
Right now I have the vacuum retard can (I think - the one closer to the dizzy body) hooked up like my '77 is normally hooked up (I guess, I don't understand why there is vacuum retard, I always thought that as your engine speed increased that more advance was needed on the ignition (which it doesn't seem that there is any provision for with a stock '77 setup), but I also know that higher speed doesn't necessarily mean higher vacuum). So are you saying if I hooked up the advance can in addition to (or instead of?) the retard can that might be a better way to go? Do I just have to be careful of pre-ignition? And I think I know what ported vacuum is, but I'm having trouble visualizing which port that is on my carb...
Much of the above is mystery for me as well, relative to the late 70s Aisan carbs. I am hoping that your thread on MUD will result in some additional insight and understanding - someone out there has to know this stuff
About all I know for sure is that automotive engineers have evolved things over time. In the 70s they started using engine vacuum, controlled by speed sensors (initially) and components/computers (next in the evolution) to advance or retard timing thereby altering the dizzy's built in advance system resulting in even better maximization of power and fuel. Like you, I'm not clear on the detailed specifics. We are now in the age where the dizzy is gone, and each cylinder has its own coil pack and the computer runs the show. Great for low smog, more power and improved fuel economy.
Not knowing how vacuum is routed through all the ports, it is not advisable to start changing connections on the dizzy. The results may likely not improve the performance of the engine. That said, I am pretty sure that if you have a ported vacuum port on the carb - it is ok to hook it to a vacuum advance port on a distributor.
Again, I'm not familiar with mid-70's and newer carbs - but on all the early carbs the ported vacuum port is just to the right of the idle mixture screw. This port does not exist on all early carbs but can be added. It is a real simple thing to do, but has to be very precise. I didn't have the confidence to do it on my own, so I sent my carb base out to get it done.
You are correct, when setting engine timing if you are pinging it has been advanced too far. Also, if it is difficult to start, it has been advanced too far.