Your distributor should have both vacuum and mechanical advance. Some in the '80s had one or the other, or neither, when they started going to computer-controlled timing.
The mechanical advance is for high-speed driving. That's what those weights and springs are for. The fuel takes time to burn and you want the most pressure pushing on the piston just after top dead center. At higher speeds the fuel takes the same amount of time to develop maximum "head pressure", so for that to occur at the right time the fuel has to start burning sooner. The centrifugal weights turn the top part of the two-piece shaft so the breaker points open sooner or the trigger wheel induces a signal into the pickup coil sooner. That makes the spark occur sooner.
The vacuum advance is for light throttle and better fuel mileage. Under heavy throttle the fuel / air mixture is relatively rich and it's easy for the flame front to jump from one molecule to the next. All the fuel will burn rather quickly. Under light throttle, like cruising or coasting, the mixture is leaner for better fuel mileage and the molecules of gas are further apart. It's harder for the flame to jump from one molecule to the next so it takes longer for the fuel to burn and to build maximum pressure. For that reason we use engine vacuum, which will be high under those conditions, to advance the spark so the fuel has the time it needs to burn completely. Under heavy acceleration vacuum advance is not desired because the mixture will be rich, and as luck would have it manifold vacuum is low at that time, so ported vacuum will be low too and the vacuum advance will go away.
Most engines use "ported" vacuum for the vacuum advance. The port is in the carburetor, above the throttle blade when it's at idle or real low speed. As the throttle blade begins to open past the port, intake manifold vacuum gradually appears there and begins to advance ignition timing.
There were some GM engines that used straight manifold vacuum for the vacuum advance. The idea there, I was told, was that part of ignition timing advance was always there, even at idle, and that's when you adjusted timing, then that advance would go away during periods of acceleration or heavy load when vacuum was low and the fuel mixture was rich. That was just a little before my time and I never really learned if that's how GM designed it or if some people hooked the vacuum hoses to the wrong place. I do know I saw a number of cars that had so much vacuum while just cranking the engine that when they were hot they had a hard time starting. They'd act like they had a weak battery, then the starter would take off after a few seconds of struggling. I don't ever recall running into that on a Chrysler or Ford.
I probably told you a bunch of stuff you already know but I just couldn't help myself. Happy to hear you solved the problem.
Friday, September 6th, 2013 AT 1:40 AM
