On a Vacuum advance distributor,my question is how does it work exactly,you have the hose going from the carb to the vacuum advance thingy on the distributor and from there on where does the vacuum go into?and my other question is,i went to time the stock `67 GS today which has vacuum advance i noticed that whether the vacuum advance is plugged at the carb or whether its hooked up normally i am get the same reading at idle,is that normal?or is there something wrong? Thanks
George, I think at idle its normal. The advance is to advance the timing as the engine increase in rpms . With that increase the vacuum decreases and advances the timing. Some 80 cars had the fuel economy gauges in the dash and that was basicly a vacuum gauge. The harder into the gas the less the vacuum. I am no expert but this is my understanding. Note I have been wrong before.
George, Vacuum advance is mostly for economy. Under light loads at a steady cruise speed, an engine can tolerate more spark. Vacuum canisters typically provide another 10-20* more advance above the mechanical advance. Ther are 2 ways to hook up your vacuum advance, to ported vacuum, and full manifold vacuum. With ported vacuum, there is no vacuum at idle. As you open the throttle, vacuum increases and advances the spark. With full manifold vacuum, your advance is fully in at idle. Both kinds go away when you go to wide open throttle, vacuum drops towards 0 and the advance drops off. Which type you use is up to you. I like ported better, I like the way my car runs with ported. I've been told that full manifold vacuum is better, I ve tried it, I still like ported better. Some guys leave the vacuum advance disconnected, and run without it. On a street car, I think you should run it. If the advance doesn't fall off fast enough when you punch it, you can get some part throttle ping. Adjustable vacuum advance cannisters help alot in that case. You can limit the advance to say, 8-10* and adjust the spring tension inside the cannister so that the advance drops off faster. That way, you can dial out the ping. George make sure that your vacuum cannister is working. Connect it up to full manifold vacuum, the engine speed should increase, and you should see the timing mark move up, when viewed with a timing light.
