General Motors vacuum advance degree stop information


Ever wonder why that new vacuum advance gives 20, 30, 40 plus degrees of timing, when it shouldn't? It is because when the cars were new, they came with a rubber stop bushing on the vacuum advance pull pin inside the distributor. Over the years, those rubber bushings degrade, fall off, rot away, allowing the pull pin to pull farther, result, more degrees of vacuum advance than the engine likes.


Parts store replacement General Motors point type, and, HEI vacuum advances do not come with the rubber, or, any stop bushing.


When these engines do not perform correctly, and over heat from the distributor not being set up correctly, most "tuning experts" will then tell the customer to "disconect that vacuum advance, and jack the initial timing up, you don't need that vacuum advance, anyway".  Dead wrong in most applications. Only very radically cammed engines don't make enough vacuum to run a vacuum advance. Also, engines that are "dyno tuned" usually do not like vacuum advance, even though the engine does. Dyno's do not like vacuum advances, they prefer considerably too much timing, and very rich carburetor jetting than real world driven engines. This is another reason a dyno hero will tell you the same, "rip that vacuum advance off, it doesn't do anything, anyway". Wrong again.


When properly set up for correct degrees given, and sourced correctly on full manifold vacuum, the vacuum advance becomes a valuable asset in tuning the street driven engine correctly.


Aftermarket stock G.M. replacement vacuum advances have different length stop specifications to meet different engine-power train applications, but, NONE of them come with the rubber, or, any other stop bushing in place on the pull pin, which allows them to severely over advance the timing past reasonable levels. 


The complete set of instructions I offer give you the basic outline of this issue, and, both text and pictures on how to install a stock rubber stop bushing, and, to fit either a home made metal stop for a set number of degrees, or, a Crane 99619-1 stand alone scroll type adjustable degree stop plate.


These instructions will also allow you to properly fit the Stand Alone Crane Stop Plate  to stock G.M. advances, and the same for mounting those plates to any Crane adjustable vacuum advance. In the case of the Crane, my stop plate mounting instructions installs the stop plate on the other side of the pull pin, so it does not preload the spring as degrees are taken out of the advance. This separates the stop degrees adjustment from the vacuum pull setting of the diaphragm. This stops the degrees stop plate from changing the vacuum setting as degrees of timing are taken out of, or, added to the advance unit. If the degrees stop is pushing on the pull pin to drop degrees off the advance, it is also changing the spring setting for the vacuum pull. Yes, this is OK...for a small amount of adjustment, but, soon, the stop starts to preload the spring past any adjustment, and that renders the vacuum pull rate adjustment moot. This modification stops the pin as it advances through its travel, as G.M. originally did it with the rubber stop on the pull pin.


These instructions sets are free of charge, and available by email, please use this email address:  gmvacuumadvancemodifications@gmail.com