The motor is not original to the truck, and does not currently have a vacuum advance on the distributor that is installed.
The rebuild guide that I have does not show the distributor, but does mention that setting the timing should be done with the vacuum port covered... so I guess I need one.
Its just new to me that you can simply bolt on a vacuum advance diaphragm and it magically works..... most of the dist's I've seen the vacuum advance was integral to the dist, and not simply 'added' to it.
