SAAB Distributor Test Machine

 As promised, here is the SUN distributor test machine, with a Bosch/Saab V4 distributor installed for testing. When the machine is running, an arrow appears on the large black INNER ring below the distributor. Just outboard of that is another black ring with degrees etched into its surface. As distributor RPM increases [by turning the knob at the bottom, below the black rings] the arrow moves anti-clockwise, indicating degrees of advance for that RPM. The top meter on the right measures the RPM. I plot the RPM for each degree of advance, in order to see the mechanical advance curve.  

A typical advance curve--taken right from my distributor reconditioning records book--is shown in RED above, plotted over a set of minimum/maximum curves, as shown for that particular distributor, in the Saab V4 Service Manual.

When the actual plotted curve [RED] is NOT within the factory spec's, I have to disassemble the distributor and change one [or both] of the wee springs in the advance mechanism. Depending upon the distributor type, there may or may not be an additional mechanical adjustment that I can make.  Remember---there are at least a dozen different Bosch distributors used by Saab in their V4 model run from 1967 to 1980.
Without plotting the mechanical advance curve, using the SUN machine, you have no idea what the distributor is doing and I guarantee you--if the curve is significantly outside the minimum/maximums, the engine will run poorly, no matter what tune-up and/or repair you have done to it.

The bottom curve is a plot of the vacuum advance, also done on the SUN machine, or with a special machine I built for the purpose. Again, Saab shows the ideal minimum/maximums for that curve. The RED plot shows the curve of the vacuum advance unit I used for this particular recondition job.  One thing to note:  if the vacuum advance does not work, the engine will run like crap even if you have the timing and carburetion set perfectly.  How is YOUR V4 engine running?

One thing I have found out over the years of reconditioning distributors---the SUN machine never lies.