Remember I asked about a "delay flash timing light"? At any set engine speed, the control on the light is turned, and the timing marks are brought back into alignment by delaying the flash of the strobe. Then a scale or a digital readout indicates how much advance the spark has for that speed. My Snap-On timing light has a built-in tach, and a timing indication so I can toggle back and forth. I usually check the advance in 500 RPM increments from idle to cruise speed, and compare my readings to the factory specifications for the engine I'm checking.
Vacuum advance only works at light throttle cruise, so by disabling the vacuum during the advance check, only the operation of the centrifugal weights is being checked by increasing speed. It's impossible to put enough load on an engine in the shop to drop the manifold vacuum so the vacuum advance drops out, unless the shop is equipped with a chassis dyno. I've got a dyno that bolts onto a bellhousing and puts load onto the engine with water, but I haven't gotten around to building a set of rollers and a load cell to check the whole vehicle.
Jerry
