How to Use and Interpret a Vacuum Gauge

This was pointed out to me by Walter Higgins and the original site is gone.  So he found it in the way back machine and I'm gonna repost it here.   Originally from "Second Chance Garage".

Very important and interesting. The original site may still be available in the Internet archive here:

https://web.archive.org/web/20170327191027/http://www.secondchancegarage.com/public/186.cfm

If not here is a copied version:

Few tools or test devices are more useful and versatile than a vacuum gauge, yet very few hobbyists own one. A vacuum gauge can tell as much about the internal and external workings of an engine as the combination of a voltmeter, compression gauge, stethoscope and timing light. Really!

An engine's vacuum readings can tell an awful lot about its running parameters, provided you know how to read the gauge in the first place. Unfortunately, most vacuum gauge manufacturers don't provide easy-to-follow instructions and diagrams to help users interpret gauge readings, rendering the gauges themselves virtually useless. 

We're here to change all that, so follow along with us on a tour of vacuum gauge readings and we'll show you all the neat things you can do with one.