You can use a vacuum gauge to set ignition timing, but it is best used along with a tachometer. I have found the best results by advancing the timing until either the RPM or vacuum reaches a peak, then retard the timing to give about 1"Hg drop in vacuum. I have found that method gives a close approximation to manufacturer's specifications. Using a vacuum gauge alone, going for peak vacuum will usually leave you with timing that is advanced more than is desirable for your engine and operating conditions- engine load, altitude, ambient air temperature, etc. Others have their preferred method of setting timing using a vacuum gauge, but they also advise to listen for detonation/spark knock/pinging, and to retard timing until that no longer occurs. The best approach is to use a timing light.
The information on vacuum gauge readings contained in the link in the previous post is weak or incorrect on several points.
If you search the forum using "How much vacuum?", there are responses there to give you more insight on the topic. Make sure you use the quotation marks, otherwise the search produces more responses than you will care to read.
Harvester
