It is acceptable to set ignition timing using a vacuum gauge.
Adjust the distributor until you get the highest vacuum reading and then back off the adjustment 2 in.Hg.(inches Mercury) from highest reading.
Example: if highest reading is 21 in.Hg...
Adjust until 19 in.Hg is reached.
This method is similar to timing by ear (pinging), although you are looking at a vacuum gauge rather than detecting evidence of pinging.
Timing using a vacuum gauge is a way to set the timing according to what type of fuel is being used and other engine modifications. Also if the timing marks are gone or harmonic balencer has slipped or flywheel marks are not visible, this method can be used.
Hope this helps.
