Phil---The vacuum retard, when workling properly, will retard timing
around 16 deg.. So if you set the timing (with the vacuum plugged) at
12deg. BTDC and then reconnect the vacuum line, the timing wil retard to
4 deg. ATDC, or close to it. This will only be true at idle speed of
900rpm or slower, as the proper port for vacuum retard is only present
when the throttle plate is closed.
If you have less than 16 deg. of retard, don't worry about it. It's more
important to have proper timing when OFF idle.
The retard feature was intended to reduce NOx. Strangely enough, this
test is not performed during emissions checks on older cars. At least
not here is Ca. when my own car needed to go thru this.
Dick
'73
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