A member a few days ago asked about how to set the ignition timing.
He was told in two different reply’s to make sure that the vacuum line to the trailing vacuum unit was plugged before setting the timing.
I have never removed the vacuum line when setting the timing I double-checked my 79 Factory Manual and the Haynes Repair Manual neither say to remove the vacuum line Just curious where this information is coming from?

 

This is basically a precaution. If your vacuum line is connected to the right nipple, and your idle is set correctly, then you should be okay without disconnecting it. But if you're plugged into the wrong vacuum port, then it will throw off the timing at idle. Hope this helps...