Why Control Pressures Higher with Vacuum Line Connected
 

We all know that the vacuum line on 78/79 WURs is there to lower the warm control pressure for enrichment @ WOT. This line is @ the bottom chamber and attaches to the lower port on the throttle body. The top line is for atmosphere and can either be left open or attach to the top port on the TB. Why then do the charts show a higher wcp w/ the line attached when the engine is warm and idling? Isn't there always going to be some vac applied to the chamber when the engine is running? Is pressure somehow actually added to the WUR @ idle via the vac line? I don't get it. Is it possible the vacuum that lowers the wcp only comes on during the overswing of the sensor plate that occurs@ WOT?

OK. I think I've got it now. According to Bosch, @ idle the vac on the WUR should be 18 to 20" (of hg I guess). Part throttle SB 12 and WOT should be 0. So the amount of vac actually decreases from idle to WOT.

More vacuum (idle) equals a higher cp equals a leaner mix.

Less vac (WOT) equals a lower cp equals a richer mix.