yep it does give me some more thoughts :o)
that has the vacum line connecting to the ecm directly so this is actually pre tier1 or early tier 1, the later versions of the tier1 updated came without the vacum line and calculated dwell via the ecm instead of reading the vacum from the intake/carb/mixer

check the mixer diaphram, if its dirty clean it, pay close attention to where the button in the center seats in the center, if the seat is dirty it will not close properly and cause a leak inside the mixer. It will not want to idle very well if this is found.
clean the mixer body and also down inside where the butterfly in the throttle body is. You can use brakecleaner or if your not comfortable using that just use some carb cleaner.
also check the vacum line going from the carb base over to the air cleaner pipe at the engine, does it have a check valve inline? not the pcv valve but there is another line that should have a check valve inline the hose that goes over to the air intake hose just on top of the engine. I think the older versions had a check valve in the fitting where the hose connects at the air intake hose on top of the engine where the bracket holds the hose to the valve cover.

it sounds to me like your not getting vacum on this line to the ecm, ya wanna make sure the hose is intact and no cracks. make sure the pcv valve isnt clogged or stuck, if there is that vacum check valve in the hose to the air intake pipe make sure its not damaged or dirty or blown out.

if you find all the above good then there could be something causeing low vacum like bad valves or lifters, worn out engine.
I'm just throwing out all this stuff to give you things to check that MAY cause this problem.
If anyone else can think of anything please chime in ;o)

Good Luck