So my 1985 f350 box truck with the i6 in it. I am having a hard time understanding the vacuum system and the mechanical nature of this carb'd engine. I think I have a vacuum problem and its causing all kinds of engine havoc. when I leave a vacuum leak on one port of the PCV valve, I get an ok, but tad higher idle. If I plug it or plug the line in the engine sounds like its going to die. theres tons of vacuum on this hose, that I have no idea where its supposed to go, as one already goes to manifold vacuum. Someone before me removed most of the emissions stuff, except the air pump which is frozen so I disconnected it from the belt. There is this filter that looks like it connected to the gas tank with a line. I have absolutely no idea what it does, but if I connect it the emissions light comes on. connecting it also made it run worse! So does plugging in the EGR valve as I know it was connected to that ported vacuum switch.
This is a nightmare sort of. all the little brittle vacuum lines are toast and the ported vacuum switch that connected to the heater portion of the thermostat housing snapped.
Connecting vacuum advance on the dizzy seems to do nothing but I just left it.
Does anyone know what vacuum is essential? I know the brake booster is and the PCV valve is needed but I'm not sure what else. I'm definitely no mechanic but I am not afraid of this. I have no ECU to get codes from either, just a duraspark II I think it is. I think my starter may be going bad too because it was very loose and I found out because I was getting whirling but no engine turn over and it freaked me out but I went underneath the truck and it was loose so I tightened its bolts back down and it starts easy now, but stumbles a bit when first accelerating or at low rpms now. the rabit hole just keeps getting deeper. No overheating and it doesn't die though.

 

You said you connected a vacuum line from the dist to the carb, so it does sound like you have a duraspark II ignition system. I would disconnect all vacuum lines except the brake booster, PCV valve, transmission if it needs it (automatic) and you will need the one vacuum line that feeds the HVAC stuff under the dash. I would also take the EGR off, make a solid metal gasket for it, and put it back in place.

 

What DaveF said is what I would do first.
When doing this check to see if the carb is lose. Grab the top and try and move it, it should not.
Once the vacuum lines are taken care of and the carb checked if you still have the same issue use carb cleaner with the straw and spray at the base of the carb and the manifolds to head.
If the RPM changes you have a vacuum leak in that area.
Dave ----