79' Truck not idling

I have a 1979 Gmc truck 350 motor and its not idling. when driving it runs fine. but if i stay at a stop sign for to long it dies just kinda sputters out. and when the truck sits for over a day or 2 without driveing it needs jump started whats wrong?

First, get the battery cables cleaned and have the charging/starting system tested at the nearest McParts store. It probably needs a battery. Get back when that issue is settled.

would the battery be the reason its not idling?

Is it a matter of it idles fine for a while, but then dies? Or does it always idle low and rough?

The one specific suggesion I have is the anti-diesel solenoid. This is a little gizmo that snaps the throttle shut another few degrees when you shut the ignition off to prevent the engine from continuing to run due to heat in the cylinders, which was a common problem on early smogged engines like this. If it’s stuck shut, the truck will idle like crap if at all. You should be able to see it move the throttle a little bit if you have an assistant turn the ignition between off and on. On my '76 Chevy (with the 250), you can actually see the pedal move ever so slightly.

However, I think it’s more likely that doing basic tune-up stuff (check the plugs, timing, idle mixture, idle speed) and changing the fuel filter will resolve the problem. The battery problem probably isn’t related.

Just to add, do check the anti-diesel solenoid before you mess with the idle adjustments. If you just turn up the idle, the thing will run on pretty bad when you turn it off hot, which is very embarassing in crowded parking lots!

You may have 2 problems, battery or alt getting bad and maybe vacuum leak.

How to find the leak and fix it?

You cannot diagnose the idling problem if you cannot start the engine and low voltage MAY be a part of the stalling problem. I am reading between the lines here. Often DIYer don’t want to spend $80 on one issue because they fear the other issue may cost hundred$. Other than suggest that you install a good, fully charged battery from another car to check for the idling problem there’s not much else I can advise. You must get the starting cranking problem solved first, even if only temporarily.