Yep, first thing I thought of too based on your description is a vacuum leak or three.
Did you test the vacuum lines, or just visually inspect them? Sometimes that latter is not enough.
The PCV can rattle all it wants and not be serviceable anymore. If your engine truly has only 40k original miles, then it's probably ok. But if it's got more, then by all means replace the PCV valve anyway. You can keep the old one as a spare if the new one does not help. But they do wear out, and can still pass the rattle test.
The EGR can have failed too. It's a diaphragm device and things like that fail.
And speaking of diaphragms, you need to test your vacuum advance diaphragm as well. Including just watching a timing light while the truck sits idling. If you see a timing change when the idle changes, you may have to find the culprit there too.
Does your '73 still have all it's smog stuff intact? And I mean all of it?
If so that's quite a few extra items that use vacuum, so are all suspect at this point. If you have any vacuum caps over unused ports too, those are extremely suspect.
Even my Explorer engine started surging under light throttle and slow speeds the other day. Been keeping track of it and I'll bet money it's one of my vacuum caps again. The modern ones are crap and fail about every six months or so. Whether you run the engine or not it seems.
So that's what I get to check on mine. Definitely do the test on yours with some sort of spray fluid. Flammable or not, chemical or just water, if your idle changes at all when you spray an area, you've found a leak.
Paul
