The distributor and other ignition component are new. Not that new always means good. Before I ran into this erratic idle speed, I was playing hell with the ignition. Turns out my brand new condenser was bad and burned the points up in less than a hundred miles. Also the quality of the new points was fairly suspect. Installed pertronix. No more issue. Had it running well until I got the advice that my distributor should be on manifold vacuum. All that extra advance at low rpm and my best attempt at tuning it like that made it run a little rougher and backfire pretty hard a number of times.

 

A quick re-read of this thread, says you found the engine vacuum dropping from 19 to 16, this is part of the issue. Don't take anything for granted, start from square one.

Determine and verify for certain #1 piston at TDC on compression stroke equals "0" on crankshaft damper. Everything else is predicated on this being correct.

Sometimes damper rings slip off their clock, timing chains jump a tooth, distributors get installed off a tooth, wires installed 180 out, etc. Make sure breaker plate moves smoothly and freely and vac diaphragm holds. I would also assume several carb backfires has lunched the powervalve and replace it on general principles.

 

Update. Took the carb off and cleaned up under spacer. Removed the cover from power valve and that chamber was full of gas. Gasket around power valve was not sealing and letting vacuum pull gas directly past. Like having a vacuum leak and over fueling in one. I am guessing that is why it surged. Variation in speed caused a variation in vacuum, which would pull more or less fuel around that valve, which cased the rpm to raise and drop. Vicious cycle. Not sure why it didn't rear its head earlier. Put it all back together and it seems to be running well. Thanks for all the help.

 

Back in the mid 70's bought the wife a little Toyota wagon a few yrs old for the wife to run around selling her Amway crap. Ran good for a couple months then the idle started doing like your trucks condition of surging high & then lower rpm.

But it ran good after the speed of 30 mph. I looked & looked for a vacuum leak with no luck so finely pulled the carb apart and carefully inspected it and found a main jet had backed out just sitting on top of the threaded body.

So it was bouncing or rocking around at idle then pull back into place when to throttle was open little run smooth. I messed with searching for a vacuum leak after work for about a month before pulling the carb top.

But it drove me nut under the hood on those 112*-117* warm summer day time temps

but the wife kept those cold ones coming.
Orich

 

Sounds to me you put too much vacuum signal to the Dizzy's vac advance when you switched to the carb's rear port meant for a PCV system or Brake booster. Ruptured the diaphragm inside it. So now you have both a vacuum leak and no advance from that unit. Take the vacuum hose leading to the Dizzy's vacuum advance off the carb. Suck on that hose and I'm betting you will find no resistance.