I’m putting my money on a leaky vacuum booster. When it’s running badly, run a rubber hose from your ear to where the brake pedal goes though the cab into the booster and listen for a hissing noise.
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..... just got a vacuum gauge and connected it for the first time this evening - was getting about 15 with variance (up and down) 1-2......
Out of curiosity, have you tried retarding the ignition a bit? Or even disconnecting the vac advance and blocking it off...
Also make sure your ignition system, coil and leads etc are in good nick...
Don't know if this has been asked, but is the pcv valve ok? If it has one?
With it idling ( I guess you have to choke it, so hard to see I guess), is gas spewing out the venturi on the passenger side? Maybe you can back the choke out enough to see in the top of the carb. You should not see gas there when at idle. The gas should be getting to the intake via the idle circuit, which dumps the gas out below the butterfly at the bottom. If you see gas being dumped out the venturi during idle then the idle circuit is blocked some place.
In troubleshooting mine I traced the path of the idle circuit. Saw where the passages lead to the fuel cut solenoid. I energized the solenoid and then blew carb cleaner through the passage at one end and verified the cleaner spit out the hole that leads to the bottom section of the carb. This will verify that the fuel cut solenoid is opening.
I had an idle problem that sounds like yours and I ended up finding a piece of crud in the bottom section of the carb that was blocking the idle circuit. If you have the carb off, take off the top. Now remove the idle mix screw from the base and blow compressed air into that hole. This should force air out a hole in the bottom section of the carb that leads up to the top section of the carb, leading to the fuel cut solenoid passage. Make sure the idle circuit passage that leads up to the top of the carb is clear. That way you know that the gas coming from the fuel cut solenoid is getting all the way down.
And trace it through the bottom part of the carb where it leaves the bowl in the first place. It leaves the bowl through a passage, then goes through a jet and up into the top of the carb. Make sure that jet is clean. Once mine had the jet clogged and would not idle.
The gas being supplied for idle leaves the bowl through a passage, through a jet in the bottom part of the carb and then up to the top of the carb where it goes through the fuel cut solenoid. Then through another passage and down to the bottom of the carb on the passenger side where it gets metered by the idle mix screw and dumped out and goes to the intake.