Still hesitates off throttle...

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well as stated before way back , some carbs have a vacuum pickup off a point in the venturi
i.e mid way up the carb.

carb basically has a wide section foillowed by a narrow section with the main circuit nozzle near the bottom of it and then a wide section. The point after the narrow section has the area of lowest presure/highest vacuum. with some air flowing through the carb it will allways be below atmospheric pressure in this spot. which is why fuel rises up out of the bowl and out of the nozzle, fuel in the bowl is being pushed by atmospheric pressure making it it step out of the nozzle into the air flow

a vaccum takeoff here would see vacuum at all rpms apart from idle, and it would get greater with RPM.
i dunno what this vacuum source would be used for. but some cars have vacuum control for

1) retard on the dizzy
2) air directing flaps in the air cleaner
3) headlamp flaps and aircon control
4) vacuum activated swicthes for EGR or converter lock up

etc

if the vacuum can on the dizzy is plugged into this, you may see the symptoms you describe.

or it could be as simple as a lazy vacuum can, quick to pull in advance, and very very slow to let it go once the vacuum source is gone . seems to be a feature of chinese dizzys, vac can doesn't work..

have you got a mitivac brake bleading pump. you can check your vacuum advance with it. but clean any brake fluid from the pump and pipes first.

but i agree that the actions taken so far sound like a move that makes sense

bigger main jet will fill in a gap between progression and main operation, in some cases
raising the level in the bowl by adjusting float height means the main system will start earlier

both would help if the issue is caused by a lean spot

but then again
a better igntion system and more spark power could also help here
a marginal igntion can not ignite as weak a mixture as a better one can.
and this varies with igntion advance as well. if the timing is too retarded you start the fire too late.. expect hot idle temps.
if the timing is too advanced you have not given the crappy just off idle lumpy mixture a chance to swirl and vapourise . its like flicking lit matches into a bucket of diesel.... or trying to start a fire with the biggest log in the pile, rather than the kindling left after splitting the logs.

it will be a combo of igntion and carb and spending time or money on the igntion first kinda makes sense to me... new set of plugs check the igntion leads clean up the cap contacts etc.
 
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Sounds like my 1969 Dart slant. It would stumble off the line and sometimes die when leaving a stop light, which was embarrassing. I would drive with both feet to goose the throttle so it wouldn't die. I also had to fool with the choke setting between summer and winter, since I didn't understand the choke pull-off then, thinking it was a damper. No internet then to ask and learn.

I dealt with it for 18 years, trying 3 rebuilt carburetors. I long suspected a vacuum leak and even tried a different manifold. Mechanics said "valves", so I had the head rebuilt twice. Finally, when same issue with a new long-block, I tried a 4th carburetor and it purred like a kitten. The Holley 1920 has a sealed metering block which gets clogged (idle metering circuit) and most rebuilders don't touch it. Some here said they know how to blow it clean with an air nozzle. I no longer have that car (stolen in 1994) and my current 1964 Valiant Slant has a Carter BBS which works fine.

One test for running lean is to close the choke plate by hand (or just partially cover the top inlet). If the idle smooths, you know it was running lean. Of course, installing an O2 sensor, especially a wideband type, is the ultimate diagnostic. Forget looking at spark plugs since that only indicates the average O/F, not conditions at idle.
 
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