"Throttle blades" are not the same as "idle mixture screws".
The "thinner" the charge that goes into the cylinders is, where "thinner" = less dense. it takes more time for the mixture to light-off and burn by the time it gets near TDC, so generally, more initial timing acts better than less timing might. Just like a combustion chamber that has better burn characteristics will need less total advance than one which is not quite as efficient. The normal Chrysler chamber, which is similar to another popular V-8, takes about 38* BTDC total whereas that same popular V-8s sibling W-motor takes more like 31* total, for example.
If you read some of the carb tuning books, they claim that you can get a smooth idle with the timing advanced at idle, yet have an off-idle hesitation. This is due to the idle transition ports not having enough of them exposed below the throttle plates at hot base idle. Ideally, about .040". Less that than, off-idle hesitation. More than that, off-idle hesitation/flat spot. OEM carbs are set-up for that spec, but aftermarket carbs might not be and have more lee-way. Or perhaps the aftermarket carbs have a bit more feed fuel in the transition ports than the OEMs do? Not sure.
This is the basis for my suggesting a more normal (for back then) 12.5* BTDC initial timing setting. Which should put the OEM carb's throttle plates in the place they were designed to be in, or near it, to decrease/minimize any off-idle drivability issues. At the lower initial timing, to maintain the spec rpm (probably about 600rpm in "N"?), the throttle blades will be open a bit more to expose a bit more of the lower part of the transition ports, so they come online as designed.
As to setting the hot base idle speed mixture, with the idle mixture screws lightly seated and then turned 1.5 turns out, each, set the 600rpm idle speed. Then, fine tune the mixture screws, individually, to seek the highest rpm with each screw. If the hot base idle speed might increase, then adjust the idle speed screw to get the rpm lower. Cycle repeat with the mixture screws. At that highest rpm, the manifold vac should be maxed, too.
At this point, you can verify "lean best idle" by carefully turning each mixture screw "in" until you get a 20rpm drop, then go back to the prior "max speed" setting, each one that way. To me, this is a more accurate method than just using a vac gauge alone. I found the vac gauge method to have a bit too much "margin for error" for me, as to getting things "just right".
Then, with an assistant to help, have them firmly apply the foot brake and shift the trans into "D". Walk around to the exh pipe and check for smooothness of the gas flow. If you can feel the pulses, increase the rpm about 25rpm. If it is already one smooth, gentle flow (but HOT), that's fine as is. This step can work better on a single exh system, obviously.
This "hesitation" you are speaking of, is it when coming off-idle slowly, with a 1/3 throttle punch, a 3/4 throttle off-idle punch, or WOT off-idle? At our local 1000' elevation, the normal=spec accel pump setting seemed to work better for me.
These things work for me. Your experiences might vary.
CBODY67
