Results: with ported vacuum driving the vacuum advance, throttle modulation off idle is MUCH improved, and the need to brake torque at stops is reduced (though not eliminated). Manifold vacuum is 15" at idle, ported vacuum is 12" (this is with the throttle blades completely closed). When the throttle is opened, both drop, though manifold vacuum drops much more than ported does, probably because the ported side has all of Earth's atmosphere to suck on. Both vacuum levels then increase as engine RPM increases (all this is with no load). The spark advance is thus not being retarded so much off idle when using ported vacuum, and so it makes sense that power comes on more gradually. That was a very insightful suggestion, thanks!

I have also noticed a "chug-chug" mode when starting warm without opening the throttle- there isn't enough vacuum to advance timing, so the engine chugs along like a steam engine at low rpm until it either dies or you open the throttle a little, which develops vacuum, which advances the timing, and then the engine idles smoothly with the throttle closed. Is this why they had throttle stop solenoids "back in the day"?

Carb numbers are 3437S front and 3858S rear. The dies used to make the stampings are very different, so I suspect they are not of the same vintage.

Now if I can make it smoke a little less...

3437 is for a 383, the other is 413 but they are small secondary carbs and both are rear carbs. Not sure how you have progressive linkage with them. All extra vacuum ports plugged? Which carb has distributor connected? You would be better off with some clone carbs or real hemi carbs in my opinion.

Vac advance should be on primary carb, which is the rear one, and ported vacuum.
Smoking ? Oil or fuel ?
On mine, we had to stagger-jet, change the metering springs, and use silicone umbrella valve seals from Silver Seal; cleaned it right up.

After all this fiddling I don't doubt it. Vacuum advance is now connected to the ported vacuum port on the 413 carb (rear), all other ports blocked or capped except the big one on the back of the rear carb for the brake booster.

I made the throttle linkage with spare parts and stuff from Ace Hardware. It seems to work fine. The photo was taken before the rear carb went for refurb; it has a choke and a fast-idle linkage now.

Something is not right as you should have NO vacuum at the front (PORTED) opening at idle. If you look at the two pictures below, In the 1st pic you will note the side port is much lower than the front one. That is because the side port sees manifold vacuum for an external use such as a choke pull off. The front port is higher as its opening is ABOVE the butterflies and NOT exposed to manifold vacuum. The vacuum at that port is created by a "Venturi" effect. IE: as more air passes by the opening it begins to try and draw air in creating vacuum.

The second pic shows the path for the lower port. The Front port cannot be seen as it is ABOVE the butterflies. If yo have the carb off, and open the primary butterflies you should see a small round hole that feeds the front port. Blow air or carb clean through it
It cannot have any vacuum being drawn at idle unless, A. the butterflies are too far open or, B. the butterflly for that side is not closing in conjunction with the other side

Your chugging when starting is likely due to your not opening the throttle a bit when starting. These cars are not fuel injected and need to be dealt with differently than today's vehicles. The other possibility is that fuel is percolating in one or both of the carbs

I am not trying to be offensive but you have a 25K motor with mismatched carbs and are asking for more expensive problems if you don't either A. get a pair of carbs meant for a dual 4 street setup. Or B. Drop back to a single 4 intake. Most will not notice with the air cleaner your running. Either way you will need to spend some $$ to do it properly. If you happen to have a nice wife🙄🙄🙄 PSST, Santa's coming Actually you might need this one

BTW, I have pre made factory appearing throttle cable brackets and kick down adapters for the single 4 setup

Here is a video showing an alternative to drilling throttle plates. DISCLAIMER: This is an Uncle Tony video. He seems to be controversial. None the less- here it is:
https://www.youtube.com/watch?v=JvL0-pBWb44

I'm just having a hard time trying to understand how a multi carb setup would ever need drilled plates. There's at least one extra idle circuit that can be used.

Somewhere you are getting extra air (I still say there are a pair of butterflies not closed somewhere) Two holes in the butterfly don't cause it to idle at 2000.

Vacuum at a part-time port means that those primaries are open.

Maybe you think your closing the butterflies but your linkage is hanging them up.

Your linkage is attached wrong. The slotted side goes to rear carb. Rear throttle can open without pulling on front. At WOT you adjust that linkage that both carbs full open. It looks like your linkage is too long and already out of adjustment and the front may not be opening soon enough or go full. But that is just a guess from the picture.

The choke pull off tube should be plugged on both carbs but I am not sure what type of choke and how they attain pull off. On the hemi integral choke, there is no tube and the housing mates to the pulloff port. You can then add the electric to the housing. I attached a 70 hemi rear that has the electric inplace of a heat tube type.

I think they ruined that rear carb for you. You can't get a decent balance anymore. They should be able to replace the primary blades for you.

Everything about hemi had to do with proper distribution with the hemi manifold. Staggered jetting and rods for some. Long down stream venturis and venturi tabs on some bores.

The linkage works as you describe. The slot is at that end because it made the hardware work out. I also made sure neither accelerator pump either held a throttle open or kept it from opening fully. Installed choke is electric and covers a vacuum port behind the housing. Seems to work OK. Rods and jets are staggered but that's just copying the values from a book, not experience or tuning.

What carbs would you recommend?

Edelbrock makes a pair that are for a dual 4 setup But I don't have any experience with them.
The bore spacing may be an issue as well as the air cleaner opening size. The early AFB's had a smaller opening than the carterbrocks and holleys. I believe the bore spacing is also different but not 100% on either. I have both 5/16 and 3/8 inverted flare fittings for the edelbrocks to retain a stock appearing fuel line.

What year is your car? You can pick up early version Hemi carbs at a more reasonable price then later carbs. I have a set of 66 carbs but they are a little pricey as they are cadmium plated so would be more for a points car. I do have a 67 set but have not finished them yet.

If you went the Edelbrock or early carter 9625 types, I could provide you with the correct early tops that are 4 1/8" opening like original.

Hemi had 1 7/16" primary with 1 11/16" secondary. 4139/4140 for the 66 cars. 4139/4343 for 67 cars. These would be the pre emission carbs and air bypass type.

I think Mancini sells the hemi clones with reproduction levers and jetting for hemi. I also think someone had a set on ebay for sale recently.

More about your motor and I am sure folks could help steer you.

Engine is a '66 out of a drag car and rebuilt years ago. Carbs I have been messing with came with it. Has lopey cam and headers, stock intake. About 7-9" of vacuum at 900 rpm, which can only be reached with no vacuum advance attached. Current rear carb (rebuilt 413) shows about 12" ported vacuum at 1150 rpm, which is where it idles if vacuum advance is hooked up. Jetting is as close to stock as I can get it, runs OK on 91 CA gas plus octane booster.

Anyone get the clones from Harms Auto? Engine is not in an original Hemi car, so functional equivalents are OK.

Since you have a bumpy cam, stock jetting would only be a starting place, at best.

Those tiny 413 carbs will hold you back a bunch
I would sell them and either find a set of Carters for dual in line carbs or a set of the Edelbrock's and set them up like the OEM Carters were with that weird stagger jetting that works really good