Slant 6- Idle and Running

Good luck w/ your Holley 1920. My 1969 Dart idled poorly for decades, and it didn't have the complications of your later engine. I tried 2 rebuilt carbs, plus tried rebuilding them myself. After I changed to a whole new long block, and same thing, I tried a 4th carb (from Carquest I recall) and it ran like a kitten. All the others ran lean at idle. I think the problem is there is a sealed metering block for the idle circuit which gets clogged and most rebuilders don't touch it. Some guys here say they know how to blow one clean on an assembled carb by applying shop air to a hole in the base or throat (forgot). My 1964 Valiant slant came with a Carter BBS. That engine idles so smooth you can barely hear the engine is running with traffic going by. I also hated the side bowl on the Holley which can leak fuel and gaskets can be a pain. Overall, I would say the 1920 was designed to be ultra-cheap. Other manufacturers used it too. Many here say the later Holley 1945 is better (never tried). Another idea is the special Holley Economaster after-market they sold ~1980 to improve mileage. I recall a slant version. You occasionally see them on ebay. A harder option is to change to a 4 bbl intake (Offy or Edelbrock) and find a rare 390 cfm carburetor.

Another thing I should have known at the time was about the choke pull-off at the top. Insure the diaphragm isn't torn. Push the stem in, block the vac supply tube w/ your finger and see if the stem stays in. The choke should be set to fully close when cold. As soon as the engine fires and builds vacuum, the pull-off should pop the choke open just slightly (1/8" drill bit). Mine was bad, so I always had to fuss with the choke adjustment summer to winter. They go bad because people spray carb cleaner on them, or get a little gas in them.

After fixing the fuel issue, I suggest considering an electronic ignition upgrade. Points are a maintenance pain, tricky to adjust, especially in the dark grub zone of a slant, and give a weaker spark. I had a Crane XR700 optical retrofit in my 1969 for years, but outdated now. Pertronix is similar and Ignitor II better (no ballast). In my 1964 slant, I installed an e-distributor which triggers a GM 8-pin HEI module and their coil. Too bad nobody makes a ready-to-run HEI distributor for the slant, as they do for SB and BB. I bought one for only $45 for my 273.