Holley 1920 Stumbles

Check to make sure your distributor vacuum advance and its hose are in good shape (holds vacuum). Carburetor operation and repair manuals and links to training movies and carb repair/modification threads (including a lot of Holley 1920 material) are posted here for free download. Note that the 1920 has a lot of tiny blind passageways in its metering block; once these get full of powdery corrosion they cannot be cleaned out and the carburetor is well and truly Done. I have a new-in-box main metering block and, for that matter, a new-in-box (not "remanufactured") carburetor if you wind up in need -- send me a PM.

Other things worth mentioning: You say the carb is "fairly new", which suggests it is a "remanufactured" unit. These are almost always garbage that cannot be made to run right no matter how much time, money, and effort you spend trying. Also, improper valve adjustment can and will cause all kinds of driveability issues including stumble on acceleration. Use the valve adjustment procedure posted here. And tune-up parts and technique suggestions in this thread.