Sounds like dirt or corrosion in your carb's idle circuit. You can try clearing it out by propping the throttle open with the highest step of the fast-idle cam, removing the idle mixture needle, and spraying carburetor cleaner into the mixture needle hole with the engine running. Use your carb spray to clean the area around the needle before removing it, so you don't wind up flushing dirt
into the hole. Then reinstall the needle, apply some carb spray to the tiny air bleed holes that face the sky (you can see them near the edge of the carb throat when you look down it) and readjust your idle mixture—carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted
here for free download.
The first-year (1974) 1945 was a terrible carburetor, not fully baked. By a couple of years later they had fixed most of the problems with it—the cars still ran poorly, but mostly because of super-lean jetting and a pile of halfassed, thrown-on widgets to squeak the cars past federal new-vehicle emissions type-approval tests so they'd be legal to put on the market. Lean jetting is easily fixed, and widgets are easily removed or reworked (thoughtfully, not just willy-nilly, or you can make stuff worse instead of better).
The BBS was a decent-to-good carb design which had its bugs worked out many years before 1974 production, and the last few years of it—1972 through '74 in North America, a decade later in export markets—were only for trucks, so jetted for torque and driveability, not for passcar emissions compliance.
