The Carter BBS was used on slant-6s from 1960-1971 in passenger cars, 1961-1974 in trucks. The Holley 1920 was used on slant-6s from 1962-1973 in passenger cars, 1966-1973 in trucks. There were
many variations of both carbs across these year ranges; all of them will bolt up to any slant-6 1bbl intake, but the throttle hookups, vacuum ports, and other aspects are not all the same. In most years, there were Holley 1920 and Carter BBS carbs for most slant-6 configurations (displacement, transmission, vehicle application, emissions package, etc.); which carb any given car got was a function of what was in stock at the build plant.
Overall, the Carter BBS is the better design and the Holley 1920 is the cheaper design. There are certain aspects of the BBS that are objectively superior -- the pontoon front-hung brass float, for example, is better than the 1920's side-hung foam float, which can create stall-on-left-turn problems. And the 1920 has a bowl gasket always below the fuel level, a much more leak-prone design than the BBS, which has no such gasket arrangement. The main metering block in the 1920 will eventually get crudded up with corrosion that cannot be remedied or cleaned out. On the other hand, the BBS is subject to casting warp at the mating surfaces of the air horn and main body castings due to overtightening of the screws over the years, and the BBS uses a leather accelerator pump plunger cup that isn't as durable in today's alcohol-contaminated gasolines as the rubber diaphragm in the 1920.
At this late date, considering these design benefits and drawbacks is a luxury available only if one has the choice of a
new BBS vs. a
new 1920. That is not usually the case, so the best carburetor becomes the one that is in the best, least-abused condition. That may mean a Holley 1945, the carburetor model used from 1974 on up. Most of the 1945s were calibrated with lean jetting to squeak the cars past Federal emissions type approval tests so they could be certified for legal sale; this can be remedied with larger (standard Holley) jets.
"Remanufactured" carburetors are in almost every case a hopeless waste of money, time, and effort.
Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted
here for free download.