|I don't think a small amount would hurt, but the idea is unsound. The B12 is a "Gumout" style product, which often contain 80% or more of acetone. Acetone is a great solvent and fuel deposit cleaner, but why do you need it? Better quality gasoline has all sorts of cleaners in it already. If your carbs are gummed up it makes more sense to R&R them on a bench that running cleaner through them.
I'd be cautious on an older engine with these type of products- suddenly removing decades worth of internal engine deposits can also result in excessive smoking that won't go away. Acetone also dissolves rubber and plastic so the old style plastic fuel-T on the JH won't have a chance!