When I got my '66 together and generally had the carb's more or less sorted, I had a flat spot off idle. As you probably know, the slides are pretty lean in comparison to Concentrics
(5.5s as I recall). I had been setting the pilot jet with a ColorTune. I ended up ignoring the ColorTune and richening up the pilots until the flat spot was gone. Runs fine although it can load up if it idles too long. Not sure if that is your problem or not.
Second issue. You say yours idles fine. Typically, they didn't come from the factory that way. BSA published a number of service bulletins on various ways to get an idle. If yours idles because the cables were simply adjusted too tight to lift the slides enough to get an idle, the result will be a jerky delivery off idle and at low speeds because the cables are tight and the slightest movement changes the slide position. In my opinion, the best solution for idling was special cables that BSA put out that had the screw end for the slide at the bottom and a a second ferrule a few inches above that. A slotted threaded hollow rod went into the top of the carb where the cable goes through. The second ferrule fit on top of that. By turning the threaded rod, you could raise and lower the slide. Meanwhile, a regular cable adjuster in the outer cable permitted you to adjust for free play and eliminate the too sensitive issue of a too tight throttle cable. Once again, not sure if this is your issue or not.
In answer to your basic question, no, they are not all like that. Mine are a bit like slightly worn out carbs (occasional blip at stoplights) at low end, but just fine above that.
Ed from NJ