I've been interested in the subject of accelerator pumps on carbys since I first came across the Dell-Orto pumpers fitted to my 860 Ducati.
Why is it that they are there at all ? Obviously it squirts a stiff dose of pure fuel as the slide opens, grossly enrichening the mixture as it does. But when the throttle is steady the mixture is controlled by whichever jets are relevant for that throttle position.
But what I wonder is why MOST carburetors, from old Amals and Bings, to modern Mikuni and Keihin flat slides, get by fine without pumps. And similarly, although virtually all (non fuel-injected) car engines have accelerator pump carbs, the traditional choice of car hot-up folk when I got old enough to notice, was the SU which was a CV carb, with no accelerator pumps.
with a accelerator pump you can run the engine leaner, without having spitbacks when suddenly opening the throttle.
Very nice if you have to meet exhaust regulation, ( tests are done with constant throttle)
without the pump you simply set the carb a bit richer, to avoid spitbacks.
so you are unlikely going to find a keihin flat slide with accelerator pump.