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Hey FC, I found this over on Modern Vespa, in the archives.
[You would expect that when you pull on the throttle a cable opens a slider on the carb. It may. The problem is then for a bit you're running quite lean as the air speed is decreased because of the larger volume opening. Then as the engine speeds up, gradually the air speed increases and Bernoulli starts sucking gasoline again. This give poor throttle response. This is compensated for on some carbs with a pump connected to the cable which will throw some raw gas into the chamber. Very primitive, dirty, and not common on small carbs.
On a CV (constant vacuum) carb, the slider is connected to a vacuum chamber. When you yank the throttle, you're opening a butterfly. The slider stays closed until the motor starts speeding up. This maintains good airflow speed. As the motor speeds up, the vacuum increases, pulling the slider out of the way.
Better, smoother throttle response, no lurching and consistent fuel/air ratio.
Simple mechanical solution.
Looking at your carb, if the cable goes right into it, usually at the top, you're not CV.
If the cable goes to an arm on the side, and there is a sort of bump at the top, you're CV.
P.]
CVK, is constant vacume Keihin
Not that I knew, but I was curious also.
One of the mods I did to my scooter, along with rejetting, was to drill out the slide vacume port.
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