Motorcycle Repair: honda cv carbs, manifold vacuum, cv type

Question
Bill, my 1967 honda cl125a uses a cv type carburator with a rubber diaphram operating a slider valve in the carb throat.  The diaphram is attached to the top of the valve with a small plastic disc that's held on by 3 small screws yet the disc has 4 holes.  Could you inform me of the function of the 4 hole?  It is NOT a screw hole and goes through to the slide valve thus eliminating the vacuum.  How then does the slide valve work without vacuum?  

Answer
Gary, the function of the small hole is basically a controlled vacuum leak, allowing the pressure differential between atmospheric pressure and manifold vacuum pressure to equalize, depending on the throttle plate (butterfly) angle.

This helps keep throttle response in a more linear manner.
Otherwise, the strong manifold vacuum signal would just suck the slide/diaphragm wide open and then slam it down when you back off the throttle. The small hole is a bleed hole to help smooth out the transitions.

Here is a website with some carb basics...
http://oldmanhonda.com/MC/Rcarbs.html

Bill Silver
www.vintagehonda.com