The plot deepens. Just found a very clear explanation of the CV carb operation. One section deals with the pilot circuit and its operation, as below.
PILOT CIRCUIT
This is sometimes called the "idle circuit" but it does a lot more than control idle. And it is perhaps the most misunderstood of the three circuits. The role of the pilot circuit is basically to run the engine when the throttle is closed, as when the engine is idling or the throttle is closed on deceleration. But this circuit is also the main source of fuel at very small throttle openings. As the throttle is opened past 1/4, the importance of this circuit diminishes, as the main fuel supply is now provided through the main "fuel hole" and controlled by the needle/needle jet and ultimately the main jet. But the pilot circuit does remain active and makes a (progressively smaller) contribution all the way to WOT.
The typical set up is this: The pilot circuit get its fuel from the float bowl through the pilot jet. The circuit also has a pilot air jet, but the purpose here is not to provide all the needed air (as in the starting circuit), but to provide air to premix with and partially aerate the fuel before the mixture enters bore and completes the atomization process with air travelling through the bore. Such little air passages are sometimes called "air bleed" circuits.
This circuit typically has two outlets. One is called the "pilot outlet" and is located on the engine side of the butterfly valve. This outlet supplies the fuel mixture to support idling and deceleration (that is, off-throttle running). It has an adjustment screw which controls the amount of fuel mixture entering the bore under off-throttle conditions. This screw is sometimes mistaken for an "air screw" but it is not. Turning in (right) reduces the amount of fuel mixture, and turning out (left) increases it. In for lean, out for rich.
The other outlet is call the "bypass outlet" and is located right at the point in the bore where the bottom of the butterfly comes to rest when closed. Typically, the butterfly is set a tad open to permit just a slight amount of air to pass by at the bottom to support idle and deceleration, and most of the fuel for these functions is supplied through the pilot outlet. But as the butterfly is opened, more air flows past it, and the venturi effect starts to work on the bypass Additional fuel is now drawn out of the bypass to support low speed running and cruising at small throttle openings. (Note that if the butterfly valve is adjusted to 'fully closed' the engine will probably not start or idle. It needs to be open a tad. As mentioned above, these outlets continue "giving" throughout the rev range, but their contribution to the overall mixture diminishes as the slide rises.)
An addition to this circuit is found on later Hitachi and all Mikuni carbs. This is the "coasting enricher". A typical problem in earlier carbs was the fact that when you chopped the throttle (closed the butterfly) on deceleration, there would not enough fuel in the mixture at the (at that moment) high revs to allow the engine to fire consistently. You would then get a "lean misfire". That is, the engine would fail to fire, and the unburned mixture (lean though it was) would enter the exhaust header. Then when the engine next fired, you'd get a backfire. (So backfiring on deceleration is typically a lean condition, and not "loading up" as some people think.) The solution they came up with was to reduce the amount of air in the "airbleed" circuit by about half, meaning the fuel content hitting the bore from the pilot outlet was much higher than the normal idle fuel mixture you get on closed throttle. Once the revs came down, the full air bleed would be restored for proper idling. The "coaster enricher" is activated by the strong vacuum created in the carburetor holder (intake stub) by high revs when the butterfly is closed on deceleration.
On Hitachis some external piping was added to service these diaphragm driven valves. On Mikunis the needed passages were drilled into the carb bodies.
Ref the last paragraph.
So, the drillings are there intentionally to add fuel under certain curcumstances.
I am now definitely leaning towards incorrect pilot jets.
