Coming out of the red area, the center Venturi. Any way to adjust that flow?
Al,
No, no adjustment. If you look at the picture, the horizontal light dotted line is, I think, the reference line for design fuel level. Notice it runs through the float pivot point. The passage that travels to the auxiliary (booster) venturi extends to below fuel level. It also travels a distance above the fuel level so fuel will not be pulled through the main system until enough airflow at the booster venturi(BV) causes enough vacuum in the BV. The depression in the BV will not be sufficient to start drawing fuel over the high point until the upper ranges of the idle system is reached. You can sort of see where the throttle blade should be for that transition point when the throttle blade just passes the idle progression holes just above where the idle mixture screw passage is located near the base. There is nothing to adjust between the base and top of that passage (air emulsification passages don't function without airflow through the carb, either).
Your fuel bowl is filling up to a level that allows excess fuel to drain out the BV like an overflow, instead of air at normal atmospheric pressure in the fuel bowl, you might have air at a different pressure than normal that causes the fuel bowl to overfill and overflow. The vents might not be working properly, but that seems to unlikely. There are two vents, one in the top cover and one that is a passage into the top of the carb throat.
Too high an internal air pressure would tend to push fuel up and out the BV. I just don't know what would cause a consistently high internal pressure at idle.
Too low an internal air pressure would cause the pump pressure to be more effective and possibly overpower the needle and seat. If the floats are possibly in a condition where they are already lacking the authority to control fuel flow, then this condition would only be worse. The only method I envision for this is if engine vacuum is leaking past or through the economizer diaphragm and venting that is not operating properly.
Another possibility seems there might be a pressurized leak path before the fuel even gets to the needle valve and fuel bowl. The drawings seem to show a potential across the top of the emulsion block straight to an opening in the passage to the BV. Two drawings seem to disagree about this so I'm not sure. Ensure the gasket is in good condition an properly placed between cover and emulsion block and neither surface is warped.
Float level setting can't be set by checking the fuel level in the bowl. When the floats are removed the level drops making the new level unsuitable for measuring. The proper way to check is with the needle and float assembled to the top cover assembly, hold upside down and measure the distance from the highest point( bottom edge as normally installed) on the float to the gasket surface. The setting is 32-33 mm at that point. Check both sides. I guess it might be possible to install the floats so they can't close the needle valve completely, but don't think the carb could be assembled in that manner.
I'm out of time for this evening. Hope this helps to some degree. Still just some random thoughts.
Good luck,
Don