Effect of bleeds in Carbs

[Chryco Psycho]

The bleed circuits are like a straw with a hole in the side so the bottom of the straw is submerged in fuel , the hole in the side is the jet or idle screw restriction  & the top of the straw is the bleed , so the vacuum in the venturi or manifold draws in fuel through the jet or idle circuit , if you close the top of the straw vacuum will pull 100% through the idle or jet , the larger the hole at the top of the straw the less effect the vacuum has on fuel flow through the jet or idle circuit .
 So with higher engine vacuum either in the manifold or venturi you need a larger bleed to lessen the pull / fuel flow  & with lower vacuum you need a smaller bleed to increase fuel flow .
I hope this helps

[73restomod]

I will throw my  Here as well, if your getting an off the shelf carb Holley, Demon, Proform, Quick Fuel, etc. And it doesn't use a billet metering block, pull it out and check that the sizes and locations are correct. More often than not, I have found undersized holes, even the occasional oversized hole, in the emulsion bleeds, and or high speed air, Low speed bleeds, etc. Most carb manufacturers will list the "blue-printed" diameters' for the given orifices. If you have a good wire gauge drill set, and get as close as you can to the factory specs w/o going over. If you've already tuned the carb to the car skip this, however, it does make the fuel curve more consistent, and can be tuned further by knowledgeable persons in a dyno environment. When I was dynoing my 440 we blueprinted the metering blocks emulsion bleeds to factory spec we were getting very rich on high end, and really lean in lower mid range so we upped the jet size, and drilled the lowest emulsion hole .06 larger to compensate. Ended up getting a good AFR in the mid range, and leaned out on top enough to pick up a handful of power.