Jason - sorry for the short answer previous ; got busi with something else.
Some carbs vent thru the gasket in the carb.inlet , some vent out the upper side of carb , some vent straight out the top , some will have a small rubber hose loop attached , some won't . So without taking my CV 15 carb off and checking , I can't tell you exactly how it vents, but I did look at mine , (so just found out), it vents directly into the inlet throat of carb. so is invisible from the outside,as it's cast into the housing.so I doubt that would be your problem.
You must test the integrity of needle /seat with either a vacuum source,or about 1.5 or so psi air pressure in place of fuel hose at fuel inlet to carb. If it does not hold pressure / vacuum when the float is at the "full" position , then you must get back in to the needle seat interface . Must pass muster here , or your'e just gonna keep chasin your tail around this issue .
One other thing I can think of ; is to check to be sure you don't have a valve hanging open , causing intake pooling , but that is not the most likely cause . And just because it has just been re-built or even a new carb. , It could still have some junk getting caught in the needle / seat from deteriorated hose , or other debris that wasn't flush out as completely as you may have thought. Run a new fuel filter at the inlet to carb , and don't think that just cause the air filter "looks" clean , that it's not restricting air flow !! Replace it too . Check needle / seat interface . Flush every thing in the carb that you can see with brake cleaner , or other suitable spray cleaner , and maybe even things you can't see . Put it all back together . Take two aspirins ; Get some sleep . Try again tomorrow .