hi

first t you must open the choke plate and make sure you are not on fast idle cam and throttle blades closed .

then the vent arm should open the bowel vent at warm idle .

What sucks about this design is that a stuck needle/seat will fill your intake manifold area with fuel before you even know there's an issue, or if you're lucky enough to hook the vent to your valve cover breather, you can fill your crankcase with fuel. With the normal venting path, a stuck needle/seat will push fuel down the carb throat and cause the car to run like crap immediately so you can shut it off.

I had a freak occurrence with the car idling in the driveway, the needle and seat stuck on center carb, and it filled up the intake area with fuel before it started spilling over the back of the block to the ground and I saw it. Luckily no fire. After that, I capped the bowl vent tube and haven't had any hot starting issues.

So I tried Transman’s suggestion about loosening the pivot screw. With it loose the vent lever moves closer to the carb link but still doesn’t quite touch. Then in order to tighten the screw again it pulls back to the locating bosses. I’m thinking the next step will be to bend the rod or maybe add material as suggested. Seems like a weird design.

Hummm. So it sounds like if the vent doesn’t function at all it’s ok?

There's still a main vent inside the air cleaner that sticks up by the accelerator pump nozzle. The external bowl vent is supposed to keep the fuel vapors from going up the tube when the engine is off and laying in the intake manifold, leading to a richer (tougher) hot start. Considering there are two other carbs doing the same thing (and contributing to idle) without a bowl vent, I'm not sure why secondary venting the center carb matters so much.

Mostly wrong. The external vent was mainly designed for emission vapors. Sending the vapors into the system and returning them to the tank.

Since I'm "mostly wrong" please explain how the first iteration of this design (a plug on the top of the float bowl that vented to the engine compartment) did anything to capture emission vapors.

Carbs built after Jan 1, 1970 all generally have the same vent as the OP picture. It is correct that it was a design made for emissions control. Previous to Jan 1, they were not required. I’m sure it was an EPA thing. As to why it’s not working as it should direct from Holley, is a mystery. I restore six pack carbs now that Bill O has passed and have many carbs in the works right now. I’ve got some original carbs that are off of my AAR so I’ll inspect them when I go to the shop. I’ve done multiple sets of the early style bowl lately but I have a set in progress with the emission bowl so maybe I can check it out today. I did have a problem with the last emission bowl set and I used a new Holley vent arm. I had to bend it quite a bit to get it in the proper specs. I did not try and compare an original arm. Looks like I may have to do that. Be careful in bending that rod. It is made out of hard material and does not bend without a lot of force. You can mar the plating very easily, though.