I get asked about the external fuel vents on 1960s Ford Holleys pretty regularly, so lets have a chat about them. I am not going to discuss each weird idea Holley had, I will only focus on performance carbs.
I will write the whole deal here as one massive wall of text but I will also have info with each picture, so maybe scroll through if you are interested.
There were several styles of bowl vents.
Firstly, we all know the two bowl vents that are part of the main body casting. These vent full time inside the air cleaner ring. Every 4150/4160 Holley has these.
There is no real need to discuss these in depth, they provide a vent to atmosphere. Without these fuel could not enter or exit the bowl. Carbs are fully based on pressure, both positive and negative inside the venturi, the bowls need to be at a standard pressure. The easiest stable pressure to attain is provided by the weight of the atmosphere.
Ok, so why did Ford feel the need to add additional vents outside of the air cleaner ring?
If fuel were to boil or get hot enough to begin to vaporize in the fuel bowl, that fuel would emerge inside the air cleaner ring and would change the air fuel ratio of the running engine. In 1961 there were no high powered multispark ignition systems. Single point distributors provided spark and did so sufficiently. However this makes low speed air fuel ratios more critical. if the engine got a little out of range due to an overly rich condition, engine running issues resulted.
Imagine if you will: You own a brand new 1969 car with a 428ci FE engine. The engine comes with a 735cfm carburetor. Heck yeah! Performance engine fun!!!! Yes. But this is also a car sold off a dealer lot to anyone who walked in with the money. The car was under warranty and anyone from the 18year old performance enthusiast to the 70 year old lady who wanted to do grocery runs was expected to be pleased with performance during all usage. We in 2022 often forget this. For us these are toys, hotrods, etc. In 1969 this may have been a daily driver. The car was expected to start and function in freezing temps as well as the heat of summer. So often the cars had exhaust heated intakes, coolant heated carb spacers, emissions equipment that made the engine run hot, etc. People tend to think that annoying issues like vapor lock or fuel boilover on shutdown are a modern phenomenon caused by bad fuel, this is not the case. But the OEM manufacturers had to make these systems function well.
Here you are, driving your mean ole Mustang with a 428 out in the country at speed. Of course a few sprited acceleration runs happen, the engine is hot as you roll into town at 35mph.
At this point the heat in the intake is rising and the carb is going from ice cold to very hot. As you pull into town a light turns red and you have to idle for a minute. As you begin to accelerate the engine is a little rough. 1/4 mile later and you are stopped again due to a train slowly rolling through town. Idling for three minutes becomes difficult. The fuel in the carburetor is at a full boil. Keeping the engine idling at the stock 625rpms is difficult and you find you have to speed engine to 1200rpms to keep it from stalling.
This would be an issue if all vapors vented inside the air cleaner through the main body vents. But Ford and Holley found a work around, an external fuel bowl vent. This allows fuel vapors to exit the boiling carburetor without impacting air fuel ratio.
Styles: -The first I’ve seen was 1957-58. The vent rod was closed by spring pressure on the side of the bowl as the accelerator pump lever pushed down. The spring was retained by a small Eclip. The vent seal was plated steel. The rod passed through the bowl casting itself. Tho only used for a short period, the side hung bowls retained the spot on the casting for decades. It is pretty rare to see this style as it was only used for a short period.
-1959 until 1963 I have seen the lever activated side hung bowl vent on the primary fuel bowl. The vent seal itself was metal and retained with a small 1/8in Eclip. These function by being held open by a small lever on the throttle shaft. As the throttle opens a small spring closes the vent. If set properly by the book spec, typically the vent is open at idle and closed by 1200-1500rpm. List 1848, 1848, 1850 all used this system.
-In 1960/61 Ford came out with a few 600cfm carbs for performance usage. They also offered their 3x2 setup for Galaxies and Thunderbirds. These carburetors all had a small metal cap pressed onto the primary bowl. Under the cap was a 1/4inch hole. This vented full time and did not require a lever. Carbs that used this system were all of the 3x2 setups, even including later 260/289ci engine setups. List 2112, and 2328 also used this vent cap on the primary bowl. In late 62/early 63, Ford made their first 2x4 550cfm setups with List 2652-1 both having this same venting arrangement.
-1963 arrived and perhaps there was an issue with only venting the primary bowl, so the metal cap was retained for new models, but a secondary bowl vent was introduced. This had a small hollow brass insert that stood up from the center of the secondary bowl. This vent had two holes in it to vent vapors to atmosphere. Ford used this setup on three carbs, List 2599 a 600cfm 4v, List 2668 a 700cfm 4v, and List 2804/2805 which was a 2x4 600cfm setup. I have only seen this on 1963 engineered carbs. These Holleys were used later both as over the counter use as well on GT500s etc, but the engineering was all from 1963.
-For whatever reason, Ford decided to seal the bowls on all carbs engineered in 64/65, so no introductions had external vents at that time. Ford’s race carbs, and all their 2x4 designed created in this time had sealed bowls. Never again did a 2x4 setup have full time vents. (Except for ones that utilized 1963 engineered carbs)
-1966/67/68 Ford 390 GT carbs were 600cfm. These all went to a sealed secondary bowl and a primary that used a vent level actuated by the primary throttle shaft. This vent rod is for center hung bowls and is different from the previous side hung. They look very similar but had some differences that make it hard to use one with the other. The metal vent seal was replaced with rubber. Dozens of List were just like this, 3557, 3530, and even 4088. Too many to list, but from what I have seen, nearly all of the 66-68 Performance 390s had this venting system.
-1968 the 428 Cobrajet made it appearance with a new 735cfm carb. All of these had the same venting system as the GT390 carbs. 68/69 and some early 1970 cars used this. Boss429 and Boss302 both used this as well.
-1970, something changed. All Boss and 428cj engines used pretty much the same carburetors, but all have the bowls sealed. Due to other minor changes, we can only guess there were emissions reasons that prevented dumping raw fuel vapor into the air.
In 2022, I still like having the bowl vents. They do prevent some fuel vapor issues. That said, with fuel insulating technology, bypass fuel regulators and return lines, phenolic carb spacers, electric fuel pumps, I honestly think we do not need them. In a performance only application, there is no reason to retain exhaust heated intakes or any of the cold start tricks. I like to keep the vents on for appearance reasons, but being that they cover a 1/4in hole, they are easy enough to plug. A metering block well plug works nicely, or simply bend the arm slightly so the vent never opens.
Leaving it open is not a huge deal, but it can cause fuel staining on top of the carb. A stuck needle can also very quickly soak the entire engine bay with hot raw gasoline. Given the option, I prefer to do without these issues.