950 3 barrel holes in main body

[Ndean]

Hi all . This is my first post I’ve been learning and reading for a few years on this forum and the old one it’s great can’t get enough . Anyways I picked up a 3916 950 3 barrel to play with and I researched some old threads but no discussion on it . Why is there a hole in each Venturi close to base of the main body ? I thought someone drilled them in there at first but I googled some images and they are factory . Just curious what the reason is for them . Thanks .

[rgalajda]

Approximately .088" holes in the side of the main body into each throttle bore near the bottom?

This is the Vapor vent, so the engine won't be so rich after a hot soak.

[Ndean]

Yeah those are the ones . That’s a different way of doing it for the Vapour, first time I’ve seen it done that way , must have been cheaper to do it like that instead of the extra bowl vent with the flap .

[GTO Geoff]

I presume these holes are above the t/blades. Have seen them in AFB carbs also. Do not see how 0.088" holes will do much for vapour release when there arehuge holes, the venturiis, above them for vapours to dissipate. Do not see the logic unless I am missing something.

[rgalajda]

I have a 4131s carter AFB / 1966 440 on the shelf here which has all four veturis drilled.

Throttle Bore Vents: To insure quick hot engine starting, four holes are drilled through the main casting body above the primary and secondary throttle valves. These vents in the primary and secondary sides of the carburetor permit any accumulated fuel vapors present in the bores to escape to atmosphere.

Fuel vapor is heavier than air and will ‘spill’ out the hole instead of flow down into the engine and fill it with a too-rich mixture. This was so a hot re-start would fire on the bump of the starter button. Emission requirements prohibited venting vapor and carbs newer than about 1966 don’t have the feature.